LINOLEIC-ACID METABOLISM IN THE RED ALGA LITHOTHAMNION-CORALLIOIDES - BIOSYNTHESIS OF 11(R)-HYDROXY-9(Z), 12(Z)-OCTADECADIENOIC ACID

Incubation of [1-C-14]linoleic acid with an enzyme preparation obtained from the red alga Lithothamnion corallioides CROUAN resulted in the formation of 11-hydroxy-9(Z), 12(Z)-octadecadienoic acid as well as smaller amounts of 9-hydroxy-10(E),12(Z)-octadecadienoic acid, 13-hydroxy-9(Z),11(E)-octadecadienoic acid and 11-keto-9(Z), 12(Z)-octadecadienoic acid. Steric analysis showed that the 11-hydroxyoctadecadienoic acid had the (R) configuration. The 9- and 13-hydroxyoctadecadienoic acids were not optically pure, but were due to mixtures of 75% (R) and 25% (S) enantiomers (9-hydroxyoctadecadienoate), and 24% (R) and 76% (S) enantiomers (13-hydroxyoctadecadienoate). 11-Hydroxyoctadecadienoic acid was unstable at acidic pH. In acidified water, equal parts of 9(R,S)-hydroxy-10(E),12(Z)-octadecadienoate and 13(R,S)-hydroxy-9(Z),11(E)-octadecadienoate, plus smaller amounts of the corresponding (E),(E) isomers were produced. In aprotic solvents, acid treatment resulted in dehydration and in the formation of equal amounts of 8,10,12- and 9,11,13-octadecatrienoates. The enzymatic conversion of linoleic acid into the hydroxyoctadecadienoic acids and the ketooctadecadienoic acid was oxygen-dependent; however, inhibitor experiments indicated that neither lipoxygenase nor cytochrome P-450 were involved in the conversion. This conclusion was supported by experiments with O-18(2) and (H2O)-O-18 which demonstrated that the hydroxyl oxygen of the hydroxyoctadecadienoic acids and the keto oxygen of the 11-ketooctadecadienoic acid were derived from water and not from molecular oxygen.