Cytosolic 5’-nucleotidase II (cN-II) is a 6-hydroxypurine-specific nucleotidase, most active with IMP and dIMP. This is a widespread enzyme. Human cN-II gene shows strikingly high conservation with at least 89% homology to the other vertebrates’ cN-II genes. Open reading frames with similarities to human cN-II of 60-67% were found in genomes of some arthropods. Physiological roles of cN-II have not been elucidated yet. Regarding several vertebrates (mammals, birds, fish and reptiles), its activity in various tissues has been assessed by immunotitration to eliminate the interference of non-specific phosphatases and other nucleotidases. Among the animals examined, cN-II activity is markedly high in the liver of birds, snakes, a crocodile and lizards. These are constitutively uricotelic animals, which are characterized by the absence of carbamoyl-phosphate synthase I and presence of glutamine synthetase in mitochondria. In these animals, the de novo synthesis of IMP and its dephosphorylation constitute the pathway leading to excretion of nitrogen from excess amino acids in the form of uric acid. The activity of cN-II in chicken liver increases two-fold in response to high protein diet. This was not observed in the livers of the rats, which are ureotelic. The high preference to IMP of cN-II, its high activity in the liver of constitutively uricotelic animals, its increase in response to a high protein diet observed in chicken and its ubiquitous distribution strongly suggest that a role of cN-II is regulation of intracellular purine nucleotide levels through dephosphorylation of excess IMP.
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