Thymidine Catabolism by Normal and Leukemic Human Leukocytes*

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Abstract
Human leukocyte homogenates inhibit the uptake of trltiated thymidine by chronic myelocytic leukemic cells in short term culture. This inhibition is the result of thymidine phosphorylase activity and the degree of inhibition correlates with the level of enzyme activity. Normal leukocytes contain significantly higher levels of thymidine phosphorylase activity than leukemic leukocytes. Leukocytes from patients with acute leukemia have the lowest levels. Enzyme values were higher in leukocytes from patients with chronic myelocytic leukemia in remission than in leukocytes from patients in relapse. Thymidine phosphorylase activity is also found in the medium after intact leukocytes are incubated in phosphate buffer. In addition to demonstrating thymidine phosphorylase activity converting thymidine to thy mine, leukocytes of patients with chronic myelocytic leukemia in vitro can reduce thymine to a compound corresponding chromato-graphically to dihydrothymine and/or [beta]-ureidoisobutyric acid.