The PRPP concentrations, PRPP formation, and phosphorylation of 6-mercaptopurine in leukocyte suspensions and homogenates prepared from leukemic patients were studied. The rate of phosphorylation was 3 orders of magnitude higher in leukocyte homogenates (329 ± 56 nmoles/3 × 107 cells/hr) than in leukocyte suspensions (0.01 to 0.52 nmole/3 × 107 cells/hr). Leukocytes contained less than 0.03 nmole PRPP/3 × 107 cells, which was less than the amount of thioinosinic acid formed in the leukocyte suspensions. Therefore, PRPP must be synthesized during the incubation. The amount of PRPP formed in leukocytes from 10 patients with acute leukemia varied from 5.2 to 38.0 nmoles/3 × 107 cells/hr. The cells which had higher PRPP formation also had higher thioinosinic acid formation. Thus, the availability of PRPP may play a key role in thioinosinic acid formation in human leukemic leukocytes.


Supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education and the Ministry of Health and Welfare of Japan.

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