Arginyl residues are involved in acyl-CoA binding to the elongase from etiolated leek seedlings

Abstract

The C 18:0-CoA elongase from etiolated leek seedling microsomes was inactivated by treatment with phenylglyoxal, a reagent which specifically modifies arginyl residues. In the presence of 20 mM phenylglyoxal, 95% of the C 18:0-CoA elongation was inhibited. The condensation and dehydration reactions of the overall elongation were totally inhibited, whereas enoyl-CoA reductase activity was diminished by 75%, but the nature of the final elongation product was unchanged. Phenylglyoxal did not modify the C 18:0-CoA partition between membrane and aqueous compartments; moreover, [1- 14C]phenylglyoxal labeling experiments showed a covalent binding of the inhibitor to membrane proteins. The ability of several substrates to prevent the inactivation by phenylglyoxal was investigated. NADH and NADPH had no effect. CoA led to a 75% protection, and the incorporation of [ 14C]phenylglyoxal was strongly affected by 10 mM CoA. The acyl chain length of the acyl-CoAs played also a crucial role in preventing the binding of phenylglyoxal. The maximal prevention 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.