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not. As discussed in detail later, dietary purines are

degraded by a battery of enzymes in the intestinal

mucosa and enter the circulation as uric acid. By

contrast, dietary pyrimidines can be absorbed in the

nucleoside form and incorporated by salvage into the

nucleotide pool, as evidenced by the lifelong treat-

ment of patients with the pyrimidine de-novo salvage

defect, hereditary oroticaciduria, with oral uridine.

Nucleotides in infants

0006 Over the past two decades, there has been consider-

able interest in the role of dietary ribonucleotides

in infant feeds, in which significant effects have

been claimed. This debate stemmed from reports

that ribonucleotides were present in human milk,

but not in cows’ milk or infant formulas. However,

claims that nucleotide concentrations were much

lower in cows’ than in human milk were apparently

related to the state of lactation, with concentrations

higher during lactation. Infant formulas were con-

firmed as being devoid of nucleic acid.

0007 These findings stimulated investigations to deter-

mine: (1) whether the nucleotides found in human

milk result from degradation of nucleic acids, or are

actively secreted as a response to a nutritional

demand of the infant, and (2) the newborn infant’s

endogenous capacity to digest nucleic acids to ab-

sorbable products. Nucleotides were found in the

greatest quantity in human milk followed by nucleic

acids, nucleosides being a minor component. Import-

antly, the nucleotide/nucleosides were predominantly

pyrimidines, purines being present only as uric acid.

This specific profile must result from catalysis in

the breast. Enzymes capable of degrading nucleo-

tides, including xanthine dehydrogenase (XDH), are

known to be present in human milk. Evaluation of the

endogenous capability of infants to metabolize RNA

and nucleotides confirmed that the intestine of

infants, as in adults, digested RNA to cytidine, uri-

dine, and uric acid in vitro. This study confirmed the

extensive data derived from dietary purine loading in

adults that dietary purine nucleotides (if not already

degraded by gut bacteria) would be degraded to uric

acid in the intestinal mucosa. However, uridine and

cytidine would be absorbed (although the latter

would be degraded rapidly to uridine by cells in vivo).

0008 This research into the importance of human versus

cows’ milk, or formula feeding, was stimulated by

animal studies that had indicated that dietary nucle-

otides may be required for maintenance of normal

immune function in neonatal mice. A subsequent

study in healthy term infants demonstrated that in

those either breast-fed, or fed formula supplemented

with nucleotides, some indices of immune function

were indeed significantly higher (natural killer cell

cytotoxicity and interleukin-2 production by stimu-

lated mononuclear cells) compared with nonsupple-

mented formula groups. However, this was found

only in the newborn. The rate of growth and inci-

dence and severity of infections did not differ

significantly among these different dietary groups at

2 months. It was concluded that nucleotides may be a

component of human milk that contributes to the

enhanced immunity of the breast-fed infant. Another

study implied that allergic diseases develop during

feeding of cows’ milk, but not human milk. It may

be that the dearth of uridine nucleotides in cows’ milk

after 1 month of lactation could be implicated, since

these effects were independent of any difference in the

gut flora demonstrated in breast-fed, as distinct from

formula-fed, infants.

0009This debate and the problems addressed arose from

the use of animals. As indicated elsewhere in this

chapter, rodents have both XDH and uridine phos-

phorylase everywhere. By contrast, in humans, uri-

dine phosphorylase is confined to the liver, and XDH

is confined to the liver, intestinal mucosa, and breast

milk. This problem highlights the general lack of

awareness of the two curious and unexplained differ-

ences in the synthesis of nucleotides in humans. First,

pyrimidines derived from nucleotide degradation are

salvaged at the nucleoside (uridine) level, purines as

the base. Second, whereas purine nucleotides are de-

rived exclusively from endogenous sources in

humans, pyrimidine nucleotides can be formed from

uridine orcytidine ingested in the diet as well. These

important differences are underlined by the successful

treatment with oral uridine of patients with uridine

monophosphate synthase (UMPS) deficiency, which

presents generally as macrocytic anemia. A few

patients are also immunodeficient. UMPS is the

complex catalyzing the last two steps of pyrimidine

de-novo synthetic (DNS). Long-term follow-up of

such patients confirms that humans cannot only

survive without pyrimidine biosynthesis, but also

reproduce. Thus, uridine certainly can restore

immune function in humans as well.

Role of Endogenous Nucleotides, Nucleosides, and

Bases in Cellular Metabolism

0010Purines and pyrimidines are effectively anchored

inside the cell as the nucleotide by attachment to a

pentose linked to a mono-, di- or triphosphate group

(Figure 1a and 1b), principally the triphosphate. It

was originally assumed that all reactions of biological

significance took place intracellularly at the nucleo-

tide level. Attention has been focused recently on the

extracellular regulatory functions of purine nucleo-

sides (base plus pentose), or the bases themselves. The

pentose may be either ribose (ribonucleoside) or

4154 NUCLEIC ACIDS/Physiology

-deoxyribose (deoxyribonucleoside) bound by the

C1 atom through a glycosidic linkage to the N9

atom of the purine group, or to the N2 of the pyrimi-

dine group (Figure 1).

