Voet D., Voet Ju.G. Biochemistry

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that are collectively known as statins. Indeed, Lipitor is

presently one of the most widely prescribed drugs in the

United States. The initial decreased cholesterol supply in

the cell caused by the presence of statins is again met by in-

duction of LDL receptors and HMG-CoA reductase so

that, at the new steady state, the HMG-CoA reductase

level is almost that of the predrug state. However, the in-

creased number of LDL receptors causes increased re-

moval of both LDL and IDL (the apoB-containing precur-

sor to LDL), decreasing serum LDL levels appreciably.

Lipitor-treated FH heterozygotes routinely show a serum

cholesterol decrease of 40–50%.

The combined use of these agents, moreover, results in a

clinically dramatic 50 to 60% decrease in serum cholesterol

levels.

e. Overexpression of LDL Receptor Prevents

Diet-Induced Hypercholesterolemia

Experiments are well underway toward the treatment

of hypercholesterolemic individuals by gene therapy

(Section 5-5Hb). A line of transgenic mice has been de-

veloped that overproduce the human LDL receptor.

When fed a diet high in cholesterol, fat, and bile salts,

these transgenic animals did not develop a detectable in-

crease in plasma LDL. In contrast, normal mice fed the

same diet exhibited large increases in plasma LDL levels.

Evidently, the unregulated overexpression of LDL re-

ceptors can prevent diet-induced hypercholesterolemia,

at least in mice.

C. Cholesterol Utilization

Cholesterol is the precursor of steroid hormones and bile

salts. Steroid hormones, which are grouped into five

categories, progestins, glucocorticoids, mineralocorti-

coids, androgens, and estrogens, mediate a wide variety

of vital physiological functions (Section 19-1G). All con-

tain the four-ring structure of the sterol nucleus and are

remarkably similar in structure, considering the enormous

differences in their physiological effects. A simplified

Section 25-6. Cholesterol Metabolism 991

Figure 25-63 Some competitive inhibitors of HMG-CoA

reductase used for the treatment of hypercholesterolemia. The

molecular formulas of lovastatin (Mevacor), pravastatin

(Pravachol), simvastatin (Zocor), and atorvastatin (Lipitor), all

of which are potent competitive inhibitors of HMG-CoA

CH(CH

)

COO

–

HMG-CoA

X = H

X = CH

R = CH

R = OH

R = CH

Lovastatin (Mevacor)

Pravastatin (Pravachol)

Simvastatin (Zocor)

CoA

COO

–

COO

–

Mevalonate

Atorvastatin (Lipitor)

reductase, are given.The structures of HMG-CoA and

mevalonate are shown for comparison. Note that lovastatin,

pravastatin, and simvastatin are lactones, whereas atorvastatin

and mevalonate are hydroxy acids.The lactones are hydrolyzed

enzymatically in vivo to their active hydroxy-acid forms.

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 991

biosynthetic scheme (Fig. 25-64) indicates their structural

similarities and differences. We shall not discuss the details

of these pathways.

The quantitatively most important pathway for the excre-

tion of cholesterol in mammals is the formation of bile

acids. The major bile acids, cholate and chenodeoxycholate,

992 Chapter 25. Lipid Metabolism

12 17

21 22 24 27

20 23 25

Cholesterol

Pregnenolone

17-Hydroxypregnenolone

Testosterone

Dehydroisoandrosterone

11-Deoxycortisol

Estradiol

An estrogen

17-Hydroxyprogesterone

Cortisol

A glucocorticoidAn androgen

11-Deoxycorticosterone

Corticosterone

A mineralo-

corticoid

OHC

Aldosterone

Progesterone

A progestin

3 4

6, 78

Figure 25-64 Simplified scheme of steroid biosynthesis. The

enzymes involved are (1) the cholesterol side chain cleavage

enzyme, (2) steroid C17 hydroxylase, (3) steroid C17, C20 lyase,

(4) steroid C21 hydroxylase, (5) steroid 11-hydroxylase,

(6) steroid C18 hydroxylase, (7) 18-hydroxysteroid oxidase, and

(8) aromatase.

