ITP pyrophosphohydrolase and IDP phosphohydrolase in rat tissue

The existence of a nucleoside triphosphate pyrophosphohydrolase specific for ITP has been demonstrated in the cytosol fraction of a variety of rat tissues. The enzyme, stable to moderate heat treatment, was present in erythrocytes as well as brain, heart, kidney, liver, lung, muscle, ovaries, spleen, testes and thymus. The specific activity of the enzyme ranges from 26 to 150 mumoles/min/g protein. In addition, evidence is given for a heat labile nucleoside diphosphate (IDP) phosphohydrolase present in most rat tissues, and particularly high in the adrenal (137 mumoles/min/g protein). An "ITP-IMP cycle" is proposed as a rgulating mechanism for intracellular levels of ATP.