Properties of cytosol 5′-nucleotidase and its role in purine nucleotide metabolism

Abstract 5′-Nucleotidase which was found first in chicken liver and found to be located in cytosol was purified and characterized. This enzyme is termed cytosol 5′-nucleotidase for convenience. Some properties of this enzyme are summarized in Table 7. 7 . Molecular weight: 205,000 Molecular weight of subunit: 51,000 (four identical subunits) Sedimentation coefficient: 9.7S Stokes radius: 5.1 nm K m for IMP: 0.31 mM (in Tris/maleate buffer, pH 6.5), 1.4 mM (in imidazole/HCl buffer, pH 6.5) Optimal pH: 6.5 Substrate specificity: IMP > GMP > dGMP > XMP > AMP > UMP > CMP Allosteric activator: ATP > dATP > ADP > GTP Allosteric inhibitor: Pi Metal ion requirement: Mg 2+ > Co 2+ > Ni 2+ ⩾ Mn 2+ Subcellular distribution: cytosol Adaptive increase of the activity by high protein diet The specific activity of cytosol 5′-nucleotidase in chicken liver cytosol is higher than that in rat liver cytosol. In response to a high protein diet the activity of cytosol 5′-nucleotidase in chicken liver increased, concurrently with those of purine nucleoside phosphorylase and xanthine dehydrogenase. Of the three enzymes, the activity of cytosol 5′-nucleotidase reached a maximum most rapidly. In rat liver, the activities of these three enzymes did not increase on administration of a high protein diet. From these results the principal physiological function of the cytosol 5′-nucleotidase is assumed to be dephosphorylation of IMP as the first step in the pathway of uric acid formation from IMP, which is important in the elimination of nitrogen of amino acids and protein in a uricotelic animal. An allosteric property of this enzyme is considered to be important for control of adenine and guanine nucleotide pools, especially in connection with the biosynthetic activity of the purine nucleotides in uricotelic animals.