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 992

are synthesized in the liver and secreted as their glycine or

taurine conjugates (Fig. 25-65), which are known as bile

salts, into the gallbladder. From there, they are secreted

into the small intestine, where they act as emulsifying

agents in the digestion and absorption of fats and fat-

soluble vitamins (Section 25-1). An efficient recycling

system allows the bile salts to reenter the bloodstream and

return to the liver for reuse several times each day. The

1g day

1

of bile salts that normally escape this recycling

system are further metabolized by microorganisms in the

large intestine and excreted. This is the body’s only route

for cholesterol excretion.

Comparison of the structures of cholesterol and the bile

acids (Figs. 25-44 and 25-65) indicates that biosynthesis of

bile acids from cholesterol involves (1) saturation of the 5,6

double bond, (2) epimerization of the 3-OH group, (3) in-

troduction of OH groups into the 7 and 12 positions,

(4) oxidation of C24 to a carboxylic acid,and (5) conjugation

of this side chain carboxylic acid with glycine or taurine.

Cholesterol 7-hydroxylase catalyzes the first and rate-

limiting step in bile acid synthesis and is closely regulated.

7 EICOSANOID METABOLISM:

PROSTAGLANDINS, PROSTACYCLINS,

THROMBOXANES, LEUKOTRIENES,

AND LIPOXINS

Prostaglandins (PGs) were first identified in human semen

by Ulf von Euler in the early 1930s through their ability to

stimulate uterine contractions and lower blood pressure.

von Euler thought that these compounds originated in the

prostate gland (hence their name) but they were later

shown to be synthesized in the seminal vesicles. By the time

the mistake was realized, the name was firmly entrenched.

In the mid-1950s, crystalline materials were isolated from

biological fluids and called PGE (ether-soluble) and PGF

(phosphate buffer–soluble; fosfat in Swedish). This began

an explosion of research on these potent substances.

Almost all mammalian cells except red blood cells pro-

duce prostaglandins and their related compounds, the

prostacyclins, thromboxanes, leukotrienes and lipoxins,

known collectively as eicosanoids since they are all C

com-

pounds; Greek: eikosi, twenty). The eicosanoids, like en-

docrine hormones, have profound physiological effects at

extremely low concentrations. For example, they mediate

the following: (1) the inflammatory response, notably as it

involves the joints (rheumatoid arthritis), skin (psoriasis),

and eyes; (2) the production of pain and fever; (3) the reg-

ulation of blood pressure; (4) the induction of blood clot-

ting; (5) the control of several reproductive functions such

as the induction of labor; and (6) the regulation of the

sleep/wake cycle. The enzymes that synthesize these com-

pounds and the receptors to which they bind are therefore

the targets of intensive pharmacological research.

The eicosanoids are also hormonelike in that they bind to

G-protein-coupled receptors (Section 19-2B), and many of

their effects are intracellularly mediated by cAMP. Unlike

endocrine hormones, however, they are not transported in

the bloodstream to their sites of action. Rather, these

chemically and biologically unstable substances (some de-

compose within minutes or less in vitro) are local media-

tors (paracrine hormones; Section 19-1); that is, they act in

the same environment in which they are synthesized.

In this section, we discuss the structures of the

eicosanoids and outline their biosynthetic pathways and

modes of action. As we do so, note the great diversity of

their structures and functions, a phenomenon that makes

the elucidation of the physiological roles of these potent

substances a challenging research area.

A. Background

Prostaglandins are all derivatives of the hypothetical C

fatty acid prostanoic acid in which carbon atoms 8 to 12

Section 25-7. Eicosanoid Metabolism: Prostaglandins, Prostacyclins, Thromboxanes, Leukotrienes, and Lipoxins 993

Figure 25-65 Structures of the major bile acids and their glycine and taurine conjugates.

