FACTORS CONTROLLING MEDIUM-CHAIN FATTY-ACID SYNTHESIS IN PLASTIDS FROM MATURING CUPHEA EMBRYOS

The colorless embryos of Cuphea wrightii A. Gray accumulate capric (ab out 30%) and lauric acid (about 50%) in their storage lipids. Fraction ation studies show that the capacities for the synthesis of these medi um-chain fatty acids (MCFA) from [1-C-14]acetate were strictly bound t o intact plastids. These, in turn, obligately required the addition of ATP. ATP could partially be substituted by ADP. Reduction of the pyri dine nucleotide pool, required for optimum MCFA formation within the p lastids, was driven by glucose 6-phosphate. Under these conditions the plastids were capable of synthesizing MCFA like the intact tissue. Th e presence of CoA in the incubation medium induced acyl-CoA formation. The observed accumulation of unesterified capric and lauric acid in t he absence of CoA suggests that acyl-ACP thioesterase activity is invo lved in the chain termination. Treatment with cerulenin led to an unex pectedly small reduction of total fatty acid synthesis while the chain elongation of capric acid was clearly inhibited. A similar accumulati on of capric acid at the expense of longer chain fatty acids has been observed after replacing ATP by ADP. These findings implicate that eve n the condensing enzymes are involved in the control of chain terminat ion.