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

Specific medium-chain fatty acids (about 30% capric and 50% lauric acid) in developing seeds of Cuphea wrightii A. Gray originate from non-green plastids. Their optimum biosynthesis in vitro requires intact plastids which are exogenously supplied with ATP as energy source and glucose 6-phosphate for maintaining the plastidial pyridine-nucleotide pool in reduced forms. In Cuphea plastids optimum formation of long-chain fatty acids occurs under alkaline conditions (pH > 8), while medium-chain products accumulate in the neutral range (pH 7). In cell-free extracts neutral conditions favor not only medium-chain specific acyl-ACP thioesterases but also short-chain condensing enzyme (KAS) activities triggered by increasing ratios of acetyl- to malonyl-CoA at neutral pH. Treatment of Cuphea plastids with either cerulenin (inhibitor of 3-ketoacyl-ACP synthase I) or ADP (inhibitor of acetyl-CoA carboxylase) causes an accumulation of capric acid. In the absence of cerulenin, unesterified lauric acid predominates. Transesterification on to CoA of neosynthesized fatty acids and recombination with Cuphea microsomes induce triacylglycerol incorporation of medium-chain fatty acids in vitro,