15-hydroxy-11-alpha-9-alpha-(epoxymethano)prosta-5-13-dienoic-acid and 12-hydroxy-5-8-10-heptadecatrienoic-acid

Cyclooxygenases catalyze the oxygenation of arachidonic acid to prostaglandin endoperoxides. Cyclooxygenase-2- and the xenobiotic-metabolizing cytochrome P450s 1A and 3A are all aberrantly expressed during colorectal carcinogenesis. To probe for a role of P450s in prostaglandin endoperoxide metabolism, we studied the 12-hydroxyheptadecatrienoate (HHT)/malondialdehyde (MDA) synthase activity of human liver microsomes and purified P450s. We found that human liver microsomes have HHT/MDA synthase activity that is concentration-dependent and inhibited by the P450 inhibitors, ketoconazole and clotrimazole with IC(50) values of 1 and 0.4 microM, respectively. This activity does not require P450 reductase. HHT/MDA synthase activity was present in purified P450s but not in heme alone or other heme proteins. The catalytic activities of various purified P450s were determined by measuring rates of MDA production from prostaglandin endoperoxide. At 50 microM substrate, the catalytic activities of purified human P450s varied from 10 +/- 1 to 0.62 +/- 0.02 min(-1), 3A4 >> 2E1 > 1A2. Oxabicycloheptane analogs of prostaglandin endoperoxide, U-44069 and U-46619, induced spectral changes in human P450 3A4 with K(s) values of 240 +/- 20 and 130 +/- 10 microM, respectively. These results suggest that co-expression of cyclooxygenase-2 and P450s in developing cancers may contribute to genomic instability due to production of the endogenous mutagen, MDA.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Animals; Catalysis; Cells, Cultured; Chromatography, Thin Layer; Cytochrome P-450 Enzyme System; Fatty Acids, Unsaturated; Heme; Humans; Malondialdehyde; Microsomes, Liver; Mutagens; Prostaglandin Endoperoxides; Prostaglandin H2; Prostaglandins H; Rabbits

Thromboxane synthase forms thromboxane (TX) A2 and 12(S)-hydroxyheptadeca-5(Z)-8(E)-10(E)-trienoic acid (HHT) at equimolar amounts. Twelve-oxoheptadeca-5(Z)-8(E)-10(E)-trienoic acid (Oxo-HT) is the primary metabolite of HHT and has been described to be an inhibitor of platelet aggregation. Functional studies, Schild analysis and competitive binding studies were performed to clarify its mode of action. Oxo-HT was prepared biosynthetically as well as chemosynthetically, purified and characterized by gas chromatography and mass spectrometry. Platelet activation was assessed by determination of shape change, aggregation, fibrinogen binding and P-selectin expression using optical aggregometry and flow cytometry. Oxo-HT 0.1 nM to 50 microM did not induce platelet activation. Furthermore, it had no effect on platelet activation induced by thrombin, ADP or PAF. In contrast, Oxo-HT inhibited platelet aggregation, fibrinogen binding and P-selectin expression induced by U46619 in a competitive manner. Schild analysis for U146619-induced fibrinogen binding and P-selectin expression revealed pA2 values of 6.1 and 6.6, respectively, which correspond to Kd values of approximately 0.8 microM and 0.3 microM, respectively. Oxo-HT also inhibited U46619 induced shape change (IC50 is approximately equal to 10 microM). However, Oxo-HT over a concentration range of 0.1-1 microM enhanced the partial shape change induced by low concentrations of U46619. Thus Oxo-HT seems to possess a minimal agonistic potential, which alone is not sufficient to trigger a platelet activation but can enhance low levels of platelet activation. Oxo-HT blocked the binding of [3H]SQ 29548 in a concentration-dependent manner, whereas HHT did not displace [3H]SQ 29548. The Kd of Oxo-HT determined from competition binding studies was 7.7 microM, about 10-25-fold higher than the apparent Kd determined by Schild analysis. This discrepancy might be due to a desensitization of the TXA2 receptor triggered by the minimal intrinsic activity of Oxo-HT. We conclude that Oxo-HT is a naturally occurring specific TXA2 receptor antagonist with minimal intrinsic activity. Oxo-HT may contribute to the regulation of TXA2-induced platelet activation in vivo.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Binding, Competitive; Cell Size; Fatty Acids, Unsaturated; Humans; Platelet Activation; Platelet Aggregation; Receptors, Thromboxane; Thromboxane-A Synthase

Both polymorphonuclear (PMN) leukocytes and metabolites of arachidonic acid, especially lipoxygenase products, have been reported to contribute to myocardial damage after coronary artery occlusion and reperfusion. While canine models of myocardial ischemia were used in many of these studies, very little is known about arachidonic acid metabolism by canine PMNs. Moreover, it is unclear whether arachidonic acid metabolites released by canine PMNs affect vascular tone. Therefore, we characterized arachidonic acid metabolism by canine PMNs and determined the effect of these metabolites on vascular tone of isolated canine coronary arteries. Suspensions of canine PMNs were incubated with [14C]arachidonic acid and the calcium ionophore A23187. The incubation media was extracted, and the metabolites resolved by HPLC. 20-Hydroxy-leukotriene B4 (LTB4), 12,20-dihydroxyeicosatetraenoic acid (diHETE), LTB4, 12-hydroxyheptadeclatrienoic acid (HHT), and 12-(S)-hydroxyeicosatetraenoic acid (HETE) were isolated, and their structures confirmed by gas chromatography/mass spectrometry. There was also evidence for the formation of 20-HETE, thromboxane B2 (TXB2), 5-HETE, and several isomers of LTB4. None of the arachidonic acid metabolites that were isolated from incubates of canine PMNs augmented vascular tone, but material migrating with 12,20-diHETE relaxed canine coronary arteries. Authentic 12(S),20-diHETE also produced a concentration-related relaxation of canine coronary artery. 12(R), 20-diHETE was inactive. 20-HETE inhibited A23187-induced PMN aggregation. Thus, arachidonic acid is metabolized in canine PMNs through the cyclooxygenase, lipoxygenases and cytochrome P-450 pathways. Whether these metabolites contribute to myocardial injury remains to be determined.

Topics: 12-Hydroxy-5,8,10,14-eicosatetraenoic Acid; 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Animals; Arachidonic Acid; Arteries; Cell Aggregation; Chromatography, High Pressure Liquid; Coronary Vessels; Dogs; Fatty Acids, Unsaturated; Hydroxyeicosatetraenoic Acids; In Vitro Techniques; Leukotriene B4; Mass Spectrometry; Neutrophils; Prostaglandin Endoperoxides, Synthetic; Thromboxane A2; Vasoconstriction; Vasoconstrictor Agents; Vasodilation