0011 The importance of purine and pyrimidine nucleo-

tides in cellular metabolism is twofold. In addition to

their role in the storage, transmission and translation

of genetic information (as the polynucleotides DNA

and RNA), as mononucleotides, they play an equally

vital role in lipid and membrane synthesis (in the

form of purine and pyrimidine sugars or lipids),

signal transduction, and translation (in the form of

guanosine triphosphate, cyclic adenosine monopho-

sphate, and cyclic guanosine monophosphate), as

well as providing the energy (adenosine triphos-

phate (ATP)) that drives many cellular reactions

and forms the basis of the coenzymes (nicotinamide

adenine dinucleotide, nicotinamide adenine dinucleo-

tide phosphate, flavin adenine dinucleotide, etc.) (See

Coenzymes.)

0012 All cells require a balanced supply of purine and

pyrimidine nucleotides for growth and survival, but

this may vary from cell to cell, depending on function.

Liver, for example, has a very complex nucleotide

profile, compared with heart. These nucleotides may

be built up by one of two routes: the energetically

expensive multistep synthetic route, or the single-step

so-called ‘salvage’ pathway. In normal circumstances,

salvage predominates over synthesis. Figure 2 illus-

trates the different metabolic pathways involved in

the de-novo synthesis of these nucleotides, as well as

the efficient recycling of the nucleotides or bases de-

rived from them during the wear and tear of daily life

(muscle work, wound healing, erythrocyte senes-

cence, protein glycosylation, providing essential

nourishment for the brain, etc.). Interestingly, whilst

this recycling takes place at the base level for purines,

it is the pyrimidine nucleosides that are actively re-

cycled in humans, with only a small proportion being

degraded further in either case (Figure 2). The import-

ance of these different pathways to the overall control

of nucleotide concentrations in the body is the subject

of many excellent reviews and for this reason is sum-

marized only briefly here.

Nucleotide Triphosphate Production and Nucleic

Acid Synthesis

0013 Purine and pyrimidine ribomononucleotides built

up by either the de-novo or synthetic routes are

phosphorylated via the correponding diphosphate to

the triphosphate, which is the active intracellular

form of most mononucleotides. In addition to the

variety of vital individual cellular functions described

above, these ribomononucleotides are essential inter-

mediates in the synthesis of the polynucleotides RNA

and DNA respectively (Figure 2), being incorporated

into DNA following the formation of the deoxyribo-

nucleotide from the corresponding diphosphate by

the enzyme ribonucleotide reductase. The latter is an

allosteric enzyme; its activity and specificity are con-

trolled in a complex manner by both purine and

pyrimidine ribo and deoxyribonucleotides. This pro-

cess is particularly active in cells and tissues with a

high rate of turnover (e.g., lymphocytes, gut epithe-

lium, skin, bone marrow, etc.). There is approxi-

mately five times as much RNA and DNA in the body.

Breakdown of Nucleic Acids

0014The polynucleotides DNA and RNA, although rela-

tively stable in most tissues, turn over rapidly in

dividing cells. Both DNA and RNA first must be

degraded to the constituent mononucleotides, which

are themselves degraded further. A variety of enzymes

capable of hydrolyzing the phosphodiester bonds

have been described and include ribonucleases spe-

cific for RNA and deoxyribonucleases for DNA as

well as nonspecific nucleases, phosphorylases and

phosphomonoesterases. The mononucleotides have

the highest turnover rate, DNA the lowest. Further

catabolism of the resulting monophosphate will differ

depending on whether it is a pyrimidine or purine

ribonucleotide of deoxyribonucleotide. For example,

whereas adenine ribonucleotides are predominantly

deaminated at the nucleotide level in humans by

AMP-deaminase, deoxy-AMP is not a substrate for

this enzyme and must first be degraded to deoxyade-

nosine and deaminated at the deoxynucleoside level

(Figure 2).

0015Purine and pyrimidine (deoxy) nucleotides are de-

graded to the corresponding (deoxy) nucleosides by

specific 5

nucleotidases. Different purine endo- or

ecto-5

nucleotidases have been identified with

different substrate specificities and may be of particu-

lar importance in providing bases for nucleotide

resynthesis in tissues where there is rapid cell turnover

and massive cell death (e.g., thymus, spleen, bone

marrow). As mentioned above, whereas the normal

metabolic route for pyrimidines is salvage at the nu-

cleoside level, that for purine nucleosides and deoxy-

nucleosides is degradation to the corresponding base

by purine nucleoside phosphorylase, prior to salvage.

This degradation is favored by the high intracellular

inorganic phosphate and low ribose 1-phosphate

levels in most tissues. Interestingly, these phosphory-

lases are not reactive toward either adenosine or

cytidine, or their analogs, in human cells and first

must be deaminated at the (deoxy) nucleoside (or

nucleotide) level.

