Experiments on the biosynthesis of thiamine. Isotope-competition experiments with [35S]sulphate in cultures of Saccharomyces cerevisiae

Abstract

Yeast grown on a defined medium containing [35 S]sulphate produces [35S]thiamine of high specific activity in small amounts. Yeast has been grown in a defined medium containing [35S]sulphate and a non-radio-active sulphur-containing substance as sole sulphur sources. These 2 substances may, or may not, compete in contributing the sulphur required for incorporation into the thiazole ring of thiamine; the specific activity of the biosynthesized thiamine compared with controls gives a measure of the efficiency of the non-radioactive sulphur-containing compound under test as a donor of sulphur for the biosynthesis of thiamine. The following compounds are utilized by the yeast, in the presence of sulphate, as sources of sulphur for the biosynthesis of thiamine very efficiently: 5-(2-hydroxyethyl)-4-methylthiazole; efficiently: S-adenosylmethionine, sodium sulphate, glutathione, metnionine, thiazolidine-4-carboxylic acid, 5''-deoxy-5''-methylthioadenosine; fairly efficiently: [alpha]-amino-[beta]-(4-methylthiazol-5-yl)propionic acid, sodium sulphide, thioacetic acid; inefficiently: cysteine, homocysteine, 4-amino-2-methyl-5-thioformamidomethylpyrimidine, methanethiol and ethane-thiol. Other compounds tested which do not appear to compete significantly with sulphate are: homocystine, thiourea, biotin, thioglycollic acid, djenkolic acid, S-methylthiuronium chloride, 4-methylthiazole, (4-methylthiazol-5-yl)acetic acid, benzylpenicillin, taurine, l:2-dimer-captopropan-3-ol (BAL), l-mercaptopropan-2:3-diol, 1-acetyl-l-mer-captopropan-3-ol. Of the compounds examined, none[long dash]apart from 5-(2-hydroxyethyl)-4-methylthiazole[long dash]could be recognized as a major precursor of the thiazolium ring in thiamine.