Concerning Enzymic Reactions in Heavy Water. II. Deuterium and the Hydrolysis of Starch. ∗

One of us 1 called attention to the contradictory results reported by writers who have investigated various influences of heavy water upon living organisms. It was suggested that studies of numerous biochemical reactions in vitro, supplementing investigations upon complex metabolic processes such as respiration, growth, motion, reproduction, etc., in entire organisms, should indicate possible mechanisms involved. Experiments described in the above-mentioned report showed definitely that no retardation of various enzymic reaction rates occurred, but that on the contrary, certain of such processes, notably the hydrolysis of starch by amylase from the crystalline style of the mussel Mytilus californianus, were slightly accelerated in high concentrations of D₂O. Steacie 2 reports that the inversion of cane sugar by saccharase was retarded by 25% in concentrated heavy water, buffered at pH 4.6, but that the breakdown of the glucoside salicin by emulsin was accelerated to an equal degree in the presence of the pure isotope, buffered at pH (pD?) 4.5.

Hornel 3 cites the work of Moelwyn-Hughes, who found that the catalytic influence of acid in the hydrolysis of cane sugar was greater in D₂O than in ordinary water; also the investigations of Schwartz, who observed that both methyl and ethyl acetates were hydrolyzed more rapidly by 50% in acidic solutions of heavy water than in light water under similar conditions. Hornel's own work on the rate of acid hydrolysis of methyl acetate showed that the catalytic coefficients (assuming that complete dissociation of sulfuric acid took place in both kinds of water) were in the ratio, K_D3O+/K_H3O+ = 1.86 at 15°, and 1.68 at 25°.

While, in the earlier work of the senior author, it seemed that the rate of production of maltose from starch was at least slightly enhanced in 99% D₂O, it was observed (by the achromic point method) that there was an unmistakable increase in the rate of the first conversion, i. e., that of starch to erythrodextrin, in the same solutions.