Fat Metabolism in Higher Plants

Oleyl-CoA is the specific substrate for the microsomal oleyl-CoA desaturase from maturing seeds of Carthamus tinctorius L. Although there is a very high acyltransferase activity in the microsomes, which rapidly transfers the oleyl moiety to an endogenous acceptor to form β-oleylphosphatidylcholine, this oleyl-oxygen ester does not serve as a substrate for the desaturase. Since tritium-labeled CoA does not exchange with nonlabeled oleyl-CoA in the presence of the microsomal system, the possible roles of ACP and enzyme-bound oleyl complexes are also excluded. Hen liver desaturase utilizes stearyl-CoA as the substrate and the product is oleyl-CoA. There is no evidence for an oxygen ester substrate. Thus the substrate for the two desaturases is an acyl-CoA and the immediate product is an acyl-CoA. Acyl-oxygen esters (phosphatidylcholine) do not serve as substrates for the desaturation reaction.