Nicotinamide adenine dinucleotide activation of the esterase reaction of horse liver aldehyde dehydrogenase

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Abstract
The esterase reaction catalyzed by horse liver aldehyde dehydrogenase is activated with NAD(H) by factors of 2 under a Vmax assay and of 6.8 at low ester concentrations. Stopped-flow experiments suggested that an initial burst of 0.4 mol followed by a 2nd burst of 1 mol of nitrophenol/mol of tetrameric enzyme occurred in the absence of NAD, while the magnitudes increased to 2 and 4 mol/mol of enzyme in its presence. If the enzyme was incubated for 1 min with NAD, the burst phase was 4 mol/mol of enzyme. Nonlinear Lineweaver-Burk plots were found in the absence and presence of NAD, but incubation with NAD for 1 min abolished the biphasic response. Mg2+ ions activate the dehydrogenase reaction of horse liver aldehyde dehydrogenase. The metal neither increased the esterase reaction nor affected the NAD activation. The rate-limiting step for the esterase reaction was thought to be the formation of an acyl intermediate, while that for the dehydrogenase reaction was deacylation. Finding that a full burst exists for the esterase reaction in the presence of NAD shows that the deacylation step or product dissociation can become rate limiting. The major kinetic alteration produced by NAD is to increase the rate of acylation while not affecting deacylation. The presence of NAD appears to activate the attack of the active-site nucleophile on the carbonyl group of the substrate.