The origin of the sulfur atom in thiamine.

The mode of biosynthesis of the thiazole moiety of thiamine, 4-methyl-5beta-hydroxyethyl thiazole (MHET) was studied using Salmonella typhimurium as test organism. It was shown by isotope incorporation experiments, that the sulfur atom, but not carbon-3, of cysteine is incorporated into MHET, indicating a separation of the sulfur atom of cysteine from the carbon chain during incorporation. Isotope competition experiments revealed that the incorporation of [35S]cysteine is not significantly diluted by the presence of methionine, homocysteine, and glutathione. No incorporation of label from [14C]glutamate and [14C]formate was observed, leaving the origin of the five-carbon unit still in doubt.