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76 ISOLATION AND CHARACTERIZATION OF HELA CELL MUTANTS RESISTANT TO AN ADENINE ANALOG 4-CARBAMOYLIMIDAZOLIUM-5′-OLATE (CIO)

It is known that CIO is metabolized into bredinin-5′-monophosphate (Brd-MP) by adenine phosphoribosyltransferase (APRT) and that this metabolite is a specific inhibitor of IMP dehydrogenase. We have reported that mouse cell mutants resistant to the cytotoxicity of CIO have lost the enzyme activity (Cancer Res., 42, 4210, 1982). Recently, we isolated the same type of resistant mutants from mutagenized HeLa cells. In contrast, the HeLa cell mutants had a normal level of APRT and were cross-resistant to bredinin, a ribosylated form of CIO, but not to mycophenolic acid and virazole. These mutants showed a normal level of IMP dehydrogenase and the sensitivity to Brd-MP of the parental and mutant cell enzymes were similar. The mutants gradually reduced its resistance after passage without the drug for more than 2 months. We also found that in 105 g supernatants prepared from the mutants there existed a 28 k protein by SDS polyacrylamide gel electrophoresis and its amount was proportional to the degree of the resistance. These results suggest that this protein should be involved in the resistance observed in the HeLa cell mutants.