The kinetics and regulation of nucleoside phosphorylation by highly purified human deoxycytidine kinase from leukemic lymphoblasts were studied. The phosphorylation of purine nucleosides by this enzyme showed sensitivity to the endogenous inhibitors dCTP and UDP three times greater than the phosphorylation of dCyd. Examination of nucleotide pools in human T and B lymphoblasts disclosed that the levels of dCTP and UDP in these cells were sufficient to regulate kinase activity. The enhanced sensitivity of the kinase to dCTP and UDP was related to its reduced ability to interact with purine nucleosides. Comparison of the phosphorylation kinetics for pyrimidine and purine dideoxynucleosides used in antiviral therapy showed that the purine nucleosides were at least 50-fold less efficient as enzyme substrates. These results suggest that the phosphorylation of pharmacologically active purine nucleosides by deoxycytidine kinase is regulated by cellular nucleotide pools.
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