A purification procedure has been developed for the cytosolic aldehyde dehydrogenase of Saccharomyces cerevisiae that yields homogeneous enzyme. The enzyme seems to be a tetramer of identical 58 kDa subunits. The enzyme reaction is strongly stimulated by Mg2+ at low NADP+ concentrations but there is no absolute requirement for bivalent cations. The kinetics of the reaction have been studied in the presence and absence of MgCl2. NADP+ binding studies of the quenching of protein fluorescence in the presence and absence of MgCl2 show that the effect of Mg2+ is to increase the affinity of the enzyme for NADP+ by approx. 100-fold. NADP+ binding causes a slow conformational change in the enzyme and converts the enzyme from the inactive or low-activity form in which it is isolated into the fully active form. This conformational change seems to explain the marked lag-phases seen in enzyme assays. The enzyme is strongly inhibited by disulfiram and pyridoxal 5-phosphate.

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