The Pathways of Thiamine Biosynthesis

Incorporation of 4-amino-5-hydroxymethylpyrimidine into thiamine by microorganisms

One possible route for the biosynthesis of the (4-amino-2-methyl-5-pyrimidyl)-methyl moiety of thiamine would involve the formation of a methyl group on the demethylated pyrimidine, 4-amino-5-hydroxymethylpyrimidine, before its incorporation into thiamine. This possibility was tested by preparing the 4-amino-5-hydroxymethylpyrimidine and feeding it to Escherichia coli. Bacillus subtilis, and Saccharomyces cerevisiae. Analysis of the thiamine produced by these organisms showed that 4-amino-5-hydroxymethylpyrimidine was readily incorporated into thiamine without the addition of a methyl group, and no evidence was found for the conversion of this pyrimidine into normal methylated pyrimidine. Substitution of the demethylated thiamine for thiamine had no effect on the growth rate or the yield of E. coli cells. Complete substitution of the thiamine with the (4-amino-5-pyrimidyl)-methyl moiety was possible in an E. coli pur I mutant. The extent of incorporation of the demethylated pyrimidine decreased in some organisms and increased in others by the addition of adenine to the growth medium; this difference led to a simple test to separate organisms that use 5-aminoimidazole ribonucleotide for the biosynthesis of thiamine pyrimidine from those that do not.

Production of a precursor to the pyrimidine moiety of thiamine

The supernatant fluid from cultures of Escherichia coli W-11, a pur E mutant, prevented the inhibition of growth of E. coli B in a medium containing adenine or adenosine. Adenine inhibition was prevented more readily than adenosine inhibition. More than 90% of the biological activity of the supernatant fluid was recovered in the anionic fraction after treatment with Dowex-50 (NH4+). The cationic fraction, containing large amounts of 5-aminoimidazole ribonucleoside (AIRS), did not prevent adenine inhibition. The W-11 supernatant fluid was shown by bioautography to contain only one compound that prevented adenine inhibition. Proliferating and non-proliferating cultures produced only one compound that prevented adenine inhibition. The compound was shown to be an intermediate (int-1) in the biosynthesis of the pyrimidine moiety of thiamine, Int-1 was stable during sterilization at 121 C for 15 min, during concentration by either flask evaporation or lyophilization, and after storage for several days at 4 C or at -- 20 C. Int-1 was distinguishable from other known derivatives or intermediates of the pyrimidine moiety. A scheme is presented that illustrates the proposed relationship between int-1 and the synthesis of thiamine.