Further evidence for an elongation-decarboxylation mechanism in the biosynthesis of paraffins in leaves.
In isolated tobacco leaves l-valine-U-(14)C gave rise to labeled even-numbered isobranched fatty acids containing 16 to 26 carbon atoms and iso C(29), iso C(31), and iso C(33) paraffins. l-Isoleucine-U-(14)C on the other hand produced labeled odd-numbered anteiso C(17) to C(27) fatty acids and anteiso C(30) and C(32) paraffins. Trichloroacetic acid inhibited the incorporation of isobutyrate into C(20) and higher fatty acids and paraffins without affecting the synthesis of the C(16) and C(18) fatty acids. Thus the very long branched fatty acids are biosynthetically related to the paraffins. In Senecio odoris leaves acetate-1-(14)C was incorporated into the paraffins (mainly n-C(31)) only in the epidermis although acetate was readily incorporated into fatty acids in the mesophyll tissue. Similarly only the epidermal tissue incorporated acetate into fatty acids longer than C(18) suggesting that the epidermis is the site of synthesis of both paraffins and the very long fatty acids. In broccoli leaves n-C(12) acid labeled with (14)C in the carboxyl carbon and (3)H in the methylene carbons was incorporated into C(29) paraffin without the loss of (14)C relative to (3)H. Since n-C(18) acid is known to be incorporated into the paraffin without loss of carboxyl carbon these results suggest that the condensation of C(12) acid with C(18) acid is not responsible for n-C(29) paraffin synthesis in this tissue. Thus all the experimental evidence thus far obtained strongly suggests that elongation of fatty acids followed by decarboxylation is the most likely pathway for paraffin biosynthesis in leaves.
Kolattukudy
P E
PE
Department of Biochemistry, Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504.