Nucleoside deaminase activity during the recovery of X‐irradiated, bone marrow‐protected mice

Increased activity of the enzyme nucleoside deaminase is found in the spleens of mice protected from lethal whole‐body irradiation by transfusion of isogenic bone marrow cells. As few as 10,000 nucleated bone marrow cells produce a significant elevation in enzyme level. This increase precedes any detectable increase in hemoglobin synthesis and is temporarily associated with an increase in protein synthesis during the early stages of recovery. Hypertransfusion of the irradiated, protected mice eliminates most of the stimulus to erythroid differentiation and prevents the increase in nucleoside deaminase. The present studies indicate that this enzyme is present in a differentiated red cell precursor not found in the normal stem cell population.