Control of Fatty Acid Incorporation into Chloroplast Lipids in vitro

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Abstract
1.In isolated 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. 2.Furthermore high rates of glycerolipid biosynthesis appear to be accompanied by a preferred oleate incorporation as compared to palmitate. 3.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. 4.A comparison of calculated stromal sn-G3P levels under physiological conditions (0.1- 0.3 mᴍ) with those, required for half saturation of the primary acylation reaction either with oleate (Km(sn-G3P) = 0.3 mᴍ) or palmitate (Km (sn-G3P) = 0.6 mᴍ) in chloroplast extracts suggests, that both fatty acids to be involved in lysophosphatidic acid formation within chloroplasts, although oleate would be preferred. 5.The latter observation facilitates the understanding of a palmitate accumulation in chloroplast lipid fractions, induced by increasing sn-G3P concentrations in chloroplast sus­pensions. 6.Although stimulating fatty acid synthesis from acetate in intact chloroplasts, acyl-CoA- synthesizing-conditions (presence of CoA and ATP) in the applied chloroplast extracts appear to inhibit fatty acid incorporation into sn-G3P and thus to exert a regulatory function between the plastidary and extraplastidary glycerolipid biosynthesis.

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