Deoxynucleotidase Activity in Rat Liver and Certain Tumors2

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Abstract
The deoxynucleotidase activity in the liver tissue of the adult rat is shown by enzymes distinct from 5′-nucleotidase and resides primarily in the cytoplasmic structures. The “mitochondrial” deoxynucleotidase has an activity optimum at pH 5.5 and dephosphorylates 5′-deoxycytidylic acid (dCMP), 5′-deoxyadenylic acid (dAMP), and 5′-deoxyguanylic acid (dGMP); the ergastoplasm-bound deoxynucleotidase acts on thymidylic acid (TMP) and 5′-deoxyuridylic acid (dUMP) with optima at pH 6.0 and 8.0, respectively. The mitochondrial deoxynucleotidase is low in embryonal liver cells, increases in the postnatal life, is high in adult rat liver, but decreases during regeneration. Its course of activity during the embryonic and postnatal period is reversed to that of dCMP deaminase. On the other hand, dCMPase is low in proliferating tissues, e.g., lymphatic organs. During experimental carcinogenesis in rat liver and resulting tumors, the scarcity of cytoplasmic structures depletes the dCMPase activity. In such tumors as the Novikoff rat tumor, mouse Sarcoma 180, and Ehrlich ascites tumor cells, dCMPase is low. Inorganic phosphate inhibits mitochondrial deoxynucleotidase, which can be extracted by water from fresh and acetone-powdered tissue and is found in the “albumin” fraction of these extracts. It is concluded that the deoxynucleotides may be the natural substrates of certain “acid phosphatases” in liver and that these deoxynucleotidases may regulate the content of deoxynucleotides in the cell and participate in the regulation of cell proliferation.