= OH

= NH

Cholic acid

Glycocholic acid

Taurocholic acid

Chenodeoxycholic acid

Glycochenodeoxycholic acid

Taurochenodeoxycholic acid

COOH

= H

C R

22 24

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 993

form a cyclopentane ring (Fig. 25-66a). Prostaglandins A

through I differ in the substituents on the cyclopentane ring

(Fig. 25-66b): PGAs are ,-unsaturated ketones, PGEs are

-hydroxy ketones, PGFs are 1,3-diols, etc. In PGF



, the C9

OH group is on the same side of the ring as R

; it is on the

opposite side in PGF



. The numerical subscript in the name

refers to the number of double bonds contained on the side

chains of the cyclopentane ring (Fig. 25-66c).

In humans, the most prevalent prostaglandin precursor is

arachidonic acid (5,8,11,14-eicosatetraenoic acid), a C

polyunsaturated fatty acid that has four nonconjugated dou-

ble bonds.The double bond at C14 is six carbon atoms from

the terminal carbon atom (the  carbon atom), making

arachidonic acid an –6 fatty acid.As Sune Bergström and

Bengt Samuelsson demonstrated, arachidonic acid is syn-

thesized from the essential fatty acid linoleic acid (also an

–6 fatty acid). This occurs via its desaturation with a 

desaturase to yield -linolenic acid (GLA), followed by

elongation and a second desaturation, this time with a 

desaturase (Fig. 25-67; Section 25-4E). Prostaglandins with

the subscript 1 (the “series-1” prostaglandins) are synthe-

sized from dihomo--linolenic acid (DGLA; 8,11,14-

eicosatrienoic acid), whereas “series-2” prostaglandins are

synthesized from arachidonic acid. -Linolenic acid

(ALA), another essential fatty acid since the 

-desaturase

required for its synthesis occurs only in plants, is a precursor

of 5,8,11,14,17-eicosapentaenoic acid (EPA) and the

“series-3” prostaglandins. Since arachidonate is the primary

prostaglandin precursor in humans, we shall mostly refer to

the series-2 prostaglandins in our examples. Note, however,

that when dietary linoleic acid and -linolenic acid are

equally available, the relative activities of the 

- and 

desaturases are important in determining the relative

amounts of these prostaglandin precursors.

a. Arachidonate Is Generated by

Phospholipid Hydrolysis

Arachidonate is stored in cell membranes esterified at

glycerol C2 of phosphatidylinositol and other phospho-

lipids. The production of arachidonate metabolites is

controlled by the rate of arachidonate release from these

phospholipids through three alternative pathways (Fig.

25-68):

1. Phospholipase A

hydrolyzes acyl groups at C2 of

phospholipids (Fig. 25-68b, left).

2. Phospholipase C (Section 19-4B) specifically hy-

drolyzes the phosphatidylinositol head group to yield a

1,2-diacylglycerol (DAG) and phosphoinositol. DAG is

phosphorylated by diacylglycerol kinase to phosphatidic

acid, a phospholipase A

substrate (Fig. 25-68b, center).

(Recall that DAG and the various phosphorylated forms

of phosphoinositol are also important signaling molecules

in that they mediate the phosphoinositide cascade; Sec-

tion 19-4.)

3. The DAG also may be hydrolyzed directly by dia-

cylglycerol lipase (Fig. 25-68b, right). Corticosteroids are

used as anti-inflammatory agents because they inhibit

phospholipase A

, reducing the rate of arachidonate pro-

duction.

b. Aspirin Inhibits Prostaglandin Synthesis

The use of aspirin as an analgesic (pain-relieving), an-

tipyretic (fever-reducing), and anti-inflammatory agent has

been widespread since the nineteenth century. Yet, it was

not until 1971 that John Vane discovered its mechanism of

action. Aspirin, as do other nonsteroidal anti-inflammatory

drugs (NSAIDs), inhibits the synthesis of prostaglandins

from eicosanoid precursors (Section 25-7Ba). These in-

hibitors have therefore proved to be valuable tools in the

elucidation of prostaglandin biosynthesis pathways and

have provided a starting point for the rational synthesis of

new anti-inflammatory drugs.

