Interaction of thiamine and pyridoxine in Neurospora. II. Competition between pyridoxine and the pyrimidine precursor of thiamine

It has been shown that pyridoxine inhibits thiamine biosynthesis. This inhibition is overcome efficiently and noncompetitively by thiamine, competitively by pyrimidine, and inefficiently and noncompetitively by thiazole. In view of these findings, the fact that 4-deoxypyridoxine (a known antimetabolite of pyridoxine) can act in an identical fashion, the great structural similarity of these pyridine compounds to pyrimidine, as well as findings published elsewhere (1–3, 6), it is concluded that pyridoxine competitively prevents pyrimidine utilization, probably by acting as a competitive substrate for the enzyme which couples pyrimidine (or pyridoxine) to thiazole.