ARGINYL RESIDUES ARE INVOLVED IN ACYL-COA-BINDING TO THE ELONGASE FROM ETIOLATED LEEK SEEDLINGS

The C-18:0-CoA elongase from etiolated leek seedling microsomes was in activated by treatment with phenylglyoxal, a reagent which specificall y modifies arginyl residues. In the presence of 20 mM phenylglyoxal, 9 5% of the C-18:0-CoA elongation was inhibited. The condensation and de hydration reactions of the overall elongation were totally inhibited, whereas enoyl-CoA reductase activity was diminished by 75%, but the na ture of the final elongation product was unchanged. Phenylglyoxal did not modify the C-18:0-CoA partition between membrane and aqueous compa rtments; moreover, [1-C-14]phenylglyoxal labeling experiments showed a covalent binding of the inhibitor to membrane proteins. The ability o f several substrates to prevent the inactivation by phenylglyoxal was investigated. NADH and NADPH had no effect. CoA led to a 75% protectio n, and the incorporation of[ C-14]phenylglyoxal was strongly affected by 10 mM CoA. The acyl chain length of the acyl-CoAs played also a cru cial role in preventing the binding of phenylglyoxal. The maximal prev ention of phenylglyoxal inhibition was obtained with C-18:0-CoA. This suggests that arginyl residues could be present in the vicinity of the acyl-CoA binding site of the subunits of C-18:0-CoA elongase. (C) 199 8 Elsevier Science B,V.