Although the structures of mammalian cytosolic and mitochondrial ALDH have been determined, several differences, mainly functional, between these two 70% identical isozymes remain unexplained. A major difference is the differential effect of Mg2+ ions that inhibits the cytosolic and activates the mitochondrial isozyme. Here, we have investigated the effect of Mg2+ ions on each individual kinetic step of ALDH1 and ALDH2. The metal ions were found not to affect either acylation or hydride transfer for either isozyme. The lack of a Mg2+ ion effect on hydride transfer was further demonstrated with an E399Q mutant of ALDH1 whose rate-limiting step had been changed from NADH dissociation to hydride transfer. The other steps, however, were affected by Mg2+ ions for both isozymes. The metal ions inhibited NADH dissociation, the rate-limiting step for ALDH1, and enhanced deacylation, the rate-limiting step for ALDH2. Our results indicated that, with both isozymes, Mg2+ ions tightened the binding of NADH, and by binding to the coenzyme, they increased the nucleophilicity of the nucleophile Cys302. The inhibition of ALDH1 and activation of ALDH2 at pH 7.4 are due to their different rate-limiting steps. Mg2+ ions affected similarly the NADH activation of the esterase reaction for both isozymes. In contrast, the metal ions affected only the NAD+ activation of ALDH1. This latter finding and other features described here can be rationalized on the basis of the known three-dimensional structures of the isozymes.
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