0016Salvage is an active process for both pyrimidines

and purines. Consequently, only a small fraction of

NUCLEIC ACIDS/Physiology 4155

GDP-sugars

dCDP-lipids

CDP-lipids

UDP-sugars

UMP UDP dUDP dTTP

dTMPdUMP

cytidine

CMP

thymidine

dCTPCDP

CTP

d-cytidine

GTP

GDP

guanosine

guanine

GMP

XMP

IMPDH

p53

cGMP

dGTP

DNA

uridine

PPribP

ribose-5-p

+ ATP

p53

CPS II

AMPS

AMP

IMP

inosine

hypoxanthine

xanthine uric acid

adenosine

ADP

ATP

cAMP

dATP

HCO

ATP

UTP

uracil

Pyrimidine de novo synthesis Purine de novo synthesis

RNA

−

Figure 2 Multistep purine and pyrimidine nucleotide de novo synthetic (DNS) routes and single-step salvage (hypoxanthine, guanine, uridine, cytidine, D-cytidine) pathways, indicating the

enzymes inosine monophosphate dehydrogenase (IMPDH) and carbamoyl phosphate synthetase 11 (CPS 11) targeted by analogs which deplete nucleotide pools, reportedly associated with

a p53-dependent Go/G1 arrest. Note the involvement of 5-phosphoribosyl pyrophosphate (PPribP) in both purine DNS and salvage as well as in pyrimidine DNS. The role of ATP, GTP, UTP,

and CTP in DNA and RNA synthesis, of ATP and GTP in 2nd messenger synthesis, of UTP, CTP, and GTP in the synthesis of the glycoconjugates (dashed box) essential for membrane

synthesis, structure, and function and protein glycosylation, is also evident.

fig0002

the nucleotides turned over daily is actually degraded

and lost to the body. The pyrimidine bases, uracil and

thymine, derived from nucleosides not recycled are

degraded further to the b-amino acids, and there is

thus no measurable end product. However, such loss

is probably comparable with that for purines, the

normal end product of which in humans is uric acid,

formed from the precursor purine bases xanthine and

hypoxanthine by the action of xanthine dehydrogen-

ase (Figure 2).

Rates of Nucleotide Synthesis and Degradation in

Different Cells

0017 It has become apparent that the original concept of

endogenous metabolism and its overall control, in-

volving a complex interplay between de-novo synthe-

sis and salvage, does not apply to all cells but is

governed by a tissue- or cell-specific complement of

enzymes and/or controls on them, depending on the

function of that cell or tissue. The human erythrocyte,

for instance, is anucleate and lacks the ability to use

either salvage or de-novo synthesis to maintain its

ATP levels, being dependent on adenosine scavenged

from other tissues for this. In addition, the pyrimidine

nucleotides found in nucleated cells are absent from

mature erythrocytes, the only pyrimidines normally

present being in the form of uridine diphosphate

(UDP) sugars. ATP is also the most important purine

in both skeletal and heart muscle, adenine nucleotides

making up 95 and 90% of the total nucleotide com-

plement, respectively. Although DNA in most tissues

is considered relatively stable, it is evident from the

two inherited disorders associated with immunodefi-

ciency that cell death and rapid turnover of cells of

the hemopoietic system (e.g., extrusion of the nucleus

during erythrocyte maturation, or mounting an

immune response) normally produces significant

amounts of deoxyribonucleosides, as well as ribonu-

cleosides, which must be degraded further. These

disorders have highlighted the fact that removal of

metabolic waste from DNA catabolism is vital to the

normal immune response; failure to do so can result

in the accumulation of deoxy-ATP and deoxy-GTP.

These deoxynucleotide triphosphates are extremely

toxic to T-lineage stem cells, resulting in severe

combined immunodeficiency affecting both T- and

B-cells in subjects deficient in adenosine deaminase,

or a T-cell-specific immunodeficiency, in purine nu-

cleoside phosphorylase deficiency (Figure 2).

Endogenous Nucleic Acid Synthesis in the Gut

0018 It was originally reported from studies in guinea-pigs

that the gastrointestinal tract was incapable of

de-novo synthesis. However, subsequent workers

have shown significant synthetic activity in rat intes-

tine, as evaluated by the incorporation of radio-

labeled glycine into RNA. The nucleic acid content

of intestinal mucosa is high, as is the rate of cell

turnover in the luminal villi, and it has been calcu-

lated in rat that about 30 mg of endogenous nucleic

acid enters the lumen daily. This implies a consider-

able loss of both purines and pyrimidines, which can

only be replaced by de-novo synthesis. Studies of

nucleotide concentrations in rat intestine have shown

both pyrimidine and purine nucleotides at concentra-

tions equivalent to those in liver, supporting active

nucleotide metabolism in the intestine.

