Regulation of cytosol 5'-nucleotidase by adenylate energy charge

In the physiological range of the adenylate energy charge in liver (0.7-0.9), th rate of AMP-hydrolysis catalysed by rat liver cytosol 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) increased sharply with decreasing energy charge. In addition, a decrease in the concentration of Pi caused marked acceleration of the AMP-hydrolysing activity over the physiological range of adenylate energy charge. These responses seem to serve to protect the cells against a metabolic stress which could result from sudden utilization of ATP by removal of AMP. The AMP-hydrolysing activity of this enzyme decreased sharply as the size of the adenine nucleotide pool decreased in the physiological range. This effect may be a self-limiting response to prevent excess depletion of the pool. IMP-hydrolysing activity of this enzyme increased with increasing adenylate energy charge. But no marked response to its variation within the physiological range was observed. On the basis of the data obtained in this study, the IMP-hydrolysing activity of the cytosol 5'-nucleotidase in rat liver cells seems to be comparable to that of AMP deaminase reaction, but the AMP-hydrolysing activity was estimated to be less than 10% of AMP deaminase reaction at energy charge value of about 0.7. This strongly suggests that the AMP leads to IMP leads to inosine pathway is more significant that the AMP leads to adenosine leads to inosine pathway in rat liver.