Inhibition of prostaglandin endoperoxide synthetase by thiol analogues of prostaglandin.

Abstract

A variety of thiol compounds inhibited the enzymatic bis-oxygenation of 8,11,14-eicosatrienoic acid to prostaglandin GI, as examined with a purified preparation of prostaglandin endoperoxide synthetase (prostaglandin synthase; 8,11,14-eicosatrienoate, hydrogen-donor:oxygen oxidoreductase; EC 1.14.99.1) from bovine vesicular gland. The hydroperoxide cleavage of prostaglandin G1 producing prostaglandin H1 was not affected by these thiol compounds. Several prostaglandin analogues with a thiol group (9,11-dihydroxy-15S-or 15R-mercaptoprosta-5,13-dienoic acid, 1-mercapto-9,11,15-trihydroxyprosta-5,13-diene, and 1-mercapto-9-oxo-11,15-dihydroxyprosta-5,13-diene) were most potent inhibitors, showing almost complete inhibition at concentrations on the order of 1 t&M.Other thiol compounds, such as 2,3-dimercaptopropanol, dithiothreitol, and dihydrolipoic acid, were also inhibitory but were much less effective. The inhibition, as examined with 9,11-dihydroxy-15S-mercaptoprosta-5,13-dienoic acid and 2,3-dimercaptopropanol, was noncompetitive. The hypothetical mechanism of prostaglandin biosynthesis proposed earlier (1, 2) has recently been substantiated by the isolation of two prostaglandin endoperoxides (3-5). The intermediary nature of these endoperoxides was further supported by the solubilization and purification of two enzyme fractions in our laboratory (6-8). One catalyzes the bis-oxygenation of an unsaturated fatty acid producing prostaglandin G, which has both an endoperoxy and a hydroperoxy group. Heme is required for this reaction. The same enzyme preparation also catalyzes the cleavage of the hydroperoxy group of prostaglandin G, producing prostaglandin H with an endoperoxy and a hydroxy group. This reaction also requires heme and is markedly stimulated by tryptophan (7). The other enzyme isomerizes the endoperoxy group of prostaglandin H to produce prostaglandin E (6, 8).In view of the role of these endoperoxides in prostaglandin biosynthesis, attempts have been made to synthesize analogues of prostaglandin endoperoxides that would have any biological activity and effect on enzyme (9)(10)(11)(12) Synthesis of (5Z,9a,lla,13E,15)-l-Mercapto-9,11,15-trihydroxyprosta-5,13-diene (V in Fig. 1). Reduction of I by diisobutylaluminum hydride produced the 1,9-diol II, which was subjected to selective tosylation using 1.2 equivalents of p-toluenesulfonyl chloride in pyridine at 00 for 20 hr (68% yield). The tosylate III was converted to the thioacetate IV with excess sodium thioacetate in dimethyl sulfoxide-dimethylformamide at 250 for 2 hr (83% yield). Hydrolysis of IV by K2CO3 in methanol, followed by removal of tetrahydropyranyl group in acetic acid-tetrahydrofuran-water, gave the 1-mercapto derivative V (in 73% yield from IV): nuclear magnetic resonance spectrum in CDC13 due to 2H of CH2S at S 2.55 ppm.Synthesis of (5Zlla,13E,15S)l-Mercapto-9-oxo-11,15-dihydroxyprosta-5,13-diene (VIII). The alcohol III was oxidized by Collins reagent in methylene chloride at 00 for 15 min. The resulting ketone VI was transfor...