0019Intestinal absorption is obviously related to intes-

tinal motility and local blood flow. The presence of

adenosine receptors on rat jejunal mucosal cells may

be particularly important in the regulation of fluid

and electrolyte absorption as well as the active trans-

port of nutrients. Such adenosine may be generated

slowly from nucleotides by nucleotidases physically

or functionally coupled to the adenosine membrane

translocator.

0020It is evident from the above that the pathways

leading to the formation and degradation of nucleic

acids are complex, and that many factors determine

the origin as well as amount of endogenous pyrimi-

dine or purine nucleotides turned over daily. More-

over, this will differ depending on the cell or tissue.

Role and Fate of Dietary Nucleic Acids

0021The metabolism of nucleic acids ingested in the diet

difers from that of endogenous nucleic acids. The

intestinal mucosa plays an important role in this.

The above studies confirm that pyrimidine as well as

purine nucleotides in intestinal mucosal cells are de-

rived from endogenous sources. However, a growing

body of evidence supports a role for dietary nucleo-

tides derived from the gut in intestinal development,

turnover, and repair. Suggested effects include en-

hancement of the host mucosal defense system and

an influence on neonatal lipid metabolism and on

iron bioavailability, implying a novel role for nutri-

tion in the modulation of gut function.

0022Most of our knowledge relating to dietary nucleic

acid metabolism in the intestine is derived from studies

of the absorption of exogenous purine in different

species – mouse, dog, rat, pig, etc., as well as humans

– which date back as far as the latter half of the

nineteenth century. Investigations in animals have

shown that whereas the ribose attached to nucleosides

derived from RNA was further metabolized, the

phosphate was absorbed and excreted in the urine.

0023Recent studies using

C-labeled nucleic acid to

supplement the diets of rats and chickens have

NUCLEIC ACIDS/Physiology 4157

provided further evidence for the incorporation of

dietary pyrimidine nucleosides, but not purine

nucleosides, directly into hepatic RNA. The success-

ful lifelong treatment with uridine of patients with

hereditary oroticaciduria – a defect in pyrimidine

de-novo synthesis – confirms that dietary uridine is

certainly absorbed and salvaged into mono- as well as

polynucleotides in humans. The lack of effect of oral

uracil in this disorder also confirms that uracil is a

metabolic waste and that pyrimidine salvage occurs

at the nucleoside level in humans.

0024 Recent investigations have addressed the sup-

posedly beneficial effects of dietary CDP-choline

(citicholine) in patients with stroke or trauma, theor-

etically by increasing acetylcholine production in

cholinergic neurons, as well as the amount of cell

membrane. These studies confirmed that, when

taken orally, CDP-choline elevates plasma levels of

both choline and uridine (not cytidine) in humans,

but no benefit was demonstrated in clinical trials.

Metabolism of Dietary Purines by Gut Bacteria

0025 Animal studies have shown that the metabolism of

dietary nucleic acid and the corresponding purine

nucleotides, nucleosides, and bases in the gut is

rapid. Isotope studies demonstrated that up to 50%

of the radiolabeled purine is recovered as carbon

dioxide within 30 min. The role of gut bacteria in

this process was indicated by experiments using the

XDH inhibitor, allopurinol, concomitantly, when the

radiolabel was recovered in toto (in urine and feces

only), presumably relating to inhibition of bacterial

XDH. Allopurinol not only inhibited purine degrad-

ation but also decreased the absorption of dietary

purine, a seemingly beneficial effect that may explain

the reduction in total purine excretion noted in

human subjects on a normal diet taking allopurinol.

Intestinal Mucosa Degrades Dietary Purines to Uric

Acid

0026 Interestingly, the above radiolabeling studies in pigs

(where, as in humans, XDH activity is significant

only in intestinal mucosa and liver), confirmed a

lack of any radiolabel incorporation from dietary

purine into tissue nucleotides. These studies demon-

strated conclusively that dietary purine is degraded to

a nonreutilizable form by the intestinal mucosa. Stud-

ies in other animal species have confirmed this, purine

nucleotides, nucleosides, and bases absorbed from the

gut lumen being largely converted to uric acid during

passage across the mucosa and released as such in

serosal secretions, prior to further degradation by

urate oxidase (uricase) in the liver. Thus, in contrast

to pyrimidines, humans have no apparent require-

ment for dietary purines. The intestine thus serves as

an effective barrier through the activity of a battery of

enzymes capable of rapidly degrading purines already

partly processed by gut bacteria to the nonreutilizable

metabolic waste, uric acid. Such rapid degradation of

purines by the gut presumably reflects an important

evolutionary development to protect the integrity of

the human genome. The uric acid produced daily in

humans thus derives from two sources: catabolism of

exogenous as well as endogenous mono and polynu-

cleotides (Figure 3).