994 Chapter 25. Lipid Metabolism

COOH

(a)

(b)

(c)

Prostanoic acid



G+H



COOH

PGE

COOH

PGE

COOH

PGF

2

Figure 25-66 Prostaglandin structures. (a) The carbon

skeleton of prostanoic acid, the prostaglandin parent compound.

(b) Structures of prostaglandins A through I. (c) Structures of

prostaglandins E

, and F

2

(the first prostaglandins to be

identified).

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 994

c. Arachidonic Acid Is a Precursor of Leukotrienes,

Thromboxanes, and Prostacyclins

Arachidonic acid also serves as a precursor to com-

pounds whose synthesis is not inhibited by aspirin. In fact,

there are two main pathways of eicosanoid metabolism.

The so-called cyclic pathway, which is inhibited by

NSAIDs, forms prostaglandin’s characteristic cyclopen-

tane ring, whereas the so-called linear pathway, which is

Section 25-7. Eicosanoid Metabolism: Prostaglandins, Prostacyclins, Thromboxanes, Leukotrienes, and Lipoxins 995

Linoleic acid

(9,12- octadeca-

dienoic acid)

-Linolenic acid

(GLA; 6,9,12-

octadecatrienoic

acid)

DihomoGLA

(DGLA; 8,11,14-

Eicosatrienoic

acid)

rachidonic acid

(5,8,11,14-eicosa-

tetraenoic acid)

5,8,11,14,17-

Eicosa-

pentaenoic acid (EPA)

COOH

–2H 

-desaturase



-desaturase

(plants only)

–2H

+2C elongase

PGH

synthase

PGH

synthase

+2C elongase

–2H 

-desaturase

–2H 

-desaturase

20:3

COOH

20:4

COOH

20:5

18:3

18:2

COOH

–2H 

-desaturase

18:3

–2H

COOH

18:4

-Linolenic acid

(ALA; 9-12-15-octadeca-

trienoic acid)

OCH

(b)

(a)

–

X =

OCR

Arachidonoyl

group

Phospholipid

(phosphatidylinositol)

Inositol

1,2-Diacylglycerol (DAG)

Phosphoinositol

phospholipase A

phospholipase C

phospholipase Cphospholipase A

diacylglycerol kinase diacylglycerol lipase

Phosphatidic acid

Lysophosphatidic acid

Arachidonic acid

Monoacylglycerol

Arachidonic acid

Lysophospholipid

Arachidonic acid

phospholipase A

OH H

OHH

Figure 25-67 Synthesis of prostaglandin precursors. The

linoleic acid derivatives dihomoGLA (DGLA), arachidonic acid,

Figure 25-68 Release of arachidonic acid by phospholipid

hydrolysis. (a) The sites of hydrolytic cleavage mediated by

phospholipases A

and C.The polar head group, X, is often

inositol and its various phosphorylated forms (Section 19-4D).

(b) Pathways of arachidonic acid liberation from phospholipids.

and 5,8,11,14,17-eicosapentaenoic acid (EPA) are the respective

precursors of the series-1, series-2, and series-3 prostaglandins.

JWCL281_c25_940-1018.qxd 6/8/10 9:00 AM Page 995

996 Chapter 25. Lipid Metabolism

Figure 25-69 The cyclic and linear pathways of arachidonic acid metabolism.

Figure 25-70 The cyclic pathway of arachidonic acid metabolism. This pathway’s branches lead

to prostaglandins, prostacyclins, and thromboxanes.

