Prostaglandin H synthase, activity

Enzyme Assays. Cytochrome P-450 peroxidase activity was determined by the method of O Brien and Rahimthula (27) as modified by Reddy, et al, with cumene hydroperoxide (CHP), linoleic acid hydroperoxide (LHP) and 15-HPETE as substrates and tetramethyl-P-phenylene diamine (TMPD) as electron donor. Prostaglandin H synthase activity was measured as previously described (19). 

Aqueous cigarette tar extracts increased prostaglandin H synthase activity in isolated rat pul-... 

The enzyme system responsible for the biosynthesis of PGs is widely distributed in mammalian tissues and has been extensively studied (2). It is referred to as prostaglandin H synthase (PGHS) and exhibits both cyclooxygenase and peroxidase activity. In addition to the classical PGs two other prostanoid products, thromboxane [57576-52-0] (TxA ) (3) and prostacyclin [35121 -78-9] (PGI2) (4) are also derived from the action of the enzyme system on arachidonic acid (Fig. 1). 

Figure 23-6. Conversion of arachidonicacid to prostaglandins and thromboxanes of series 2. (PG, prostaglandin TX, thromboxane PGI, prostacyclin HHT, hydroxyheptadecatrienoate.) (Asterisk Both of these starred activities are attributed to one enzyme prostaglandin H synthase. Similar conversions occur in prostaglandins and thromboxanes of series 1 and 3.)...

The first line of evidence derives from the predominant formation of quinones when metabolism of BP is conducted under peroxi-datic conditions, namely by prostaglandin H synthase (21) or by cytochrome P-450 with cumene hydroperoxide as cofactor T22). Under these metabolic conditions one-electron oxidation is the preponderant mechanism of activation. 

LOX-dependent superoxide production was also registered under ex vivo conditions [55]. It has been shown that the intravenous administration of lipopolysaccharide to rats stimulated superoxide production by alveolar and peritoneal macrophages. O Donnell and Azzi [56] proposed that a relatively high rate of superoxide production by cultured human fibroblasts in the presence of NADH was relevant to 15-LOX-catalyzed oxidation of unsaturated acids and was independent of NADPH oxidase, prostaglandin H synthase, xanthine oxidase, and cytochrome P-450 activation or mitochondrial respiration. LOX might also be involved in the superoxide production by epidermal growth factor-stimulated pheochromo-cytoma cells [57]. 

It has already been mentioned earlier that similar to LOXs, prostaglandin H synthases can be activated or inhibited by reactive nitrogen species. Nitric oxide may exhibit the inhibitory [58,65,86,101 104] or stimulatory effects [105 110] on PGHSs. Inhibitory effects depend on the ability of nitric oxide to reduce the ferric enzyme to the inactive ferrous form, competition... 

Schreiber, J., Eling, T. E. and Mason, R. P. The oxidation of arachidonic acid by the cyclooxygenase activity of purified prostaglandin H synthase spin trapping of a carbon-centered free radical intermediate. Arch. Biochem. Biophys. 249 126-136,1986. 

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

Eling TE, Mason RP, Sivarajah K. The formation of aminopyrine cation radical by peroxidase activity of prostaglandin H synthase and subsequent reactions of the radical. J Biol Chem 1985 260(3) 1601—1607. 

Phenylbutazone. - This anti-inflammatory drug inhibits prostaglandin H synthase. Earlier spin-trapping studies established that PB is oxidised to a carbon-centred radical by the peroxidase activity of the enzyme.175 The radical has since been trapped with MNP upon incubation of the drug with HRP. The intensity of the signal from the adduct was reduced by GSH, suggesting chemical repair of the radical by the thiol. The PB/HRP system induced lipid peroxidation in microsomes, which was suppressed by GSH.176... 

Lehmann, W.D. 1994. Regio- and stereochemistry of the dioxygenation reaction catalyzed by (S)-type lipoxygenases or by the cyclooxygenase activity of prostaglandin H synthases. Free Radical Biol. Med. 16 241-253. 

Fig. 9. Redox-active amino acid residues related to tyrosine, (a) Tyrosine, the redox center in ribonucleotide reductase, prostaglandin H synthase, and the photosynthetic oxygen evolving complex, (b) 2,4,5-Trihydroxyphenylalanine, the redox cofactor of the quinoprotein amine oxidase, (c) Tyrosine-cysteine (Tyr-Cys), the redox cofactor of galactose oxidase.

Kulmacz RJ, Wang LH. Comparison of hydroperoxide initiator requirements for the cyclooxygenase activities of prostaglandin H synthase-1 and -2. J. Biol. Chem. 1995 270 24019-24023. 

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