Purine mutants of mammalian cell lines. II. Identification of a phosphoribosylpyrophosphate amidotransferase-deficient mutant of Chinese hamster lung cells.

A class of purine auxotrophs blocked early in the purine biosynthetic pathway was examined. The inability of these mutants to accumulate formylglycinamide ribotide (FGAR) in the presence of azaserine suggested that one or more of the first three enzymes of the pathway were either missing or defective. By direct enzyme assay, phosphoribosylpyrophosphate (PRPP) amidotransferase (E.C. 2.4.2.14) was found to be absent in extracts of mutant cells. Thus these cells were unable to convert PRPP to phosphoribosylamine (PRA). By reacting ribose 5-phosphate with ammonium ions, PRA was generated nonenzymatically in the incubation mixture, thus enabling us to test for the presence of the two enzymes required to convert PRA to FGAR. It was demonstrated that sufficient amounts of these enzymes, phosphoribosylglycineamide synthetase (E.C. 6.3.1.3) and phosphoribosylglycineamide formyltransferase (E.C. 2.1.2.2), were present in mutant extracts to allow synthesis of FGAR to occur once PRA was so provided.