THE ACTIVITY OF LIVER ALCOHOL DEHYDROGENASE WITH NICOTINAMIDE-ADENINE DINUCLEOTIDE PHOSPHATE AS COENZYME.

1. The separation of nucleotide impurities from commercial NADP preparations by chromatography is described. All the preparations studied contained 0.1-0.2% of NAD. 2. The activity of pure crystalline liver alcohol dehydrogenase with NADP as coenzyme has been confirmed. Initial-rate data are reported for the reaction at pH 6.0 and 7.0 with ethanol and acetaldehyde as substrates. With NADP and NADPH(2) of high purity, the maximal specific rates were similar to those obtained with NAD and NADH(2), but the Michaelis constants for the former coenzymes were much greater than those for the latter. 3. The oxidation of ethanol by NADP is greatly inhibited by NADH(2), and this accounts for low values of certain initial-rate parameters obtained with commercial NADP preparations containing NAD. The kinetics of the inhibition are consistent with competitive inhibition in a compulsory-order mechanism. 4. Initial-rate data with NAD and NADPH(2) do not conform to the requirements of the mechanism proposed by Theorell & Chance (1951), in contrast with results previously obtained with NAD and NADH(2). The possibility that the deviations are due to competing nucleotide impurity in the oxidized coenzyme cannot be excluded. The data show that the enzyme reacts more slowly with, and has a smaller affinity for, NADP and NADPH(2) than NAD and NADH(2). 5. Phosphate behaves as a competitive inhibitor towards NADP.