Evidence for two-step binding of reduced nicotinamide-adenine dinucleotide to aldehyde dehydrogenase

The displacement of NADH from cytoplasmic aldehyde dehydrogenase (EC 1.2.1.3) from sheep liver was studied by using NAD+, 1,10-phenanthroline, ADP-ribose, deamino-NAD+ and pyridine-3-aldehyde-adenine dinucleotide as displacing agents, by following the decrease in fluorescence as a function of time. The data obtained could be fitted by assuming two first-order processes were occurring, a faster process with an apparent rate constant of 0.85 +/- 0.20 s-1 and a relative amplitude of 60 +/- 10% and a slower process with an apparent rate constant of 0.20 +/- 0.05 s-1 and a relative amplitude of 40 +/- 10% (except for pyridine-3-aldehyde-adenine dinucleotide, where the apparent rate constant for the slow process was 0.05 s-1). The displacement rates did not change significantly when the pH was varied from 6.0 to 9.0. Kinetic data are also reported for the dependence of the rate of binding of NADH to the enzyme on the total concentration of NADH. Detailed arguments are presented based on the isolation and purification procedures, the equilibrium coenzyme-binding studies and the kinetic data, which lead to the following model for the release of NADH from the enzyme: (formula: see article). The parameters that best fit the data are: k + 1 = 0.2 s-1; k - 1 = 0.05 s-1; k + 2 = 0.8 s-1 and k - 2 = 5 X 10(5)litre-mol-1-s-1. The slow phase of the NADH release is similar to the steady-state turnover number for substrates such as acetaldehyde and propionaldehyde and appears to contribute significantly to the limitation of the steady-state rate.