Structure and Biosynthesis of Cuticular Lipids: Hydroxylation of Palmitic Acid and Decarboxylation of C28, C30, and C32 Acids in Vicia faba Flowers 12

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The structure and composition of the cutin monomers from the flower petals of Vicia faba were determined by hydrogenolysis (LiAlH4) or deuterolysis (LiAlD4) followed by thin layer chromatography and combined gas-liquid chromatography and mass spectrometry. The major components were 10, 16-dihydroxyhexadecanoic acid (79.8%), 9, 16-dihydroxyhexadecanoic acid (4.2%), 16-hydroxyhexadecanoic acid (4.2%), 18-hydroxyoctadecanoic acid (1.6%), and hexadecanoic acid (2.4%). These results show that flower petal cutin is very similar to leaf cutin of V. faba. Developing petals readily incorporated exogenous [1-14C]palmitic acid into cutin. Direct conversion of the exogeneous acid into 16-hydroxyhexadecanoic acid, 10, 16-dihydroxy-, and 9, 16-dihydroxyhexadecanoic acid was demonstrated by radio gas-liquid chromatography of their chemical degradation products. About 1% of the exogenous [1-14C]palmitic acid was incorporated into C27, C29, and C31n-alkanes, which were identified by combined gas-liquid chromatography and mass spectrometry as the major components of the hydrocarbons of V. faba flowers. The radioactivity distribution among these three alkanes (C27, 15%; C29, 48%; C31, 38%) was similar to the per cent composition of the alkanes (C27, 12%; C29, 43%; C31, 44%). [1-14C]Stearic acid was also incorporated into C27, C29, and C31n-alkanes in good yield (3%). Trichloroacetate, which has been postulated to be an inhibitor of fatty acid elongation, inhibited the conversion of [1-14C]stearic acid to alkanes, and the inhibition was greatest for the longer alkanes. Developing flower petals also incorporated exogenous C28, C30, and C32 acids into alkanes in 0.5% to 5% yields. [G-3H]n-octacosanoic acid (C28) was incorporated into C27, C29, and C31n-alkanes. [G-3H]n-triacontanoic acid (C30) was incorporated mainly into C29 and C31 alkanes, whereas [9, 10, 11-3H]n-dotriacontanoic acid (C32) was converted mainly to C31 alkane. Trichloroacetate inhibited the conversion of the exogenous acids into alkanes with carbon chains longer than the exogenous acid, and at the same time increased the amount of the direct decarboxylation product formed. These results clearly demonstrate direct decarboxylation as well as elongation and decarboxylation of exogenous fatty acids, and thus constitute the most direct evidence thus far obtained for an elongation-decarboxylation mechanism for the biosynthesis of alkanes.

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