The concentrations of enzyme sites in cells are usually higher than the concentrations of cognate intermediary metabolites. Therefore metabolic pathways or substantial segments of pathways may proceed by the direct transfer of metabolites from one enzyme site to the next by means of enzyme-enzyme complex formation. This mechanism of metabolite transfer differs from that usually assumed where dissociation and random diffusion of metabolite through the aqueous environment is responsible for the transfer to the next enzyme site. Since the direct transfer mechanism does not involve the aqueous environment, the energetics of metabolite interconversion can differ from expectations based on aqueous solution data. Evidence is summarized suggesting that metabolite is transformed and transferred with equal facility everywhere in the direct transfer pathway.

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).

Vol. 70 ex,/3-Di-(^>-methoxyphenyl) -valeric Acid (V).-A mixture of 12.1 g. of the solid nitrile, 4 g. of sodium hydroxide, 8 ml. of water and 75 ml. of ethylene glycol was refluxed for thirty-six hours.16•16 An equal volume of water was added and the solution filtered while hot. Acidification of the cooled solution with dilute hydrochloric acid yielded a mixture of isomeric acids. Recrystallization from 95% ethyl alcohol gave 4.65 g. of white product (A), m. p.

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