The activity of hybrid ribonuclease (ribonuclease H) has been determined in mononuclear blood cells (lymphocytes plus monocytes) from 23 normal individuals and cells (pool of immature granulocytes, metamyelocytes and lymphocytes) from 35 untreated acute and chronic myelogenous leukemia cases. It was found that in 86% of the leukemic samples the activity of ribonuclease H was above two standard deviations from the mean activity level drawn for the group of normal samples along the 0–100% substrate hydrolysis scale. The activity of the enzyme in leukemic cells correlated linearly with the DNA-synthesizing activity of the cells in vitro and in the examined CML cases it paralleled the inverse relationship of the incorporation of tritiated thymidine into DNA to the size of the pool of immature granulocytes. In one CML patient who received chemotherapy with Myleran, the activity of ribonuclease H, high at the initiation of drug therapy, was reduced to a normal level at remission, but increased again at the stage of subsequent relapse. These findings indicate that the levels of ribonuclease H in leukemic cells reflect the proliferative activity of the population in the cases of untreated myelogenous leukemias.

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