Prostaglandins

Arachidonic acid

5-Hydroperoxyeicosatetraenoic acid

(5-HPETE)

12-Hydroperoxyeicosatetraenoic acid

(12-HPETE)

COOH

“cyclic pathway”

20:4

“linear pathway”

(lipoxygenases)

HOO

COOH

Leukotrienes

Hepoxilins

OOH

COOH

Lipoxins

15-Hydroperoxyeicosatetraenoic acid

(15-HPETE)

HOO

COOH

Arachidonic acid

PGH synthase

prostacyclin

synthase

thromboxane

synthase

(Aspirin inhibits)

COOH

PGI

(unstable)

6-Oxo-PGF

1

COOH

Prostacyclins

PGH

TxA

(unstable)

TxB

Thromboxanes

Prostaglandins

COOH

PGE

COOH

PGF

2

COOH

PGD

COOH

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 996

not inhibited by these agents, leads to the formation of the

leukotrienes and HPETEs (Fig. 25-69; Section 25-7C).

Studies using NSAIDs helped demonstrate that two

structurally related and highly short-lived classes of

compounds, the prostacyclins and the thromboxanes (Fig.

25-70), are also products of the cyclic pathway of eicosanoid

metabolism. The specific products produced by this

branched pathway depend on the tissue involved. For ex-

ample, blood platelets (thrombocytes) produce thrombox-

anes almost exclusively; vascular endothelial cells, which

make up the walls of veins and arteries, predominantly syn-

thesize the prostacyclins; and heart muscle makes PGI

PGE

, and PGF

2

in more or less equal quantities. In the re-

mainder of this section, we study the cyclic and the linear

pathways of eicosanoid metabolism.

B. The Cyclic Pathway of Eicosanoid Metabolism:

Prostaglandins, Prostacyclins, and Thromboxanes

The first step in the cyclic pathway of eicosanoid metabo-

lism is catalyzed by PGH synthase (PGHS; also called

prostaglandin H synthase and prostaglandin endoperoxide

synthase; Fig. 25-71). This heme-containing enzyme con-

tains two catalytic activities: a cyclooxygenase activity and

a peroxidase activity. The former catalyzes the tyrosyl

radical-mediated addition of two molecules of O

arachidonic acid, forming PGG

. The latter converts the

hydroperoxy function of PGG

to an OH group, yielding

PGH

. PGH

is the immediate precursor of all series-2

prostaglandins, prostacyclins, and thromboxanes (Fig.

25-70).The cyclooxygenase activity of the enzyme gives it its

common name, COX [not to be confused with cytochrome c

oxidase, which is also called COX (Section 22-2C5)].

PGHS, a homodimeric glycoprotein of 576-residue sub-

units, is a monotopic membrane protein that extends into

the lumen of the endoplasmic reticulum. Its X-ray struc-

ture, determined by Michael Garavito, reveals that each of

its subunits folds into three domains (Fig. 25-72a): an N-

terminal module that structurally resembles epidermal

growth factor (EGF; a hormonally active polypeptide that

stimulates cell proliferation; Section 12-3Ae); a central

membrane-binding motif; and a C-terminal enzymatic do-

main. The 44-residue membrane-binding motif has a hy-

drophobic surface that faces away from the body of the

protein [as is also true of oxidosqualene cyclase (Fig.

25-58b) and fatty acid amide hydrolase (Fig. 12-28)].

The peroxidase active site of PGHS occurs at the inter-

face between the large and small lobes of the catalytic do-

main, in a shallow cleft that contains the enzyme’s

Fe(III)–heme prosthetic group. The cleft exposes a large

portion of the heme to solvent and is therefore thought to

comprise the substrate binding site.

The cyclooxygenase active site lies on the opposite side

of the heme at the end of a long narrow hydrophobic chan-

nel (⬃8  25 Å) extending from the outer surface of the

membrane-binding motif to the center of each subunit

(Fig. 25-72b).This channel allows access of the membrane-

associated substrate to the active site. Tyr 385, which lies

near the top of the channel, just beneath the heme, has

been shown to form a transient radical during the cy-

clooxygenase reaction as does, for example, Tyr 244 of cy-

tochrome c oxidase (Section 22-2C5c). Indeed, the muta-

genic replacement of PGHS’s Tyr 385 by Phe abolishes its

cyclooxygenase activity. The Tyr 385 radical is generated

via an intramolecular oxidation by the heme cofactor.

