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

Specific medium-chain fatty acids (about 30% capric and 50% lauric aci d) in developing seeds of Cuphea wrightii A. Gray originate from non-g reen plastids. Their optimum biosynthesis in vitro requires intact pla stids which are exogenously supplied with ATP as energy source and glu cose 6-phosphate for maintaining the plastidial pyridine-nucleotide po ol in reduced forms. In Cuphea plastids optimum formation of long-chai n fatty acids occurs under alkaline conditions (pH > 8), while medium- chain products accumulate in the neutral range (pH 7). In cell-free ex tracts neutral conditions favor not only medium-chain specific acyl-AC P thioesterases but also short-chain condensing enzyme (KAS) activitie s 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, u nesterified lauric acid predominates. Transesterification on to CoA of neosynthesized fatty acids and recombination with Cuphea microsomes i nduce triacylglycerol incorporation of medium-chain fatty acids in vit ro,