FATTY ACID-ELONGATING ACTIVITY IN RAPIDLY EXPANDING LEEK EPIDERMIS

A microsomal fatty acid elongase activity measured in epidermis of rapidly expanding leek (Allium porrum L.) was 10-fold higher in specific activity than preparations from store-bought reek. These preparations elongated acyl chains effectively using endogenous or supplied primers. Elongation of C20:0 was specifically inhibited by 2 mu M cerulenin, and labeling experiments with [H-3]cerulenin labeled two polypeptides (65 and 88 kD). ATP was required for maximal elongase activity in expanding leaves but was lost in nonexpanding tissues. Both [C-14]stearoyl-coenzyme A (CoA) and [C-14]stearate were maximally elongated in the presence of ATE. Addition of fully reduced CoA, however, inhibited [C-14]stearate elongation, suggesting that stearoyl-CoA synthesis was not a prerequisite for elongation. Furthermore, microsomes preincubated with [C-14]stearoyl-CoA plus ATP resulted in loss of radiolabel from the acyl-CoA pool without a corresponding loss in elongating activity. The lack of correlation between elongating activity and the label retained in the putative acyl-CoA substrate pool suggests that acyl-CoAs may not be the immediate precursors for elongation and that ATP plays a critical, yet undefined, role in the elongation process. We propose that an ATP-dependent elongating activity may generate the long-chain fatty acids required for wax biosynthesis.