Species Specificity in the Biosynthesis of Branched Paraffins in Leaves

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Abstract
Isobutyrate-1-14C and l-isoleucine-U-14C 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 14C incorporated into the surface lipids was found in the C29 paraffin and derivatives, whereas more than two-thirds of the 14C from straight chain precursors are usually found in these compounds. The small amount of 14C incorporated into the paraffin fraction was found in the n-C29 paraffin rather than branched paraffins showing that the 14C 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-C16 acid which was formed from both branched precursors, isoleucine-U-14C gave rise to branched C15, C17, and C19 fatty acids, and isobutyrate-1-14C gave rise to branched C16 and C18 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 C19 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.