Conversion of 5-aminoimidazole ribotide to the pyrimidine of thiamin in enterobacteria: study of the pathway with specifically labeled samples of riboside.

Samples of 5-amino-1-(beta-D-ribofuranosyl)imidazole labeled with 13C at position C-1 or C-2 of the ribose part or with 15N at position N-3 or amino of the imidazole part were prepared by chemical synthesis. The incorporation of label from these samples into the pyrimidine of thiamin biosynthesized by a mutant strain of Salmonella typhimurium was studied by GC-MS. The results show clearly that in enterobacteria the methyl carbon atom and the N-1 nitrogen atom of one molecule of thiamin pyrimidine derive from the same molecule of 5-aminoimidazole ribotide. More specifically, the methyl carbon atom comes from the carbon C-2' of the ribose part and the nitrogen N-1 from nitrogen N-3 of the imidazole; furthermore, the amino nitrogen of the aminoimidazole becomes the amino nitrogen of the pyrimidine.