A biochemical genetic study of the role of specific nucleoside kinases in deoxyadenosine phosphorylation by cultured human cells

Deoxyadenosine is a toxic substrate of adenosine deaminase and accumulates as deoxyATP in cells of patients deficient in that enzyme. However, it is unclear which of the deoxyadenosine phosphorylating enzymatic activities is responsible for this accumulation of deoxyATP in humans. From a human lymphoblastoid cell line, WI-L2, mutants deficient in various nucleoside kinase enzymes have been isolated. A line of cells lacking deoxycytidine kinase had less than 10% of the deoxyadenosine phosphorylating activity of the wild type cells when measured in cell free extracts. However, in growth rate experiments, this cell line was as sensitive as the wild type cell line to the toxic effects of deoxyadenosine. Furthermore, these mutant cells could accumulate deoxyATP from exogenous deoxyadenosine as effectively as the wild type cells. A second line of cells (107A), deficient in adenosine kinase activity, had virtually wild type levels of deoxyadenosine phosphorylating activity when measured in extracts. However, the cells of this line were 3-fold less sensitive than wild type cells to deoxyadenosine and accumulated far less intracellular deoxyATP from exogenous deoxyadenosine. A cell line lacking both adenosine kinase and deoxycytidine kinase activities exhibited 3- to 5-fold greater resistance to the growth inhibitory and toxic effects of deoxyadenosine than did the adenosine kinase deficient parent and did not accumulate detectable deoxyATP from exogenous deoxyadenosine. Although deoxyadenosine phosphorylating activity in cell extracts appears to be associated with the deoxycytidine kinase enzyme, it is clear that the physiologically important deoxyadenosine phosphorylating activity is associated with the adenosine kinase enzyme. Thus, the roles of specific nucleoside kinases in the metabolism of deoxyadenosine in human cells have been delineated by these biochemical genetic studies.