Excess Dietary Levels of Nucleic Acids

and their Clinical Consequences

0027As mentioned above, pyrimidines have no measurable

end product. The potential toxicity of dietary nucleic

acids thus relates predominantly to a single muta-

tional event that has resulted in the fact that the insol-

uble uric acid, and not the much more soluble

allantoin as in other mammalian species, is the only

measurable end product of nucleic acid degradation

in humans. Although it had long been accepted that

the enzyme uricase had been lost in the course of

human evolution, more recent studies have estab-

lished that the absence of uricase activity results

from a lack of gene transcription, rather than loss of

the gene itself. However, other factors play an equally

important role in determining the pathogenesis of

nucleic acids ingested in the diet.

Role of Exogenous Purine in Determining

Circulating Uric Acid Concentrations

0028The ingestion of food now known to be rich in

purines has been noted for millennia to be high

in subjects with what has been designated ‘primary’

gout, the gout affecting predominantly the middle-

aged male. This type of gout is a disorder of affluent

societies consuming diets rich in purines; it is rare

in women or children. During times of hardship,

uric acid concentrations in the population fell consid-

erably, and ‘primary’ gout almost vanished. The fact

that gout was extremely prevalent among wealthy

Englishmen for more than three centuries up to

World War I is not surprising when the dietary habits

of the day are examined. These affluent gentlemen

habitually consumed vast meals comprising many

courses and frequently including 16 different meats,

the majority of which were rich in nucleic acids and

other purines.

Type of Nucleic Acid and Toxicity

0029Detailed studies of dietary nucleic acid absorption in

humans were carried out by Zo

llner and coworkers

in the late twentieth century and confirmed that

nucleic acids ingested in the diet exert their major

4158 NUCLEIC ACIDS/Physiology

effect on uric acid levels. These studies showed that

the nature of the nucleic acid ingested is equally im-

portant, with the effect of RNA being more than

twice that of DNA (Figure 4a). This effect is also

evident whether the purine is ingested in the form of

nucleic acids, mononucleotides, nucleosides, or bases.

Moreover, some forms of purine, e.g., guanosine – the

principal RNA degradation product in yeast-rich

beverages such as beer (especially real ale) – are

absorbed and catabolized more readily than others.

A controlled study of diet in primary gout patients

compared with control males of comparable age dem-

onstrated that the average daily intake of most

nutrients, including purine nitrogen, was similar.

However, gouty patients drank significantly more

alcohol, predominantly in the form of beer (60 g per

day, equivalent to 2.5 l of beer) and had a significantly

higher mean plasma uric acid (0.49 mmol l

1

com-

pared with 0.39 mmol l

1

Role of the Kidney or Drugs in the Genesis of

Toxicity

0030 The pathological changes that result from uric acid

being the exclusive end product of purine metabolism

in humans are due entirely to the insolubility of this

metabolic waste, coupled with the other peculiarity

that primates display, i.e., the renal tubule reabsorbs

around 90% of the filtered urate (Figure 3). Net

reabsorption is slightly higher in normal males

(92%) than in females (88%) and is lower in children

of either sex (80–85%). This explains the higher

plasma uric acid in adult males and makes them

more vulnerable to situations of overproduction,

or overingestion. Clearly, sex and age are equally

important contributory factors.

0031It is noteworthy that when normal subjects are fed

yeast RNA, the plasma urate rises little with increased

intake, the increase in the excretion of urate being

dramatic when the rise in plasma urate is modest

(Figure 4b). Thus, neither overingestion nor overpro-

duction (unless sudden and massive) is likely to raise

the concentration of uric acid in the plasma consider-

ably, if the renal response is normal. It is obviously

not so in the majority of gouty males with primary

gout, where the striking change is that, for any

plasma urate concentration, the excretion of urate is

consistently less than in normal subjects. This defect

in handling by the kidneys relates to a greatly reduced

Filtration

Body wear

and tear

Body

purine

synthesis

Plasma

uric acid

Excretion

Purines

in diet

Reabsorption

Secretion

8−18%

fig0003 Figure 3 Schematic diagram showing the factors influencing the plasma uric acid, the end product of endogenous (body synthesis

plus daily wear and tear) and exogenous (dietary) purine metabolism in humans. The different mechanisms by which this metabolic

waste is eliminated (two-thirds by the kidney, one-third by the gut) in normal individuals are also shown. At the bottom left is a

simplified version of the complex factors (involving filtration, reabsorption and secretion) interacting in the proximal tubule of the

human kidney and resulting in the urinary excretion of only 8–18% of the filtered load depending on age and sex. Reproduced from

Nucleic Acids: Physiology, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

NUCLEIC ACIDS/Physiology 4159

clearance of uric acid relative to the glomerular filtra-

tion rate (FE

); FE

is the fractional excretion of

uric acid. The mean is 5.4% in gouty males compared

with 8.1% for healthy males of comparable mean age

(52 years).

0032 The exact nature of this difference is not yet

clear, since studies must be reevaluated in the light

of recent knowledge of how complex the renal

tubular handling of urate is. However, there is no

doubt that it involves the transport of uric acid,

which occurs predominantly in the proximal tubule,

is bidirectional, and has both a secretory and

reabsorptive component (Figure 3). Current opinion

suggests that the majority of gouty patients under-

excrete urate because of a defect in tubular secretion.

