The Direct Precursor of the Pyrimidine Moiety of Thiamin Is Not Urocanic Acid but Histidine in Saccharomyces cerevisiae

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The biosynthetic route of the pyrimidine moiety of thiamin is different in prokaryotes and eukaryotes. In prokaryotes, the pyrimidine moiety is synthesized from aminoimidazole ribonucleotide, an intermediate of purine biosynthesis, while in eukaryotes, we have reported that the N-1, C-2, and N-3 atoms of the imidazole ring of histidine are incorporated into N-3, C-4, and the amino group attached to the C-4 atoms of the pyrimidine moiety, respectively, as a unit; the rest of the atoms of the pyrimidine moiety originate from pyridoxine as a unit. It has been reported that urocanic acid, the deaminated compound of histidine, is the direct precursor of the pyrimidine moiety. In the present report, we have investigated whether histidine or urocanic acid is the direct precursor of the pyrimidine moiety in Saccharomyces cerevisiae, using tracer experiments with 1) ^<13>C-formate and urocanic acid, 2) ^<15>N-NH_4Cl and urocanic acid, 3) ^<15>N-NH_4Cl and histidine, and 4) ^<13>C-histidine and urocanic acid. The GC-MS analysis revealed that the incorporation of the ^<15>N atom of ^<15>NH_4Cl was not affected by the presence of urocanic acid, although it was affected by histidine, and the incorporation of ^<13>C-histidine was not affected by the presence of urocanic acid. These results confirm that histidine is the direct precursor of the pyrimidine moiety of thiamin in S. cerevisiae.
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