Studies on nucleoside deaminase. Increase in activity in HeLa cell cultures caused by cytosine arabinoside

Cytosine arabinoside (ara C) is metabolized via two enzymatic pathways: by phosphorylation to the nucleoside triphosphate arabinosyl CTP or deamination to the inert metabolite uracil arabinoside. Resistance to cytosine arabinoside had been attributed to a decreased synthesis of arabinosyl CTP, the active inhibitor of DNA synthesis. Steuart and Burke suggested the increased deamination of cytosine arabinoside as an alternative mechanism of resistance in patients treated with this drug. In the present study the mechanism for the effect of cytosine arabinoside on nucleoside deaminase activity was investigated in cell culture. HeLa cells grown in medium with added cytosine arabinoside had a 4 fold increase in nucleoside deaminase activity. The increase is dependent on the concentration and time of exposure to cytosine arabinoside. The levels of 4 control enzymes were unaffected by this agent. No difference was found between the properties of nucleoside deaminase purified from cells grown in the presence of cytosine arabinoside and in controls. The early appearance of an increase in nucleoside deaminase in the absence of cell killing made selection of a mutant population improbable. Cycloheximide did not prevent the elevation in nucleoside deaminase indicating that the mechanism, unlike 'classical induction', may not be dependent on the synthesis of a new protein. These studies show that the presence of cytosine arabinoside in tissue culture leads to an increased activity of nucleoside deaminase which could result in accelerated drug degradation. This process may provide a mechanism for resistance to the antimetabolite.