COMPARTMENTATION OF FATTY-ACID METABOLISM IN ZYGOTIC RAPE EMBRYOS

During seed development of rape (Brassica napus L. var. Nilla), optimu m biosynthesis of fatty acids occurs in green embryos with photosynthe tically active plastids, since their radiolabeling from C-14-acetate w as significantly reduced in the dark or by DCMU. Furthermore, differen t sensitivities against the thiol-inhibitors, cerulenin and iodoacetam ide, give evidence for the strict compartmental separation of ACP-boun d <<de novo>>-synthesis of fatty acids up to chainlengths of 18 C-atom s inside the plastid and their CoA-bound elongation to erucic acid by the endoplasmic reticulum. The latter observation was supported by dec reasing contamination with acyl-CoA elongase activities of plastids du ring their purification from cell-free extracts of embryos. Fatty acid synthesis from acetate by purified plastids showed an absolute requir ement for ATP but no dependence on added reducing equivalents (NAD[P]H ). Subcellular fractionation by differential centrifugation showed tha t although acyl-CoA elongases and glycerol 3-phosphate acyltransferase were detected in all subcellular fractions from 5000 - 100,000 x g, t he highest activities were present in the 100,000 x g (microsomal) pel let. An interrelationship of these enzymes is suggested by their compe tition for the same substrate, i.e. long-chain acyl-CoAs. Acyl CoA syn thetase activities were not restricted to plastids alone but were foun d in the microsomal and cytosolic fraction as well.