Fat metabolism in higher plants: L. The biosynthesis of polyunsaturated fatty acids by isolated spinach chloroplasts

Abstract

Unlike mature spinach chloroplasts, chloroplasts isolated from immature spinach leaves incorporated [l- 14C]acetate into polyunsaturated fatty acids. In addition, they had a higher capacity to synthesize long-chain fatty acids when compared with plastids isolated from mature leaves. Coenzyme A and ATP in high concentrations stimulated polyunsaturation. While cyclic AMP markedly inhibited total lipid synthesis in both types of chloroplasts, cyclic AMP completely inhibited polyunsaturation in chloroplasts from immature leaves. Evidence for independent synthesis was obtained using potassium cyanide (1 m m), which completely inhibited linoleate synthesis but did not alter oleate or linolenate synthesis. The fatty acids accumulated predominantly as thioesters during early stages of synthesis in these chloroplasts. With longer periods of incubation with [1- 14C]acetate, a greater percentage of the labeled fatty acids was observed in oxygen ester lipids viz., phospholipids, monogalactosyl diglycerides, and diglycerides. The diglycerides formed in vitro served as substrates for galactolipid synthesis in the presence of UDP-galactose.