Observations on the biosynthesis of thiamine in yeast.

1. Methods are described for the isolation of radioactively pure thiamine from yeast and its degradation on a small scale to its cyclic components. 2. A degradation of the pyrimidine ring and a thin-layer method for the separation of thiamine, its derivatives and pyrimidine and thiazole residues are described. 3. [14C]Formate is more effectively incorporated into the pyrimidine residue than into the thiazole residue, whereas the reverse is true with l-[Me-14C]methionine. 4. Experiments with [Me-14C,35S]methionine demonstrate that methionine provides an intact unit for the biosynthesis of the thiazole ring. 5. [6-14C]Orotic acid is insignificantly incorporated into the pyrimidine residue of thiamine. 6. Experiments with [1-14C]- and [2-14C]-acetate indicate that it is incorporated as a unit into the thiazole residue, but that only C-2 is incorporated into the pyrimidine residue. 7. l-[U-14C]Alanine is also effectively incorporated into the thiazole residue. 8. These results are discussed in relation to possible pathways of biosynthesis of the two ring components of the thiamine molecule.