Synthesis of Long-Chain Acyl-CoA in Chloroplast Envelope Membranes.

The chloroplast envelope is the site of a very active long-chain acylcoenzyme A (CoA) synthetase. Furthermore, we have recently shown that an acyl CoA thioesterase is also associated with envelope membrane (Joyard J, PK Stumpf 1980 Plant Physiol 65: 1039-1043). To clarify the interacting roles of both the acyl-CoA thioesterase and the acyl-CoA synthetase, the formation of acyl-CoA in envelope membranes was examined with different techniques which permitted the measurement of the actual rates of acyl-CoA formation. Using [(14)C]ATP or [(14)C]oleic acid as labeled substrates, it can be shown that the envelope acyl-CoA synthetase required both Mg(2+) and dithiothreitol. Triton X-100 slightly stimulated the activity. The specificity of the acyl-CoA synthetase was determined either with [(14)C]ATP or with [(3)H]CoA as substrates. The results obtained in both cases were similar, that is, as substrates, the unsaturated fatty acids were more effective than saturated fatty acids, the velocity of the reaction increased from lauric acid to palmitic acid, and the maximum velocity was obtained with unsaturated C(18) fatty acids.The results obtained suggest that the acyl-CoA thioesterase associated with envelope membranes could be an ultimate control to prevent the transport (outside of the chloroplast) or the insertion (into chloroplast lipids) of fatty acids with chains shorter than C(16).