Biosynthesis of riboflavin. Structure of the purine precursor and origin of the ribityl side chain.

We studied the incorporation of 14C-labeled guanosine into riboflavin under conditions precluding the metabolic conversion of guanosine compounds to free guanine. For this purpose we isolated a mutant BM 2 of Salmonella typhimurium deficient in the enzymes IMP dehydrogenase, purine nucleoside phosphorylase, and purine nucleotide pyrophosphorylase. The mutant incorporated [ribose-14C]guanosine into riboflavin and GMP without dilution. The isolated compounds were exclusively labeled in the ribityl and ribosyl side chain, respectively. AMP and CMP were not labeled. [2-14C]Guanosine was incorporated into riboflavin and GMP without dilution. The isolated compounds were exclusively labeled in the isoalloxazine and guanine moiety, respectively. AMP and CMP were again unlabeled. We conclude that the ribose moiety of proffered guanosine is directly converted to the ribityl moiety of riboflavin. Thus the biosynthesis of the vitamin begins at the level of a guanosine compound. Guanine, ribose, ribitol, and the respective phosphates are not direct precursors of the vitamin.