Cutin Biosynthesis in Vicia faba Leaves: Effect of Age

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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.

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.

Metabolism of red beet slices I. Effects of washing

The changes in relative participation of pathways of glucose catabolism in red beet slices during washing have been examined using specifically (14)C labeled glucoses. Washing of these slices brings about an increase in participation of the pentose phosphate pathway. The composition of the washing medium influences slightly the extent of change in pathway participation. The activity level of certain enzymes participating in the initial stages of glucose catabolism has been measured in fresh and washed beet slices. Fresh slices which barely metabolized gluconate were found to have very little 6-phosphogluconate dehydrogenase activity. Washing brings about a dramatic increase in 6-phosphogluconate dehydrogenase activity and this increase was accompanied by a marked increase in the ability of the slices to metabolize gluconate. In red beet slices the TPNH generated via pentose phosphate pathway appears to be utilized for biosynthetic reductions rather than as respiratory substrate.

Metabolism of Red Beet Slices II. Effects of Aging

The rate of respiration of red beet slices increased 3- to 4-fold when the slices were aged in a moist atmosphere for 18 to 24 hours. The respiration of fresh slices was severely inhibited by 8 x 10(-5)m HCN but as slices aged the sensitivity of respiration to HCN fell rapidly. The presence of HCN stimulated the respiration of slices after 8 to 12 hours of aging.The relative participation of the pentose phosphate pathway in the glucose catabolism increased very little, if any, during aging of slices prepared from beets in the growing phase although the total metabolic rate seems to be greatly increased. However, data obtained from slices prepared from dormant beets indicated a possible increase in pentose phosphate pathway participation in glucose catabolism upon aging. The activity of 6-phosphogluconate dehydrogenase increased very markedly during aging and a corresponding marked increase in the ability of the tissue to metabolize gluconate was observed. It is concluded that to associate the aging process of storage tissue slices with a specific increase in a particular pathway is not justifiable.