Sulfolipid synthesis from 35SO 42− and [1- 14C]acetate in isolated intact spinach chloroplasts

Abstract

Chloroplasts isolated from fully developed spinach leaves and incubated in the presence of 35SO 4 2− or [1- 14C] acetate were able to synthesize sulfoquinovosyldiacylglycerol (SQDG). In experimental conditions which promote glycerolipid biosynthesis (i.e. in the presence of sn-glycerol 3-phosphate), de novo SQDG biosynthesis was observed. Newly synthesized SQDG molecules were formed from diacylglycerol having 18:1 and 16:0 fatty acids exclusively located at the sn-1 and sn-1 positions of glycerol, respectively. Therefore, SQDG formed by isolated intact chloroplasts contained the so-called ‘prokaryotic’ structure of diacylglycerol that is typical for all plastid glycerolipids synthesized within the organelle. In addition, SQDG and galactolipid biosynthesis were demonstrated to compete for the same diacylglycerol molecules that were formed via the chloroplast Kornberg-Pricer pathway. Finally, the presence of small, but consistent, amounts of 18:2 in SQDG, but not in diacylglycerol or phosphatidic acid, suggests that desaturation of 18:1 fatty acid might occur in isolated chloroplasts after formation of SQDG molecules. These results demonstrate that spinach chloroplasts are autonomous for the biosynthesis of both the polar head group (sulfoquinovose) from SO 4 2− and the diacylglycerol backbone (from acetate and sn-glycerol 3-phosphate) of SQDG.