Thiamin Biosynthesis in Saccharomyces cerevisiae: Origin of the Pyrimidine Unit

Article abstract of DOI:10.1021/ja00261a024

Radioactivity from <¹⁴C>formate, from D-<1-¹⁴C>-, D-<2-¹⁴C>-, and D-<6-³H,6-¹⁴C>glucose, from D-<1-¹⁴C>fructose and from <1,3-¹⁴C>- and <2-¹⁴C>glycerol is incorporated nonrandomly into the pyrimidine moiety of thiamin in Saccharomyces cerevisiae.The observed incorporation pattern, established by a new chemical degradation, leads to the inference that there are two biosynthetic pathways to the pyrimidine moiety of thiamin in yeast.In the major pathway, formate is the precursor of C-4 of the pyrimidine nucleus, while hexose metabolites serve as the source of the remaining five carbon atoms of the pyrimidine unit.In the minor pathway, it is C-2 and not C-4 of the pyrimidine nucleus which is derived from formate, while C-4,-5 orginates from carbohydrate.The source of C-2', C-5', and C-6 of the pyrimidine unit in this minor pathway remains unknown.Activity from 14C>citrulline and <¹⁴C>urea is not incorporated into thiamin.