Hydrogen isotope effect in the oxidation in vivo of methionine labeled in the methyl group.

The retarding effect of deuterium on biological processes has been a matter of interest to many investigators. However, the early studies in this connection were concerned mainly with the biological effects of deuterium when it was present in excess amounts in the water of the medium. Little work appears to have been done on the effect of deuterium on the rates of biological reactions in which carbon-hydrogen bonds labeled with deuterium are broken during necessary metabolic changes. In 1936, two relevant reports appeared in the literature. Erlenmeyer, Schoenauer, and Stillmann (1) studied the chemical and biochemical dehydrogenation of cr,or’-dideuteriosuccinic acid. They found that, when hydrogen is removed from the deuteriosuccinic acid by the use of selenium dioxide, the fumaric acid formed had about 10 per cent more deuterium in the (Y ,a’ positions than the initial compound. In other words, Erlenmeyer et al. had demonstrated an isotope effect on a chemical reaction when deuterium is present in a reactant. In biochemical experiments reported in the same paper, in which a crude preparation of succinic dehydrogenase from beef muscle was used, the hydrogen was transferred to methylene blue in a Thunberg tube in about 30 per cent less time from normal succinic than from the deuteriosuccinic acid, while, in the Warburg manometer, the rate of oxygen consumption with the same enzyme system was about 40 per cent less with the deuteriosuccinic acid than with the normal substrate. It was shown by Sonderhoff and Thomas (2) that the rate of oxygen uptake by yeast suspensions in the presence of trideuterioacetate in Warburg vessels was about 80 per cent of that in the presence of normal acetate. The same degree of retardation in the rate of oxygen consumption was observed by these workers in the dehydrogenation of (Y ,(r’,p ,P’-tetradeutjcriosurcinatc catalyzed by an enzyme preparation from horse heart. l‘hc kinetics of the enzymatic oxidation of deuterated succinic acaid was