Identification of the reactive cysteine residue in human placenta aldose reductase

Modification of human placental aldose reductase by iodoacetate (IAA) led to a mol/mol binding of IAA, a 40% decrease in the k_cat, a 3-5-fold increase in the K_m,NADPH and K_{m,glyceraldehyde} and a 600-fold increase in the K_i,sorbinil; determined at pH 6.0. NADPH and 2′-monophosphoadenosine 5′-diphosphoribose but neither glyceraldehyde nor sorbinil, prevented carboxymethylation-induced changes. Cleavage of [¹⁴C]IAA-modified enzyme by trypsin resulted in two radiolabeled peptides: Val-297 to Lys-307 and Val-297 to Phe-315. In both these peptides Cys-298 was the only radiolabeled residue. It is suggested that Cys-298 regulates the kinetic and inhibition properties of the enzyme, but does not participate in catalysis.