ESSENTIAL ARGININE RESIDUES IN ISOPRENYLCYSTEINE PROTEIN CARBOXYL METHYLTRANSFERASE

We used specific amino acid modifying reagents to characterize the iso prenylcysteine carboxyl methyltransferase in kidney membranes. The enz yme was inactivated by reagents specific for arginine, histidine, cyst eine, and tryptophan residues. Protection by the product and inhibitor S-adenosyl-L-homocysteine was observed for arginine modification by p henylglyoxal and tryptophan modification by N-bromosuccinimide. We foc used on modification by phenylglyoxal, a highly specific modifier of a rginine residues. The inactivation of methyltransferase by phenylglyox al follows pseudo-first-order kinetics and the order of the reaction, n, with respect to phenylglyoxal was 1.2. The inactivation increased w ith the alkalinity of the preincubation medium and was maximal at pH 1 0. Kinetic analysis showed that the K-m for S-adenosyl-L-methionine is not significantly affected by treatment with phenylglyoxal but that t he V-max is reduced. p-Hydroxyphenylglyoxal, a more hydrophilic deriva tive of phenylglyoxal, was a less potent inactivator of methyltransfer ase than phenylglyoxal, suggesting that arginine residues modified are in a hydrophobic environment. The methyltransferase is protected from phenylglyoxal modification by S-adenosyl-L-homocysteine but not S-ade nosyl-L-methionine, sinefungin, N-acetyl-S-famesyl-L-cysteine, or farn esylthioacetate. The arginine residue modified may thus be located eit her at the active site or at another additional binding site for S-ade nosyl-L-homocysteine. These results indicate that arginine residues ar e essential for the enzymatic activity of isoprenylcysteine carboxyl m ethyltransferase.