Prostacyclin Synthase, a Cytochrome P450 Enzyme

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Rearrangement reactions catalyzed by cytochrome P450s

Cytochrome P450s promote a variety of rearrangement reactions both as a consequence of the nature of the radical and other intermediates generated during catalysis, and of the neighboring structures in the substrate that can interact either with the initial radical intermediates or with further downstream products of the reactions. This article will review several kinds of previously published cytochrome P450-catalyzed rearrangement reactions, including changes in stereochemistry, radical clock reactions, allylic rearrangements, "NIH" and related shifts, ring contractions and expansions, and cyclizations that result from neighboring group interactions. Although most of these reactions can be carried out by many members of the cytochrome P450 superfamily, some have only been observed with select P450s, including some reactions that are catalyzed by specific endoperoxidases and cytochrome P450s found in plants.

Rapid reactions of peroxynitrite with heme-thiolate proteins as the basis for protection of prostacyclin synthase from inactivation by nitration

Prostacyclin (PGI(2)) synthase is a heme-thiolate (P450) protein which reacts with low levels of peroxynitrite (PN) under tyrosine nitration and inactivation. Studying heme proteins as models, we have found the heme-thiolate protein NADH-NO reductase (P450(NOR)) to be highly efficient in decomposing PN under concomitant nitration of phenol. The present study investigates two other P450 proteins, P450(BM-3) and chloroperoxidase, in order to test for the specific role of the thiolate ligand in the reaction with PN. A comparison with horseradish peroxidase and microperoxidase gives evidence of kinetic differences that classify heme-thiolate proteins, but not other heme proteins, as effective inhibitors of PGI(2) synthase nitration and inactivation. P450(BM-3) with PN catalyzes phenol nitration and nitration of its own tyrosine below 10 microM PN, whereas chloroperoxidase and P450(NOR) at such concentrations also nitrate phenol but not enzyme-bound tyrosine residues. We conclude that heme-thiolate proteins in general exhibit high reactivity with PN and turnover, probably due to the special electronic structure of the presumed thiolate-ferryl intermediate.

Site-directed mutagenesis of human prostacyclin synthase: Alteration of Cys441 of the Cys-pocket, and Glu347 and Arg350 of the EXXR motif

The possible active site Cys441 in the Cys-pocket and Glu347 and Arg350 of the EXXR motif of the human prostacyclin synthase, which catalyzes the conversion of prostaglandin H2 to prostacyclin, were subjected to site-directed mutagenesis in order to understand the role of these residues in expressing the enzymatic activity. Five expression vectors encoding the mutant enzymes with a single replacement, Cys441 Ala, Cys441 Ser, Cys441 His, Glu347 Ala and Arg350 Ala, as well as the wild-type enzyme were expressed in 293 cells. The microsomal fraction of the cells expressing the wild-type enzyme showed a specific activity of 96 nmol 6-keto-PGF1alpha/min per mg protein. All of the mutant enzymes examined showed no detectable enzyme activity, although immunoblot analysis demonstrated that levels of all the expressed mutant enzymes were similar to that of the wild-type enzyme. These results indicated that the Cys441 in the Cys-pocket, and Glu347 and Arg350 of the EXXR motif of human prostacyclin synthase are important for expressing the enzymatic activity.

Prostaglandin and thromboxane biosynthesis

We describe the enzymological regulation of the formation of prostaglandin (PG) D2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2 (prostacyclin), and thromboxane (Tx) A2 from arachidonic acid. We discuss the three major steps in prostanoid formation: (a) arachidonate mobilization from monophosphatidylinositol involving phospholipase C, diglyceride lipase, and monoglyceride lipase and from phosphatidylcholine involving phospholipase A2; (b) formation of prostaglandin endoperoxides (PGG2 and PGH2) catalyzed by the cyclooxygenase and peroxidase activities of PGH synthase; and (c) synthesis of PGD2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2, and TxA2 from PGH2. We also include information on the roles of aspirin and other nonsteroidal anti-inflammatory drugs, dexamethasone and other anti-inflammatory steroids, platelet-derived growth factor (PDGF), and interleukin-1 in prostaglandin metabolism.

Co-oxidation of salicylate and cholesterol during the oxidation of metmyoglobin by H2O2

The reaction between metmyoglobin and H2O2 proceeds with oxidation of the hemo-protein iron to a higher valence state and consumption of the peroxide. This reaction is further associated with (a) O2 evolution; (b) hydroxylation of the aromatic compound salicylate to yield a set of dihydroxybenzoic acid derivatives (analyzed by HPLC with electrochemical detection); (c) autoxidation of cholesterol with formation of 3 beta-hydroxy-5-alpha-cholest-6-ene-5-hydroperoxide; and (d) formation of electronically excited states detected by low-level chemiluminescence. The heterolytic scission of the O-O bond of hydroperoxides by metmyoglobin causes the formation of an oxidizing equivalent capable of promoting peroxidation of linoleate and arachidonate (as indicated by the parallel formation of thiobarbituric acid-reactive material and an enhancement of chemiluminescence intensity). The identity of the oxidizing equivalent(s) is discussed in terms of the formation of a relatively stable higher state of oxidation of heme Fe (FeIV-OH or FeV = O) as well as on possible intermediate species derived during the decomposition of H2O2 by metmyoglobin, such as HO.and 1O2. These species might be involved either simultaneously or sequentially in the peroxidation of fatty acids as well as in the tissue damage associated with the formation of H2O2 in ischemic-reperfusion states.

Exposure to toxic agents: the heme biosynthetic pathway and hemoproteins as indicator

The heme biosynthetic pathway is closely controlled by levels of the end product of the pathway, namely, heme, and porphyrins are normally formed in only trace amounts. When control mechanisms are disturbed by xenobiotics, porphyrins accumulate and serve as a signal of the interaction between a xenobiotic and the heme biosynthetic pathway. For example, an increase in erythrocyte protoporphyrin is a useful measurement for early detection of exposure to lead and porphyrinuria was an early manifestation of a hexachlorobenzene-induced porphyria in Turkey. In recent years a variety of additional xenobiotics has been shown to interact with the heme biosynthetic pathway, namely, halogenated aromatic hydrocarbons, pesticides, sulfides, and a variety of metals. Moreover, different xenobiotics (e.g., dihydropyridines and compounds containing unsaturated carbon-carbon bonds) interact with the prosthetic heme of cytochrome P-450 forming novel N-alkylporphyrins.