CONTROL OF FATTY-ACID INCORPORATION INTO CHLOROPLAST LIPIDS INVITRO

In isolated [spinach] chloroplasts which are provided with essential exogenous substrates for glycerolipid biosynthesis (sn-G3P and UDPgal) the incorporation of fatty acids into lipids shows the same pH dependence as the fatty acid synthesis itself with a stromal pH optimum close to 8.5. High rates of glycerolipid biosynthesis apparently are accompanied by a preferred oleate incorporation as compared to palmitate. Reinvestigations of the sn-G3P requirement of plastid lysophosphatidic acid formation with rapidly prepared substrate-free chloroplast extracts under approximately physiological conditions reveal a lower specificity of the primary sn-G3P acylation for oleate, as recently found for the fatty acid transfer from purified acyl-ACP fractions on to sn-G3P, catalyzed by purified acyl transferase 1. A comparison of calculated stromal sn-G3P levels under physiological conditions (0.1-0.3 mM) with those, required for half saturation of the primary acylation reaction either with oleate (Km (sn-G3P) = 0.3 mM) or palmitate (Km (sn-G3P) = 0.6 mM) in chloroplast extracts suggests that both fatty acids are involved in lysophosphatidic acid formation within chloroplasts, although oleate would be preferred. The latter observation facilitates the understanding of a palmitate accumulation in chloroplast lipid fractions, induced by increasing sn-G3P concentrations in chloroplast suspensions. Stimulating fatty acid synthesis from acetate in intact chloroplasts, acyl-CoA-synthesizing conditions (presence of CoA and ATP) in the applied chloroplast extracts apparently inhibit fatty acid incorporation into sn-G3P and exert a regulatory function between the plastid and extraplastid glycerolipid biosynthesis.