The biosynthesis of thiamine. 1. Enzymatic formation of thiamine and phosphate esters of the pyrimidine moiety of thiamine.

The biosynthesis of thiamine appears to be accomplished with the initial formation of the pyrimidine and thiazole moieties by independent biosynthetic pathways, followed by a final step in which 2-methyl-4-amino-5-hydroxymethylpyrimidine and 4methyl-5-(/3-hydroxyethyl) thiazole are joined together to give thiamine. This conclusion was indicated originally from the observations that certain thiamine-requiring microorganisms can utilize the combination of hydroxymethylpyrimidiner plus thiazole in place of thiamine, a fact which suggests that these organisms cannot synthesize either hydroxymethylpyrimidine or thiazole, but are able to form thiamine from these two compounds. The nutritional and genetic studies on a series of thiamine-requiring Neurspora mutants by Tatum and Bell (I) and Harris (2) tended to support the above conclusion, although some anomalous results obtained by these workers led them to postulate the existence in Neurospora of a second pathway whereby hydroxymethylpyrimidine is condensed with a thiazole precursor followed by the conversion of the unknown condensation product to thiamine (2). The first relevant enzymatic evidence was provided by Harris and Yavit (3), who reported that extracts of bakers’ yeast could catalyze the formation of thiamine from hydroxymethylpyrimidine, thiazole, and ATP in the presence of magnesium ions. They also reported that a synthetic monophosphate ester of hydroxymethylpyrimidine (pyrimidine-P) could replace the hydroxymethylpyrimidine and ATP requirements. The latter observation suggested that pyrimidine-P is an intermediate which reacts with thiazole to form thiamine and that this phosphate ester is formed enzymatically from hydroxymethylpyrimidine and ATP. However, investigations by Leder (4) indicated that the reaction is more complex than that proposed by Harris and Yavit. He found that yeast preparations could convert pyrimidine-P and thiazole to thiamine only if ATP were also supplied. Three independent groups of investigators then presented evi-