Biosynthesis of 5,6-dihydroxyprostaglandin E1, and F1α from 5,6-dihydroxyeicosatrienoic acid by ram seminal vesicles

Lipoxygenase-catalyzed transformation of epoxy fatty acids to hydroxy-endoperoxides: a potential P450 and lipoxygenase interaction

Herein, we characterize a generally applicable transformation of fatty acid epoxides by lipoxygenase (LOX) enzymes that results in the formation of a five-membered endoperoxide ring in the end product. We demonstrated this transformation using soybean LOX-1 in the metabolism of 15,16-epoxy-α-linolenic acid, and murine platelet-type 12-LOX and human 15-LOX-1 in the metabolism of 14,15-epoxyeicosatrienoic acid (14,15-EET). A detailed examination of the transformation of the two enantiomers of 15,16-epoxy-α-linolenic acid by soybean LOX-1 revealed that the expected primary product, a 13S-hydroperoxy-15,16-epoxide, underwent a nonenzymatic transformation in buffer into a new derivative that was purified by HPLC and identified by UV, LC-MS, and ¹H-NMR as a 13,15-endoperoxy-16-hydroxy-octadeca-9,11-dienoic acid. The configuration of the endoperoxide (cis or trans side chains) depended on the steric relationship of the new hydroperoxy moiety to the enantiomeric configuration of the fatty acid epoxide. The reaction mechanism involves intramolecular nucleophilic substitution (SNi) between the hydroperoxy (nucleophile) and epoxy group (electrophile). Equivalent transformations were documented in metabolism of the enantiomers of 14,15-EET by the two mammalian LOX enzymes, 15-LOX-1 and platelet-type 12-LOX. We conclude that this type of transformation could occur naturally with the co-occurrence of LOX and cytochrome P450 or peroxygenase enzymes, and it could also contribute to the complexity of products formed in the autoxidation reactions of polyunsaturated fatty acids.

Brain synthesis and cerebrovascular action of epoxygenase metabolites of arachidonic acid

The purpose of this study was to determine if whole brain makes epoxygenase metabolites of arachidonic acid and, if so, whether they are vasoactive on the cerebral microcirculation. Blood-free mouse brain slices were incubated with exogenous radiolabeled arachidonic acid, and the extracted metabolites were resolved by HPLC. Metabolite structures were confirmed by gas chromatography/mass spectrometry. In addition to prostaglandins, leukotriene B4, and hydroxyeicosatetraenoic acids, mouse brain metabolized arachidonic acid into several other compounds. Among them, we identified 5,6- and 14,15-epoxyeicosatrienoic acid. Next, we tested the effect of topical application of brain-synthesized 5,6-epoxyeicosatrienoic acid and synthetic epoxyeicosatrienoic acids on in vivo rabbit cerebral arteriolar diameter using the cranial window technique and in vivo microscopy. Brain-synthesized 5,6-epoxyeicosatrienoic acid caused a transient 28% arteriolar dilation, similar to that produced by 5 micrograms/ml of synthetic 5,6-epoxyeicosatrienoic acid. A concentration of synthetic 14,15- and 11,12-epoxyeicosatrienoic acid of 5 micrograms/ml CSF had little or no effect on diameter, whereas 8,9-epoxyeicosatrienoic acid caused a maximum dilation of 8%. These studies suggest that brain-synthesized 5,6-epoxyeicosatrienoic acid may play a role in the normal or pathophysiological regulation of the cerebral microcirculation.

On the metabolism of epoxyeicosatrienoic acids by ram seminal vesicles: isolation of 5(6)epoxy-prostaglandin F1 alpha

cis-5(6)Epoxy- and cis-14(15)epoxyeicosatrienoic acid are formed from arachidonic acid by monooxygenases. 5(6)Epoxyeicosatrienoic acid is metabolized by fatty acid cyclooxygenase of ram seminal vesicles and the major products were recently identified as 5(6)epoxy-PGE1 and two stereoisomers of 5-hydroxy-PGI1. The two isomers were likely formed from an unstable intermediate, 5(6)epoxy-PGF1 alpha. The isolation of 5(6)epoxy-PGF1 alpha is described here and 14(15)epoxyeicosatrienoic acid is shown to inhibit fatty acid cyclooxygenase of ram seminal vesicles, albeit less potently than eicosatetraynoic acid (IC50 0.18 and 0.05 mM, respectively).