Confirmation must await identification at the

molecular level of the different transporters involved

in urate reabsorption and secretion. Clearly, a com-

bination of events is needed to produce hyperurice-

mia and the clinical syndrome of gout. These are a

large intake of readily absorbed purine coupled with a

defect in the renal handling of uric acid at the kidney

level, which means that the kidney cannot respond

to a purine load without an abnormal rise in

plasma urate concentration. (See Renal Function

and Disorders: Kidney: Structure and Function.)

0033However, diet alone may not be the only culprit.

Numerous other physiological and pathological

agents such as lead are also capable of reducing

urate excretion, and hence exacerbate the rise in

Plasma uric acid rise (mg per day)

Purine derivate (mmol per 70 kg bodyweight)

Plasma uric acid (mg per 100 ml)

AMP

GMP

Hypoxanthine

RNA

DNA

Increase of urinary uric acid

excretion (mg per day)

200

400

600

800

AMP

GMP

Hypoxanthine

RNA

DNA

0.5(b)(a)

0.5

1.0

RNA (g)

Uric acid (g)

Purine N (g)

y = 3.25 + 0.9 x

y = 4.4 + 1.46 x

fig0004 Figure 4 (a) Increase in plasma and urine uric acid (mg per day) in response to a purine load from the different sources indicated

on a molar basis. (b) Influence of graded additions of RNA to a purine-free formula diet on plasma uric acid levels of either

normouricemic (-

-) or hyperuricemic (-

-) individuals (some of whom had a positive family history of gout). Reproduced from Nucleic

Acids: Physiology, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993,

Academic Press.

4160 NUCLEIC ACIDS/Physiology

plasma urate caused by diet. Any of these may lead to

an acute attack of gout in a susceptible individual

whose plasma urate is already elevated. The best

known of the physiological substances are organic

acids such as lactate; their overproduction may ex-

plain in part the hyperuricemia associated with exces-

sive alcohol consumption coupled with inadequate

food intake. Diuretics cause plasma volume contrac-

tion, and fractional urate reabsorption increases

under these circumstances. The appearance of gout

in patients treated with hypotensive agents over long

periods currently accounts for over 50% of new pre-

sentations in gout clinics. Unusually, the number of

elderly females in this group is high, and the mode of

clinical presentation is frequently atypical.

0034 Diet may also be important in drawing attention to

an unusual subset of patients with juvenile onset of

gout; a disorder affecting young males and, unusually,

young females and children of either sex equally.

Unlike primary gout, where renal function is normal

for age, unrecognized, this dominantly inherited dis-

order is associated with progressive renal disease.

Considerable variation in presentation has existed,

and there has been no consensus as to whether the

gout or the renal disease is the primary factor. How-

ever, it is now evident that there are two hallmarks for

this disease: the first is hyperuricemia disproportion-

ate to the degree of renal dysfunction; the second is

that the degree of renal hypoexcretion of urate is

generally extreme, which explains the associated ten-

dency to gout. Moreover, the mean FE

in this group

is only 5.1%, irrespective of age or sex: lower than

that of the middle-aged gouty male (5.4%), and even

more remarkable considering the high percentage of

children and young women in this group. This univer-

sally low clearance explains the added susceptibility

to dietary purine leading to the isolated attack of gout

in children as well as young adults, which has drawn

attention to families with ‘familial renal disease,’

hitherto leading to death in the 30s. Recognition of

the correct nature of the familial disorder has under-

lined the need to measure urate clearance in all

kindred members and enabled early diagnosis and

treatment of 42% of seemingly healthy children.

0035This is important because long-term follow-up

(>20 years) in these patients treated with plasma

uric acid lowering agents has shown that early recog-

nition and treatment ameliorates the progression of

the renal disease. These studies implicate uric acid in

the etiology of the renal disease, but this has been

disputed by others. However, very recent studies sug-

gest that a reappraisal of the pathogenesis and

consequences of hyperuricemia in hypertension, car-

diovascular disease, and renal disease is warranted,

which adds credence to the above hypothesis.

Role of Dietary Nucleic Acid in the Genesis of

Urolithiasis

0036Although overingestion of purines by subjects with a

normal renal response does not precipitate gout, it

can predispose to uric acid lithiasis. Uric acid stones

are relatively common in Australasia and similar

countries, where the consumption of purine-rich bev-

erages and food (e.g., beer, seafood, and meat) is high,

and in individuals addicted to health foods such as

yeast tablets. Vitamin C is also uricosuric, and acute

uric acid nephropathy and sometimes urolithiasis

have also been reported following therapy with a

variety of uricosuric drugs. Perhaps less well recog-

nized is the uricosuric effect of a high-protein diet and

the fact that the intake of purine-rich foods also pre-

disposes to calcium stone formation.

