The course of the transient kinetics for the equine liver alcohol dehydrogenase catalyzed reduction of chromophoric aromatic aldehydes by reduced nicotinamide-adenine dinucleotide has been studied in the pH region 8-10 using rapid-mixing stopped-flow spectrophotometric instrumentation. Two kinetic processes, well separated in rate, are observed for the conversion of reactants into products under conditions of excess enzyme. The amplitude of the opticaldensity change accompanying the rapid initial step corresponds to the conversion of exactly one-half of the limiting substrate (aldehyde or reduced nicotinamide-adenine dinucleotide) to product (alcohol or oxidized nicotinamide-adenine dinucleotide). The slower second process accounts for the conversion of the remaining material into product and is found to have a first-order rate numerically similar to the steady-state turnover number. When reduced nicotinamideadenine dinucleotide and aldehyde are present in excess of enzyme, there occurs a rapid initial process, prior to the attainment of the steady state, with an optical-density change corresponding to the conversion of substrates (aldehyde T J.he nicotinamide-adenine dinucleotide requiring enzyme horse liver alcohol dehydrogenase (EC 1.1.1.1) catalyzes the interconversion of aldehydes and alcohols (eq 1) with equilibrium thermodynamically favored in the direction shown for most substrates at accessible pH (Sund and Theorell, 1962).

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The pH dependence below pH 6 of kca,/K, for the papaya peptidase A catalyzed and papain-catalyzed hydrolyses of the p-nitrophenyl esters of hippuric acid, benzyl-Abbreviations: pHNP, hippuric acid p-nitrophenyl ester; Z-Gly-ONp, benzyloxycarbonylglycine p-nitrophenyl ester; Z-Lys-ONp, benzyloxycarbonyl-~-1ysine p-nitrophenyl ester; DTT, DL-dithiothreitol; EDTA, ethylenediaminetetraacetic acid.

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