Cytidine aminohydrolase activity in intact cultured transformed cells

Cultures of several cell lines convert cytidine, if present in their medium, to uridine. The reaction is rapid, being virtually complete within one hour. The enzymatic activity is that of cytidine aminohydrolase (EC 3.5.4.5). The activity is exhibited by the intact cell — substrate and products being found in the medium bathing the cells. The activity should be taken into account in studies involving cytidine, and other 6-amino pyrimidine nuclecsides. Of nine transformed cell lines, and nine primary and secondary cell strains screened for the presence of cytidine aminohydrolase activity, six cultures were positive. All the positive cultures were heteroploid transformed lines. No diploid strain cultures were positive, and two near diploid mouse lymphoid neoplasms were negative. PPLO contamination was not detected in the positive cultures. Within the limited series of cell cultures screened in this study, there appears to be a correlation between heteroploidy and the ability of the intact cultures to deaminate cytidine in excess of the cell requirement for the precursor.