Section 25-7. Eicosanoid Metabolism: Prostaglandins, Prostacyclins, Thromboxanes, Leukotrienes, and Lipoxins 997

COOH

cyclooxygenase activity

(inhibited by aspirin)

•O

•

Arachidonic acid

COOH

Tyr 385

Tyr 385HO

•O

Tyr 385

COOH

OOH

PGG

COOH

PGH

peroxidase

activity

Figure 25-71 Reactions catalyzed by PGH synthase (PGHS).

The enzyme contains two activities: a cyclooxygenase, which cat-

alyzes Steps 1 to 3 and is inhibited by aspirin; and a peroxidase,

which catalyzes Step 4. (1) A radical at Tyr 385 that is generated

by the enzyme’s heme cofactor stereospecifically abstracts a

hydrogen atom from C13 of arachidonic acid, which then

rearranges so that the radical is on C11. (2) The radical reacts with

to yield a hydroperoxide radical. (3) The radical cyclizes and

reacts with a second O

molecule at C15 to yield a peroxide in a

process that regenerates the Tyr radical. (4) The enzyme’s peroxi-

dase activity converts the peroxide at C15 to a hydroxyl group.

JWCL281_c25_940-1018.qxd 6/8/10 9:00 AM Page 997

The fate of PGH

depends on the relative activities of

the enzymes catalyzing the specific interconversions (Fig.

25-70). Platelets contain thromboxane synthase, which me-

diates the formation of thromboxane A

(TxA

), a vaso-

constrictor and stimulator of platelet aggregation (an ini-

tial step in blood clotting;Section 35-1).Vascular endothelial

cells contain prostacyclin synthase, which catalyzes the syn-

thesis of prostacyclin I

(PGI

), a vasodilator and inhibitor

of platelet aggregation. These two substances act in opposi-

tion, maintaining a balance in the cardiovascular system.

a. NSAIDs Inhibit PGH Synthase

Nonsteroidal anti-inflammatory drugs (NSAIDs; Fig.

25-73) inhibit the synthesis of the prostaglandins, prosta-

cyclins, and thromboxanes by inhibiting or inactivating the

cyclooxygenase activity of PGHS. Aspirin (acetylsalicylic

acid), for example, acetylates this enzyme: If [

C-acetyl]

acetylsalicylic acid is incubated with the enzyme, radioac-

tivity becomes irreversibly associated with the inactive en-

zyme as Ser 530 becomes acetylated (Fig. 25-74). The X-ray

structure of PGHS reveals that Ser 530, which is not impli-

cated in catalysis, extends into the cyclooxygenase channel

just below Tyr 385 such that its acetylation would block

arachidonic acid’s access to the active site (Fig. 25-72b).The

structure of PGHS, which was crystallized with the NSAID

flurbiprofen (Fig.25-73), indicates that this drug binds in the

cyclooxygenase channel, with its carboxyl group forming an

ion pair with Arg 120 (Fig. 25-72a). Evidently, flurbiprofen,

and by implication other NSAIDs, inhibits the cyclooxyge-

nase activity of PGHS by blocking its active site channel.

Low doses of aspirin, ⬃80 mg (baby aspirin) every day,

significantly reduce the long-term incidence of heart at-

tacks and strokes. Such low doses selectively inhibit

platelet aggregation and thus blood clot formation be-

cause these enucleated cells, which have a lifetime in the

circulation of ⬃10 days, cannot resynthesize their inacti-

vated enzymes.Vascular endothelial cells are not so drasti-

cally affected since, for the most part, they are far from the

site where aspirin is absorbed, are exposed to lesser con-

centrations of aspirin and, in any case, can synthesize addi-

tional PGHS.

b. COX-2 Inhibitors Lack the Side

Effects of Other NSAIDs

PGHS has two isoforms, COX-1 and COX-2, that share

a high degree (60%) of sequence identity and structural

homology. COX-1 is constitutively (without regulation) ex-

pressed in most, if not all, mammalian tissues, thereby sup-

porting levels of prostaglandin synthesis necessary to

maintain organ and tissue homeostasis such as that of the

gastrointestinal mucosa. In contrast, COX-2 is only ex-

pressed in certain tissues in response to inflammatory stim-

998 Chapter 25. Lipid Metabolism

Figure 25-72 X-ray structure of PGH synthase (PGHS) from

sheep seminal vesicles in complex with the NSAID flurbiprofen.

