Purine mutants of mammalian cell lines: III. Control of purine biosynthesis in adenine phosphoribosyl transferase mutants of CHO cells

Spontaneous and mutagen-induced 2,6-diaminopurine-resistant mutants of Chinese hamster ovary (CHO-K1) cells were isolated. Such mutants fell into two classes: spontaneous and ethylmethane-sulfonate-induced mutants had approximately 5% wild-type adenine phosphoribosyl transferase (APRT) activity, whereas ICR-170G-induced mutants had barely detectable APRT activity. Since it has been reported that human hypoxanthine-guanine phosphoribosyl transferase (HGPRT) (Lesch-Nyhan syndrome) and APRT mutants over-produce purines, we examined the control and rate of purine biosynthesis in the Chinese hamster mutants. End product inhibition by adenine could not be demonstrated in such mutants, indicating that the active feedback inhibitor is a nucleotide rather than the free purine base, HGPRT activity was normal in all mutants examined except in one isolate. Purine biosynthesis as measured by the accumulation of the purine biosynthetic intermediate phosphoribosyl formylglycineamide was not elevated in the mutants as might have been predicted from work with Lesch-Nyhan cells. The data also suggest that our strain of CHO-K1 is physically or functionally haploid for the APRT locus.