Linoleic acid metabolism in the red alga Lithothamnion corallioides : Biosynthesis of 11( R )‐hydroxy‐9( Z ),12( Z )‐octadecadienoic acid

Incubation of [1‐ 14 C]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‐hydroxy‐octadecadienoate). 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 18 O 2 and H 2 18 O, which demonstrated that the hydroxyl oxygen of the hydroxy‐octadecadienoic acids and the keto oxygen of the 11‐ketooctadecadienoic acid were derived from water and not from molecular oxygen.