Xanthine ribotide

Amino-4-imidazole carboxamide ribotide, a precursor only two steps removed (formylation and cycli-zation) from inosinic acid, can be synthesized by the direct condensation of the imidazole with 5-phosphori-bosyl pyrophosphate. The enzyme catalyzing this reaction was purified from an acetone powder of beef liver. The same enzyme (AMP pyrophosphorylase) catalyzes the condensation of adenine, guanine, and hypoxan-thine. Nucleoside phosphorylase is an enzyme that catalyzes the formation of a ribose nucleoside from a purine base and ribose-1-phosphate. Guanine, adenine, xanthine, hypoxanthine, 2,6-diaminopurine, and aminoimidazole carboxamide are known to be converted to their respective nucleosides by such a mechanism. In the presence of a specific kinase and ATP, the nucleoside is then phosphorylated to the corresponding nucleotide. 

The inhibitory effect of allopurinol on the first three steps of purine biosynthesis in Neurospora crassa cultures could be due 1. to the formation of allopurin-ol-l-ribonucleotide (by HGPRTase 1,19 nMol/mg protein/ hr in our Neurospora crassa cultures) 2. due to an oxipurinol-7-ribonucleotide effect, namely the formation of oxipurinol (by xanthine oxidase) and then oxipurinol-7-ribonucleotide (by OPRTase) and 3. by formation of allopurinol-1-ribotide through allopurin-ol-1-riboside (by a purine nucleoside phosphorylase) to allopurinol-1-ribonucleotide (by a nucleoside kinase). This latter pathway seems rather improbable because we could not demonstrate an effect on purine biosynthesis by direct addition of allopurinol-1-ribonucleoside to the incubation medium. Allopurinol-1-ribonucleotide inhibits significantly at higher concentrations. This is to explain if there is free access of the ribonucleotide form through the cell membrane (at least in these high concentrations) or if there is conversion to the ribonucleoside (which then enters the cell) by a nucleotidase located on the cell membrane. This latter mechanism however seems more improbable bcause there was no effect by allopurinol-1-ribonucleoside (which excludes the possibility for subsequent formation of allopurinol-1 -ribonucleotide by a nucleoside kinase, at least... 

---