Antagonists of nucleic acid derivatives. II. Reversal studies with substances structurally related to thymine.

One of the major problems of antimetabolite studies is the evaluation of the rale of “secondary reversing agents” (1). In a system in which there exists a competitive relationship between a metabolite and its antimetabolite, the effect of a secondary reversing agent is to increase the ratio of antimetabolite to metabolite required to produce a given inhibition; the secondary reversing agent cannot, however, replace the metabolite. One of the better known examples is the effect of methionine and purines on the sulfanilamide-p-aminobenzoic acid antagonism (2-5). Kohn and Harris (3) have suggested that methionine acts as a reversing agent in this system because the synthesis of methionine involves p-aminobenzoic acid as a catalyst. This type of explanation has been employed by Shive and associates (5, 6) and applied to the identification of the products of enzyme systems of intact cells. A variety of activities for Lactobacillus casei is found among pyrimidines structurally related to thymine. Several 5-methylpyrimidines have thymine-like activity (7, S), while a number of uracils, substituted in the 5 position with groups other than alkyl, are inhibitory (7, 8). Among these inhibitors, there exist at least four distinct types which can be differentiated on the basis of reversal experiments. During the course of these reversal studies, the chemical specificity of secondary reversing action was examined in a number of systems and found to be so highly unspecific as to render this type of action essentially uninterpretable in terms of specific enzymic activities, prosthetic groups, and products (9).