The incorporation of the pyrimidine ring of adenine into the isoalloxazine ring of riboflavin.

In the biogenesis of riboflavin by the yeast, Eremothecium ashbyii, carbon atoms constituting the pyrimidine ring of adenine were incorporated into the 6,7-dimethylisoalloxazine ring of riboflavin with much greater efficiency than was the 8 position of the purine ring (1). It remained to determine whether the carbon atoms which constitute the pyrimidine ring of the purine are incorporated directly into the pyrimidine portion of the isoalloxazine ring. This portion of the molecule has since been degraded, the specific activities of the carbon atoms have been compared, and the conclusion has been reached that adenine serves as a precursor in the biogenesis of riboflavin through the contribution of an intact pyrimidine ring. Thus it is that, while a purine may serve as a precursor in the biogenesis of the isoalloxazine ring of riboflavin, it is actually the pyrimidine portion of the ring which is incorporated.