0037Some foods rich in nucleic acid – vegetarian diets

consisting of pulses and grains and yeast extracts –

have a particularly high adenine content. Although

this is not a problem in the normal population, such

diets can be potentially lethal to a rare group of

subjects with a genetic defect leading to the inabi-

lity to recycle adenine, overexcretion of adenine,

and the even more insoluble uric acid analog

2,8-dihydroxyadenine (2,8-DHA). Patients with this

defect generally present in childhood with kidney

stones composed of 2,8-dihydroxyadenine. One

such case from a commune consuming a macrobiotic

diet presented in acute renal failure with severe renal

damage, progressing to dialysis and transplantation.

Diet can thus be an important precipitating factor in

the pathogenesis of inherited disorders associated

% males with asymptomatic hyperuricemia

Maori NZ France Japan USA Finland UK Germany

fig0005 Figure 5 Percentage of Caucasian males, compared with a

Polynesian race (the Maori), with asymptomatic hyperuricemia

(defined as a plasma uric acid in excess of 0.42 mmol l

1

,or

7.0 mg dl

1

) in four EU countries, North America, Japan, and

New Zealand (NZ).

NUCLEIC ACIDS/Physiology 4161

with the overexcretion of insoluble purines. (See

Macrobiotic Diets; Vegetarian Diets.)

Reducing Dietary Levels of Nucleic Acids

0038 Diet varies considerably in different countries and

must be taken into account when investigating pa-

tients for suspected disorders of purine metabolism.

Normal ranges for uric acid in plasma and urine differ

greatly in the healthy population depending on

the country (Figure 5). For example (and in contrast

to the nineteenth century), until recently, the majority

of subjects in the UK ingested a low purine diet con-

sisting of one meat meal a day, and urinary uric acid

excretion above 3.5 mmol per day was considered

abnormal. By contrast, the upper limit of normal in

Australia has been given as 7.00 mmol per day. How-

ever, the increasing affluence of some societies is now

leading to an increase in gout, not only in adult Cau-

casian males, but also in countries such as Japan,

where gout was once rare.

Turnover of Exogenous and Endogenous Uric Acid

0039 A useful guide to the dietary purine consumption in

different countries can be obtained from the percent-

age of males with asymptomatic hyperuricaemia.

Using this yardstick, the highest consumption in the

EU 20 years ago was in France (lovers of pa

and

seafoods), the lowest in the UK (Figure 5). The statis-

tics for Germany were from Bavaria where, as in New

Zealand and Australia, beer consumption is high.

Although similar statistics for diet-related differences

are unavailable today, plasma uric acid will have risen

along with the increase in obesity and blood pressure,

especially in the UK. Race also plays a part, and

Polynesians have a genetically low urate clearance

compared with Caucasians, as exemplified by the

New Zealand Maori (FE

4.9% in normouricaemic

males, 3.9% in asymptomatic hyperuricemic males).

0040 As we have seen, the body pool of urate, and hence

the plasma urate concentration, is the result of a

balance between production, ingestion, and excre-

tion. The method for assessing the contribution of

diet is to evaluate de-novo production of purines by

placing the subject on a purine-free diet for 5–7 days

and measuring the urinary excretion of urate, which

will equal endogenous production. In this way, less

than 1–5% of subjects in any country have been

found to excrete abnormally large amounts of urate

(>3 mmol per day). In such rare cases, an underlying

genetic metabolic defect can be generally demon-

strated in which the normal feedback controls on

de-novo nucleotide synthesis and thus endogenous

purine production are overridden, resulting in gross

uric acid overproduction. Two such defects – both of

which are X-linked and generally present in child-

hood, or adolescence, but sometimes neonatally –

have been identified.

Purine Content of Foods

0041A knowledge of the purine content of specific foods

is essential if dietary effects are to be reduced to a

minimum. Until recently, such data have been diffi-

cult to find but are now available on the web. Most

tables give only purine nitrogen, which, as demons-

trated by the purine loading studies mentioned earlier,

is not always a good guide because of the variation in

absorption. Pa

is a particular culprit, as is most

offal and organ meat (liver, kidney, heart, brains,

sweetbreads), game (venison, pheasant, partridge,

grouse), and the nucleic-acid-rich fish and seafoods

– herring, kippers, sardines, smelts, sprats, anchovies,

salmon, trout, mackerel, crustaceans (crab, lobster,

prawns), shellfish (scallops, mussels), and caviar or

roe. Purine nitrogen varies and ranges from 50 mg per

100 g in beef steak to 234 mg per 100 g in sardines.

Many fresh vegetables, e.g., spinach, peas, beans,

lentils, mushrooms, asparagus, and cauliflower, also

have a considerable purine content, as have soya and

other pulses and grains (porridge and oats, wheat and

rye cereals). All meat extracts (Bovril, Oxo) or yeast

extracts (Barmene, Tastex) are very rich in purine.

