Novel mutants of CHO cells resistant to adenosine analogs and containing biochemically altered form of adenosine kinase in cell extracts

Stable mutants which are approximately five- and eightfold resistant to an inosine analog, formycin B (Fomr) have been selected in a single-step from Chinese hamster ovary cells at a frequency of approximately 10−6. Cross-resistance studies with these mutants show that the Fomr mutants exhibit increased resistance to all adenosine analogs (N- and C-nucleosides) examined and, in accordance with their cross-resistance pattern, the mutants exhibited decreased cellular uptake and phosphorylation of formycin B and various adenosine analogs. In cell hybrids formed with sensitive cells, the drug-resistant phenotype of these mutants behaved recessively. However, unlike mutants resistant to adenosine analogs that have been obtained previously, which contain no measurable activity of adenosine kinase (AK) in cell extracts, the two Fomr mutants studied contained about 60 and 110% of the enzyme activity (compared to the parental cells) in their cell extracts. Biochemical studies with AK from the mutant cells show that in comparison to the wild-type enzyme, the mutant enzymes required much higher concentrations of the adenosine analog N7-(Δ2-isopentenyl) formycin A for similar inhibition of [3H]adenosine phosphorylation. These results indicate that AK from the Fomr mutants has lower affinity for phosphorylation of adenosine analogs in comparison to the enzyme from the parental cells. The genetic lesion in the Fomr mutants may thus be directly affecting the structural gene for AK.