Prostaglandin hydroperoxidase, an integral part of prostaglandin endoperoxide synthetase from bovine vesicular gland microsomes

The highly purified prostaglandin endoperoxide synthetase from bovine vesicular gland microsomes had two still unresolved enzyme activities; the oxygenative cyclization of 8,11,14-eicosatrienoic acid to produce prostaglandin G1 and the conversion of the 15-hydro-peroxide of prostaglandin G1 to a 15-hydroxyl group, producing prostaglandin H1. The latter enzymatic reaction required heme and was stimulated by a variety of compounds, including tryptophan, epinephrine, and guaiacol, but not by glutathione. A peroxidatic dehydrogenation was demonstrated with epinephrine or guaiacol in the presence of various hydroperoxides, including hydrogen peroxide and prostaglandin G1. Higher activity and affinity were observed with the 15-hydroperoxide of eicosapolyenoic acid, especially those with the prostaglandin structure. Both the dehydrogenation of epinephrine or guaiacol and the 15-hydroperoxide reduction of prostaglandin G1 were demonstrated in nearly stoichiometric quantities. With tryptophan, however, such a stoichiometric transformation was not observed. The peroxidase activity as followed with guaiacol and hydrogen peroxide and the tryptophan-stimulated conversion of prostaglandin G1 to H1 were not dissociable as examined by isoelectric focusing, heat treatment, pH profile, and heme specificity. The results suggest that the peroxidase with a broad substrate specificity is an integral part of prostaglandin endoperoxide synthetase which is responsible for the conversion of prostaglandin G1 to H1.

Prostaglandin endoperoxide synthetase from bovine vesicular gland microsomes. Inactivation and activation by heme and other metalloporphyrins

Prostaglandin endoperoxide synthetase purified to apparent homogeneity from bovine vesicular gland microsomes contained iron far below the equimolar amount and essentially no heme. However, the enzyme required various metalloporphyrins including hematin or several hemoproteins such as hemoglobin. Preincubation of the enzyme with hematin or hemoglobin resulted in the loss of enzyme activity. The enzyme inactivation was protected by tryptophan or various other aromatic compounds. Furthermore, the simultaneous presence of tryptophan brought about activation of enzyme; namely, the enzyme preincubated with heme and tryptophan showed an almost full activity with a heme concentration in the reaction mixture far below the saturating level. Such inactivation and activation of the enzyme were also observed with manganese protoporphyrin. An identical heme requirement, heme-induced inactivation, and activation of the enzyme were observed in three types of reactions catalyzed by the enzyme: 1) bis-dioxygenation of 8,11,14-eicosatrienoic acid to produce prostaglandin G1, 2) 15-hydroperoxide cleavage of prostaglandin G1 to produce prostaglandin H1, and 3) guaiacol peroxidation. When heme was replaced by manganese protoporphyrin, the enzyme catalyzed only the bis-dioxygenation producing prostaglandin G1 and the activities of the latter two reactions were not detectable.

Prostaglandin endoperoxide E isomerase from bovine vesicular gland microsomes, a glutathione-requiring enzyme

Prostaglandin endoperoxide E isomerase catalyzes the isomerization of the endoperoxy group of prostaglandin H and produces prostaglandin E. The enzyme was solubilized with Tween 20 from bovine vesicular gland microsomes and purified 26-fold by successive column chromatography on DEAE-cellulose and omega-aminobutyl Sepharose 4B. The previously known requirement for glutathione was further investigated. The activity of freshly prepared enzyme was destroyed rapidly with a half-life of about 30 min at 24 degrees, pH 8.0. Various thiol compounds including glutathione protected the enzyme from such an inactivation. However, several findings indicated that glutathione was possibly specifically involved as a coenzyme in the isomerase reaction, but was not oxidized in a stoichiometric quantity. The enzyme also isomerized prostaglandin G at a rate approximately half of that of prostaglandin H.

Inhibition of prostaglandin endoperoxide synthetase by thiol analogues of prostaglandin

A variety of thiol compounds inhibited the enzymatic bis-oxygenation of 8,11,14-eicosatrienoic acid to prostaglandin G1, 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 muM. Other thiol compounds, such as 2,3-dimercaptopropanol, dithiotherreitol, 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.

Purification of prostaglandin endoperoxide synthetase from bovine vesicular gland microsomes

The prostaglandin synthetase system of bovine vesicular gland microsomes was solubilized and separated into Fractions I and II. The former fraction catalyzed the conversion of 8,11,14-eicosatrienoic acid to prostaglandin H1 (9 alpha, 11alpha-epidioxy-15(S)-hydroxy-13-trans-prostenoic acid). This compound was isomerized to prostaglandin E1 (11alpha, 15(S)-dihydroxy-9-keto-13-trans-prostenoic acid) by the action of Fraction II (Miyamoto, T., Yamamoto, S., and Hayaishi, O. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 3645-3648). Fraction I was further purified by isoelectric focusing and about a 700-fold purification was achieved starting from the microsomes. When the enzyme was incubated with 8,11,14-eicosatrienoic acid in the presence of hematin, an unstable compound which was distinguishable from prostaglandin H1 accumulated. The chemical properties of this compound were identical with those of prostaglandin G1 (9 alpha, 11 alpha-epidioxy-15(S)-hydroperoxy-13-trans-prostenoic acid). The enzyme also catalyzed the conversion of prostaglandin G1 to H1 when heme and tryptophan were supplied. Thus, the purified enzyme, which was provisionally referred to as prostaglandin endoperoxide synthetase, exhibited two enzyme activities: the synthesis of prostaglandin G1 and its conversion to prostaglandin H1. Either free or protein-bound heme was required for both reactions, and only protoheme was active. Tryptophan stimulated the conversion of prostaglandin G1 to H1, and this stimulatory effect was also observed with various other aromatic compounds. Indomethacin and aspirin inhibited prostaglandin G1 synthesis, but not the other steps of prostaglandin biosynthesis.