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Purine Nucleotide Pyrophosphorylases in 6-Mercaptopurine-sensitive and -resistant Human Leukemias^*

Jack D. Davidson and Thorne S. Winter

( Clinical Pharmacology and Experimental Therapeutics Service, Medicine Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland)

The present study was undertaken to determine whether a loss of the enzyme activity responsible for converting 6-mercaptopurine, hypoxanthine, and guanine to their ribonucleotides correlates with resistance to 6-mercaptopurine in human leukemic leukocytes. Six normal and 26 leukemic patients were studied. Fifteen of the leukemic patients were felt to be clinically resistant to 6-mercaptopurine. Only one of these showed a significant reduction in inosinic-guanylic pyrophosphorylase activity. All other leukemic patients showed enzyme activities similar to those of normal patients, except that the leukemic patients had an adenylic pyrophosphorylase activity 2–3 x that of normal leukocytes. Thus, loss of inosinic-guanylic pyrophosphorylase did occur in human leukemia, but it did not explain most cases of resistance to 6-mercaptopurine in the leukemic patients studied.

^* Presented in part before the 54th annual meeting of the American Association for Cancer Research, May, 1963, Toronto, Canada, and published in abstract form in Proc. Am. Assoc. Cancer Research, 4: 72, 1963.

Present address: Peter Bent Brigham Hospital, Boston, Mass.

Received 7/31/63.

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