Effect of sodium chloride on changing the rate-limiting step in the human placental choline acetyltransferase reaction.

Abstract

The kinetic constants, Km and Vmax, for the choline acetyltransferase reaction were determined for choline and eight choline analogs under conditions of high (0.3 M) and low (approximately 0.01 M) sodium chloride. At high sodium chloride, the maximal velocities of the different substrates varied over 27-fold, while at low sodium chloride, less than a 5-fold variation was observed. Dead-end inhibition studies using acetylaminocholine as the inhibitor showed that under conditions of high sodium chloride, inhibition changes from noncompetitive to competitive as the reactivity of the substrate decreases. Under conditions of low sodium chloride, acetylaminocholine inhibition is nonlinear and noncompetitive with respect to all substrates tested. These results suggest that increased ionic strength increases the rate of coenzyme A dissociation from the enzyme. The rate-determining step of the reaction can be ternary complex interconversion, coenzyme A release, or both, depending on the ionic strength and the substrate employed.