The synthesis of lipids and acyl thioesters was studied in microsomal preparations from germinating pea (Pisum sativum cv. Feltham First) seeds. Under conditions of maximal synthesis (in the presence of exogenous acyl-carrier protein) acyl-acyl-carrier proteins accounted for about half the total incorporation from [14C]malonyl-CoA. Decreasing the concentrations of exogenous acyl-carrier protein lowered the overall synthesis of fatty acids by decreasing, almost exclusively, the radioactivity associated with acyl-acyl-carrier proteins. A time-course experiment showed that acyl-acyl-carrier proteins accumulated most of the radioactive label at the beginning of the incubation but, eventually, the amount of radioactivity in that fraction decreased, while a simultaneous increase in the acyl-CoA and lipid fractions was noticed. Addition of exogenous CoA (1 mM) produced a decrease of total incorporation, but an increase in the radioactivity incorporated into acyl-CoA. The microsomal preparations synthesized saturated fatty acids up to C20, including significant proportions of pentadecanoic acid and heptadecanoic acid. Synthesis of these ‘odd-chain’ fatty acids only took place in the microsomal fraction. In contrast, when the 18,000g supernatant (containing the microsomal and soluble fractions) was incubated with [14C]malonyl-CoA, the radioactive fatty acid and acyl classes closely resembled the patterns produced by germinating in the presence of [14C]acetate in vivo. The results are discussed in relation to the role of acyl thioesters in the biosynthesis of plant lipids.

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