HORSE LIVER ALCOHOL DEHYDROGENASE-CATALYZED OXIDATION OF ALDEHYDES - DISMUTATION PRECEDES NET PRODUCTION OF REDUCED NICOTINAMIDE ADENINE-DINUCLEOTIDE

The oxidation of aldehydes by horse liver alcohol dehydrogenase (HL-ADH) is more complex than previously recognized. At low enzyme concentrations and/or high aldehyde concentrations, a pronounced lag in the assay progress curve is observed when the reaction is monitored for NADH production at 340 nm. When the progress of the reaction is followed by H-1 NMR spectroscopy, rapid dismutation of the aldehyde substrate into the corresponding acid and alcohol is observed during the lag phase. Steady-state production of NADH commences only after aldehyde concentrations drop below 5% of their initial value; thereafter, NADH production occurs with continuous adjustment of the equilibrium between aldehyde, alcohol, NADH, and NAD+. The steady-state NADH production exhibits normal Michaelis-Menten kinetics and is in accord with earlier studies using much higher enzyme concentrations where no lag phase was reported. These results establish that the ability of HL-ADH to oxidize aldehydes is much greater than previously thought. The relationship between aldehyde dismutase and aldehyde dehydrogenase activities of HL-ADH is discussed.