Fat Metabolism in Higher Plants

A microsomal fraction from developing seeds of castor bean (Ricinus communis L.) catalyzes the hydroxylation of oleyl coenzyme A to ricinoleic acid and has the properties of a mixed function oxidase. Only preparations from seeds at a certain stage of development have hydroxylating activity. Oxygen and reduced nicotinamide adenine dinucleotide are obligatory cofactors (reduced nicotinamide adenine dinucleotide phosphate is relatively inactive), and an activated form of oleic acid, i.e. the CoA thioester, is required as substrate. Enzyme-bound iron is presumably involved in the hydroxylation, which is inhibited by thiol reagents, cyanide, azide, and chelating agents, but not by carbon monoxide. The hydroxylating enzyme shows a marked substrate specificity for oleic acid. Stearic, linoleic, vaccenic, and elaidic acids are not hydroxylated. Linoleic acid is not an intermediate in the conversion of oleic acid to ricinoleic acid.