Studies of the enzymatic deamination of cytosine arabinoside—II: Properties of the deaminase of human liver

A sensitive, radioisotopic assay for the pyrimidine nucleoside deaminase present in human liver was developed. Uracil arabinoside was the only product detected after incubation of cytosine arabinoside with a crude homogenate of human liver. The deaminase was stable at elevated temperatures and after prolonged storage under refrigeration. No cofactor requirements were demonstrated, but enzyme activity appeared to depend upon the presence of reduced sulfhydryl groups. The K m value for cytosine arabinoside was 1.2–1.6 × 10 −4M. No direct mechanism for regulating deaminase activity was detected.