Fatty Acid synthesis in endosperm of young castor bean seedlings.

Enzyme assays on organelles isolated from the endosperm of germinating castor bean (Ricinus communis) by sucrose density gradient centrifugation showed that fatty acid synthesis from [(14)C]malonyl-CoA was localized exclusively in the plastids. The optimum pH was 7.7 and the products was mainly free palmitic and oleic acids. Both NADH and NADPH were required as reductants for maximum activity. Acetyl-CoA, and acyl-carrier protein from Escherichia coli increased the rate of fatty acid synthesis, while low O(2) levels suppressed synthesis. In the absence of NADPH or at low O(2) concentration, stearic acid became a major product at the expense of oleic acid. Fatty acid synthesis activity was highest during the first 3 days of germination, preceding the maximum development of mitochondria and glyoxysomes. It is proposed that the plastids are the source of fatty acids incorporated into the membranes of developing organelles.