Abstract
The C-1, C-2 and C-3 of ribose and N-1, C-2 and N-3 of histidine imidazole ring are precursors of the C-6, C-5 and C-5' and N-3, C-4 and amino-N attached to C-4 of the pyrimidine moiety of thiamin in Saccharomyces cerevisiae, respectively. The C-5, C-4, and side chain carbons of histidine are derived from ribose. The possibility that the C-1, C-2 and C-3 of ribose are incorporated into the pyrimidine via C-5, C-4 and side chain, β carbon of histidine remains to be elucidate. The distribution of radioactivity of the pyrimidines labeled with L-[U-^<14>C] histidine and L-[2-^<14>C] histidine revealed that C-2 of histidine is incorporated into the pyrimidine and the other carbons are not incorporated. It has been reported that formate is incorporated into C-4 of the pyrimidine. However, Grue-Sφrensen et al. reported that formate is incorporated into the C-r and C-2 of the pyrimidine and they supposed the two routes, the major and the minor, for the biosynthesis of the pyrimidine. The tracer experiment with [^<13>C] formate and the inhibition by L-histidine of the incorporation of [^<13>C] formate and following GC-MS analysis revealed that formate is incorporated into C-4 of the pyrimidine via C-2 of histidine and is not incorporated into C-2. The ^<15>NH_4Cl added to the medium was incorporated effectively into N-3 and amino-N of the pyrimidine in the presence of pyridoxine. Addition of histidinol or histidine to the medium decreased the incorporation of N^<15> into the pyrimidine. However, inhibition of ^<15>N incorporation into the pyrimidine by histidinol was low compared with that of histidine. These results strongly suggest that the direct precursor of N-3, C-4 and amino-N atom of pyrimidine is histidine.
