On the mechanism of pyrimidine metabolism by yeasts.

Knowledge of the mechanisms involved in pyrimidine metabolism is fundamental to the biochemistry of the nucleic acids. Cerecedo, Emerson, and Stekol (l-7) pioneered this field in their work with dogs. Their experiments, conducted by feeding the in vitro oxidation products of uracil and the estimation of excreted urea, culminated in the proposal of a mechanism for pyrimidine metabolism (8). More recently, isotopically labeled cytosine was administered to rats and found to be catabolized partly to urea (9. The present investigation extends the previous work on the metabolism of nucleic acid derivatives by yeasts (10) to provide more complete coverage of pyrimidines and related compounds. The oxidative mechanism proposed by Cerecedo and his collaborators for the degradation of uracil into urea was found inoperative in yeasts. Unlike soil bacteria (ll-13), the yeasts were incapable of oxidizing uracil and thymine into barbituric acid and 5-methylbarbituric acid, respectively. On the basis of growth studies with yeasts the following metabolic pathway for the assimilat,ion of pyrimidines by yeasts is suggested: cytosine + uracil + hydrouracil + hydroorotic acid + urea.