Biosynthesis of guanosine 5'-phosphate. I. Xanthosine 5'-phosphate as an intermediate.

The work of Greenberg (1) and Buchanan et al. (2) has established inosine 5’-phosphate as a key intermediate in the biosynthesis of hypoxanthine in pigeon liver. The recent indication of the enzymic synthesis of adenosine 5’-phosphate from IMP1 (3-5) shows a possible role of IMP as an intermediate in the biosynthesis of other purine nucleotides. In order to investigate the possibility of the enzymic formation of guanosine nucleotides from IMP a study of the metabolism of this substance in extracts of pigeon liver was carried out. It appeared probable that the introduction of an amino group in Position 2 of IMP would be preceded by an oxidation of this position to give a xanthine nucleotide. This hypothesis was further strengthened by the finding of Magasanik and Brooke (6) that a guanineless auxotroph of Aerobacter aerogenes accumulated xanthosine. When C14-labeled IMP was incubated with a crude extract of pigeon liver in the presence of oxidized DPN a new C4-labeled compound appeared that could be identified as XMP. Upon incubation of this substance with the crude extract together with adenosine triphosphate, Mg++ ions, and n-glutamine or n-glutamate, a compound identified as GMP was formed. Further experiments implicated n-glutamine rather than n-glutamate as the amino group donor in the amination of XMP to GMP. These results are summarized in Scheme I.