Kinetics of sheep-liver cytoplasmic aldehyde dehydrogenase

1. Sheep liver cytoplasmic aldehyde dehydrogenase showed little pH dependence of V or kcat. Some buffer anion effects were noted. 2. The oxidation of aldehydes at pH 7.6 was quantitative but irreversible. The initial velocity data indicated a sequential mechanism for the addition of substrates. Inhibition by NADH and the product analogue 2-bromo-2-phenylacetic acid, together with the known tight binding of NADH to the free enzyme, indicated an ordered mechanism with NAD+ as leading substrate. 3. Values for the rate of binding and dissociation of NAD+ were obtained from the steady-state data. The values obtained were virtually identical with those which could be calculated from the data for the horse liver cytoplasmic enzyme. Close similarities are in general apparent for the horse and sheep liver cytoplasmic enzymes and with other tissue aldehyde dehydrogenases. 4. Apparent substrate activation was observed with high concentrations of both acetaldehyde and propionaldehyde, a limiting value of 0.25s-1 being obtained for kcat. No isotope effect was observed on V using [1-2H]propionaldehyde as substrate suggesting that NADH release might be rate-limiting in the steady-state. 5. The implications of the non-linear steady-state behaviour are discussed.