Species specificity in the biosynthesis of branched paraffins in leaves.

Isobutyrate-1-(14)C and l-isoleucine-U-(14)C fed through the petiole labeled the surface lipids of broccoli leaves, but the incorporation was much less than from straight chain precursors. Not more than one-third of the (14)C incorporated into the surface lipids was found in the C(29) paraffin and derivatives, whereas more than two-thirds of the (14)C from straight chain precursors are usually found in these compounds. The small amount of (14)C incorporated into the paraffin fraction was found in the n-C(29) paraffin rather than branched paraffins showing that the (14)C in the paraffin must have come from degradation products. Radio gas-liquid chromatography of the saturated fatty acids showed that, in addition to the n-C(16) acid which was formed from both branched precursors, isoleucine-U-(14)C gave rise to branched C(15), C(17), and C(19) fatty acids, and isobutyrate-1-(14)C gave rise to branched C(16) and C(18) acids. Thus the reason for the failure of broccoli leaf to incorporate branched precursors into branched paraffins is not the unavailability of branched fatty acids, but the absolute specificity of the system that synthesizes paraffins, probably the elongation-decar-boxylation enzyme complex. Consistent with this view, no labeled branched fatty acids longer than C(19) could be found in the broccoli leaf. The branched fatty acids were also found in the surface lipids indicating that the epidermal layer of cells did have access to branched chains. Thus the paraffin synthesizing enzyme system is specific for straight chains in broccoli, but the fatty acid synthetase is not.