Biosynthesis of paraffins in Brassica oleracea: Fatty acid elongation-decarboxylation as a plausible pathway

Incorporation of fatty acids of chain length C 2 to C 14 into the C 16 acid of Brassica oleracea was not inhibited by trichloroacetate at concentrations sufficient to inhibit their incorporation into paraffins almost completely. The sensitivity to trichloroacetate of incorporation of fatty acids into paraffins remained nearly the same when the chain length of the precursor fatty acid was increased from C 2 to C 16 , but incorporation of C 18 acid into paraffin was much less sensitive to trichloroacetate. Carbons 1 and 2 of stearic acid were incorporated into nonacosane at the same rate as the entire C 18 molecule, showing that C 18 acid does not undergo degradation of its carbon chain before being incorporated into the paraffin. These results support the hypothesis that the C 16 chain is elongated to a C 30 unit which on decarboxylation gives rise to the paraffin. Light and 3-(4-chlorophenyl)-1,1-dimethylurea (CMU) affect incorporation of C 18 into neither paraffins nor very long fatty acids; trichloroacetate inhibits C 18 incorporation into paraffins and to a lesser degree into very long fatty acids, suggesting that the formation of such acids is related to paraffin synthesis. The time course of incorporation of C 18 acid into very long fatty acids and paraffins failed to show a precursor-product relationship, and the very long fatty acids were found to be esterified in the phospholipids and triglycerides of the particulate structures. On the basis of present knowledge it is proposed that C 16 acid becomes the substrate for an elongation-decarboxylation enzyme complex which is situated somewhere outside the chloroplasts. This enzyme complex elongates C 16 to C 30 , decarboxylates this C 30 unit, and releases the paraffin from the complex. During the elongation process dissociation of small amounts of fatty acids occurs at each stage. These are rapidly estfieried into phospholipids and triglycerides.