(a) This homodimeric monotopic membrane protein is viewed

parallel to the plane of the ER membrane with its 2-fold axis of

symmetry vertical.The EGF-like module is green, the

membrane-binding motif is orange, and the catalytic domain is

blue. The heme (red); flurbiprofen (yellow); Tyr 385 (magenta),

which forms a transient radical during the cyclooxygenase

reaction; and Arg 120 (green), which forms an ion pair with

flurbiprofen, are drawn in space-filling form. (b) A C



diagram of

a PGHS subunit (green), the left subunit in Part a as viewed from

30° to the left.The peroxidase active site is located above the

heme (pink).The hydrophobic channel, which penetrates the

subunit from the membrane-binding motif at the bottom of the

figure to the cyclooxygenase active site below the heme, is

represented by its van der Waals surface (blue dots). The three

residues in the channel that are shown in stick form in orange

are, from top to bottom:Tyr 385, Ser 530, which is acetylated by

aspirin, and Arg 120. [Courtesy of Michael Garavito, Michigan

State University. PDBid 1CQE.]

(a)

(b)

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 998

uli such as cytokines, protein growth factors, and endotox-

ins, and hence is responsible for the elevated prostaglandin

levels that cause inflammation. The NSAIDs in Fig. 25-73

are relatively nonspecific and therefore can have adverse

side effects, most notably gastrointestinal ulceration, when

used to treat inflammation or fever.A structure-based drug

design program (Section 15-4Ad) was therefore instituted

to create inhibitors that would target COX-2 but not

COX-1. The three-dimensional structures of COX-1 and

COX-2 are almost identical.However,their amino acid dif-

ferences, specifically I523V, I434V, and H513R (COX-1

amino acid on the left and COX-2 amino acid on the right),

make COX-2’s active site channel ⬃20% larger in volume

than that of COX-1. In addition, the fourth helix of the

membrane-binding domain is oriented slightly differently

so as to provide a larger opening to the channel. Medicinal

chemists therefore synthesized inhibitors, collectively

known as coxibs, that could enter the COX-2 channel but

are excluded from that of COX-1. Two of these inhibitors,

rofecoxib (Vioxx) and celecoxib (Celebrex; Fig. 25-75), be-

came major drugs for the treatment of inflammatory dis-

eases such as arthritis because they lack the major side

Section 25-7. Eicosanoid Metabolism: Prostaglandins, Prostacyclins, Thromboxanes, Leukotrienes, and Lipoxins 999

Figure 25-73 Some nonsteroidal anti-inflammatory drugs (NSAIDs).

COOH

C CH

Aspirin

(acetylsalicylic acid)

Acetaminophen

Indomethacin

Naproxen

COOH

Flurbiprofen

COOH

CHCH CH

COOH

Ibuprofen

Phenylbutazone

O C

COOH

Aspirin

COOH

Salicylic

acid

PGH synthase

(inactive)

PGH synthase

(active)

Ser 530

Rofecoxib (Vioxx)

Celecoxib (Celebrex)

Figure 25-74 Inactivation of PGH synthase by aspirin. Aspirin

acetylates Ser 530 of PGH synthase, thereby blocking the

enzyme’s cyclooxygenase activity.

Figure 25-75 COX-2 inhibitors. Rofecoxib and celecoxib are

specific inhibitors of COX-2 (PGH synthase-2).