0042However, many studies have established that

humans addicted to diets rich in nucleic acids are

generally very reluctant to alter their dietary habits

despite the strongest advice to do so. Consequently,

therapy to reduce the pathological effects of dietary

nucleic acids, namely the elevated uric acid levels,

becomes essential.

Therapeutic Approaches to Reducing Uric Acid

Levels

0043Numerous plasma uric acid-lowering drugs are

in current use. Some act by increasing the renal elim-

ination of uric acid (uricosuric drugs, e.g., benzbro-

marone), or restrict its formation (e.g., allopurinol).

Allopurinol reduces urine uric acid levels as well. As

mentioned earlier, studies in both humans and

animals have shown that allopurinol has an add-

itional beneficial effect in reducing dietary purine

absorption. Allopurinol is usually a safe drug, but in

rare instances, mostly in renal disease, other factors

must be considered. The active metabolite of allopur-

inol, oxypurinol, is handled by the kidney in a fashion

akin to uric acid and thus, even normally, has a long

half-life. Since excretion of oxipurinol is reduced in

renal failure, the allopurinol dose must be reduced to

lower the plasma oxipurinol and minimize the risk

of bone marrow depression, or other undesirable

side-effects, which include epidermal necrolysis and

4162 NUCLEIC ACIDS/Physiology

hepatotoxicity. A recent study pinpointed the poor

response to allopurinol in heavy drinkers and related

this to the combined effect of ethanol in impairing

urate excretion and increasing production. In rare

instances, patients have had a severe allergic reaction

to allopurinol. In such cases, the potent uricosuric

drug, benzbromarone has proved beneficial, even in

renal failure patients with kidney function as low as

25% of normal.

Allopurinol and Acute Renal Failure

0044 It is important to note that in patients with genetic

uric acid overproduction, allopurinol should be used

with care. Its use to avert gout (or uric acid nephrop-

athy) has precipitated acute or chronic renal failure

due to xanthine nephropathy and sometimes xan-

thine stones instead. Xanthine is even more insoluble

than uric acid, and as with uric acid, solubility cannot

be improved by alkalinization of the urine. Xanthine

nephropathy also may occur during massive endogen-

ous nucleic acid breakdown in patients given allopur-

inol during aggressive therapy with cytotoxic drugs

for malignant disorders. In such cases, much lower

doses of allopurinol should be coupled with adequate

hydration and alkalinization of the urine.

Role in Chemical Carcinogenesis

0045 Substantial evidence from microorganisms and mam-

malian cells has implicated mutagenic events caused

by damage to endogenous DNA as the initiating

factor in carcinogenesis. Damage to critical regions

of the genome of somatic cells can be produced by a

variety of environmental mutagens. Examples of en-

vironmental factors affecting humans are cigarette

smoke, asbestos, and ultraviolet (UV) irradiation.

Recent putative additions to the list are radiation

from microwaves and mobile phones.

0046 A high correlation invariably exists between the

mutagenic activity of different chemicals and their

carcinogenic activity. Epidemiological evidence indi-

cates a relationship between exposure to benzene and

nonlymphocytic leukemia in humans, but the signifi-

cance of DNA adduct formation in this is not clear.

Normally, DNA possesses active repair systems to

protect against such damage, which can be caused

by a variety of chemical and physical agents, includ-

ing ionizing, radiation, and UV light. Strands may

become cross-linked, bases can be altered or lost,

and phosphodiester bonds may be broken. UV

light induces the formation of pyrimidine dimers,

which are recognized and cleaved by specific endo-

nucleases. Failure to repair the damage before DNA

replication occurs results in the damaged region be-

coming the site of somatic mutations, chromosomal

rearrangements or amplifications, and aberrant DNA

methylation.

0047In this context, it is of interest that the P53 gene is

altered in many human cancers. Recent studies using

analogs that target specific enzymes of either purine

or pyrimidine nucleotide biosynthesis have identified

P53 as a cellular regulator of both nucleotide syn-

thetic pathways (Figure 2). These studies have led to

the proposal that P53 is not only a sensor of DNA

damage, inducing apoptosis in response to DNA

strand breaks, but rather a sensor of nucleotide deple-

tion. Upregulation of P53 expression, activated by

nucleotide depletion or related processes, prevents

cells entering the S phase when precursor nucleotide

pools are low, thus avoiding replication of damaged

DNA through a prolonged, but reversible, G0/G1

arrest. The significance of these findings is that

elongation of the S phase during nucleotide depletion

is known to be associated with increased chromo-

some breakage. (See Carcinogens: Carcinogenic Sub-

stances in Food: Mechanisms; Carcinogenicity Tests;

Mutagens.)

See also: Carcinogens: Carcinogenic Substances in

Food: Mechanisms; Carcinogenicity Tests; Coenzymes;

Infant Foods: Milk Formulas; Macrobiotic Diets;

Mutagens; Protein: Synthesis and Turnover; Renal

Function and Disorders: Kidney: Structure and Function;

Vegetarian Diets; Viruses