Mechanism of action of NaCl on maize alcohol dehydrogenase and phosphoenolpyruvate carboxylase

Abstract

Alcohol dehydrogenase (ADH) and phosphoenolpyruvate carboxylase (PEPC) are inhibited by NaCl. PEPC is more sensitive to NaCl than ADH. Chloride ions act toward maize ADH as an inhibitor which is competitive with respect to the substrate, i.e. ethanol; the inhibition with respect to acetaldehyde is of the mixed, competitive-uncompetitive type. The inhibition constants for Cl- ions and ADH are of the order of 10-1 mol l-1. The binding site for Cl- ions constitutes most likely the central Zn atom present in the active center of ADH. NaCl also inhibits two PEPC isozymes (PEPC I and PEPC II) isolated from corn leaves. Isozyme PEPC I is inhibited more strongly than isozyme PEPC II. Sodium chloride acts on both PEPC isozymes as a competitive inhibitor with respect to the substrate, i.e. phosphoenolpyruvate, the inhibition constants being of the order of 10-2 mol l-1. NaCl also acts as an allosteric effector of PEPC I, binding not only to the active center of the enzyme but also to the allosteric site of the protein molecule of PEPC I. The reversible inhibition of PEPC isozymes is converted into irreversible inactivation of the isozymes. The irreversible inhibition is probably a result of a change in the quaternary structure in the protein molecule of PEPC.