Adenosine‐resistant chinese hamster fibroblast variants with hyperactive adenosine‐deaminase: An analysis of the protection against exogenous adenosine afforded by increased activity of the deamination pathway

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Abstract
The activity of purine salvage and interconversion enzymes was examined in two sublines of Chinese hamster cells–RA11 and RA41–isolated on the basis of their resistance to adenosine concentrations toxic to wild‐type CCL39 cells. Adenosine deaminase (ADA) activity was found to be two times higher in RA11 and three times higher in RA41 than in CCL39. Inhibition of ADA activity by coformycin reduced the level of adenosine resistance but did not restore wild‐type sensitivity, indicating that a second defect contributes to the adenosine‐resistant phenotype of these variants; evidence was indeed obtained for the presence in both lines of additional alterations protecting them against the lethal depletion of phosphoribosylpyrophosphate (Ishii and Green, 1973) imposed by adenosine to wild‐type cells. To gain better insight into the influence of ADA hyperactivity on adenosine resistance, a procedure was developed for the specific isolation of variants with increased levels of ADA activity. Cell lines with 3–5 times and then 100–500 times the wild‐type ADA activity were stepwise recovered. These investigations confirmed that amplification of ADA can efficiently contribute in protecting cells against high concentrations of exogenous adenosine. The variants isolated by this procedure again manifested, in addition to amplification of ADA activity, another alteration decreasing their sensitivity to adenosine. A possible mechanism accounting for the frequent isolation of variants that coexpress ADA hyper‐activity and a second defect contributing protection against adenosine toxicity are considered.

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