JWCL281_c25_940-1018.qxd 4/20/10 1:59 PM Page 999

effects of the nonspecific NSAIDs. However,in 2004,Vioxx

was withdrawn from the pharmaceutical market because of

unanticipated cardiac side effects arising from its attenua-

tion of PGI

formation.

c. COX-3 May Be the Target of Acetaminophen

Acetaminophen, which is among the most widely used

analgesic/antipyretic drugs (but possesses little anti-

inflammatory activity, so that it is not really an NSAID),

does not significantly bind to either COX-1 or COX-2.

Thus its mechanism of action remained a mystery until the

discovery by Daniel Simmons of a third COX isozyme,

COX-3, a splice variant of COX-1, that is selectively inhib-

ited by acetaminophen as well as by certain NSAIDs. This

suggests that COX-3 is the primary target of drugs that

decrease pain and fever.

C. The Linear Pathway of Eicosanoid Metabolism:

Leukotrienes and Lipoxins

Arachidonic acid can be converted by a linear pathway

to several different hydroperoxyeicosatetraenoic acids

(HPETEs) by the 5-, 12-, and 15-lipoxygenases (5-, 12-,

and 15-LOs; Fig. 25-69). Hepoxilins are hydroxy epoxy de-

rivatives of 12-HPETE whose functions are not as yet well

understood. Lipoxins, the products of a second lipoxyge-

nase acting on 15-HPETE, are anti-inflammatory sub-

stances. Leukotrienes, derived from the 5-LO reaction, are

synthesized by a variety of white blood cells, mast cells

(connective tissue cells derived from the blood-forming

tissues that secrete substances which mediate inflamma-

tory and allergic reactions), as well as lung, spleen, brain,

and heart. Peptidoleukotrienes (LTC

,LTD

, and LTE

)

are now recognized to be the components of the slow re-

acting substances of anaphylaxis (SRS-A; anaphalaxis is a

violent and potentially fatal allergic reaction) released

from sensitized lung after immunological challenge. These

substances act at very low concentrations (as little as 10

10

M) to contract vascular, respiratory, and intestinal smooth

muscle. Peptidoleukotrienes, for example, are ⬃10,000-

fold more potent than histamine, a well-known stimulant

of allergic reactions. In the respiratory system, they con-

strict bronchi, especially the smaller airways; increase mu-

cus secretion; and are thought to be the mediators in

asthma.They are also implicated in immediate hypersensi-

tivity (allergic) reactions, inflammatory reactions, and

heart attacks.

a. Leukotriene Synthesis

The first two reactions in the conversion of arachidonic

acid to leukotrienes are both catalyzed by 5-LO, which con-

tains a nonheme, non-[Fe–S] cluster iron atom that must be

in its Fe(III) state to be active.These reactions occur as fol-

lows (Fig. 25-76):

1. The oxidation of arachidonic acid to form 5-HPETE,

a substance that, in itself, is not a physiological mediator.

This reaction occurs in three steps:

(a) The active site iron atom, in its active Fe(III) state,

abstracts an electron from the central methylene

group of the 5,8-pentadiene moiety of arachidonate

and the resulting free radical loses a proton to an

enzymatic base.

(b) The free radical rearranges and adds O

to form a

hydroperoxide radical.

site iron, now in its Fe(II) form, to yield the hy-

droperoxide in its anionic form, which the enzyme

then protonates to yield the hydroperoxide product,

regenerating the active Fe(III) enzyme.

2. The base-catalyzed elimination of water to form the

unstable epoxide leukotriene A

(LTA

; the subscript indi-

cates the number of carbon–carbon double bonds in the

molecule, which is also its series number).

1000 Chapter 25. Lipid Metabolism

Figure 25-76 The 5-LO-catalyzed oxidation of arachidonic

acid to LTA

via the intermediate 5-HPETE.

OO•

COOH

Fe(III)–E

5-HPETE

Arachidonic acid

Leukotriene A

(LTA

)

COOH

Fe(II)–E–

•

Fe(II)–E–•

•

Fe(III)–E+

98 65

COOH

1(b)

1(a)

1(c)

BS

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