Investigation of the allene oxide pathway in the coral Plexaura homomalla: formation of novel ketols and isomers of prostaglandin A2 from 15-hydroxyeicosatetraenoic acid.

ABSTRACT

Prostaglandin A2 is a major constituent of the gorgonian Plexaura homomalla, and there is evidence that its biosynthesis involves a noncyclooxygenase pathway. The coral contains an 8(R)-lipoxygenase and an allene oxide synthase; from arachidonic acid, the sequential action of these enzymes gives an allene epoxide, the cyclization of which forms an analogue of prostaglandin A2 (PGA2) with no 15-hydroxyl group. In this study we examined the metabolic fate of 15-hydroxyeicosatetraenoic acid (15-HETE), which via analogous reactions could lead to PGA2. The 8(R)-lipoxygenase metabolized preferentially the 15(R) enantiomer of 15-HETE, and this reaction was stimulated fivefold by including 1 M NaCl in the incubation. Further enzymic steps were detected by comparing the metabolic profiles of the 8(R)-hydroperoxy-15(R)-hydroxy intermediate with that of its 8(S),15(S) enantiomer. Two main products were formed exclusively from the 8(R),15(R) enantiomer an allene epoxide and the comparatively stable epoxide, 8,9-epoxy-10,15-dihydroxyeicosa-5,11,14-trienoic acid. Formation of the allene oxide was inferred from detection of its hydrolysis and cyclization products. It cyclized to give two isomers of PGA2 which have a "cis" arrangement of the side chains. The main hydrolysis product (8,15-dihydroxy-9-ketoeicosa-5,11,13-trienoic acid) was unstable and prone to oxygenation, giving 8,14,15-trihydroxy-9-ketoeicosa-5,10,12-trienoic acids after reduction of the 14-hydroperoxide. We conclude that metabolism of a 15-hydroxy eicosanoid is a potential route to the A series prostaglandins, although the low yield and lack of stereochemical control suggest that this is not the natural pathway of biosynthesis in P. homomalla. Unexpectedly, the major end products of the pathway are trihydroxy ketols and the single diastereomer of a stable epoxyalcohol.