Altered 5-phosphoribosyl 1-pyrophosphate amidotransferase activity in 6-thioguanine-resistant HL-60 human acute promyelocytic leukemia cells.

A novel mechanism of resistance to the antileukemic agent 6-thioguanine (TGua) was demonstrated in a clone (TGuo-30-2) derived from HL-60 human acute promyelocytic leukemia cells. The clone was isolated by prescreening mutagenized HL-60 cells in hypoxanthine-amethopterin-thymidine medium, followed by selection with 6-thioguanosine. TGuo-30-2 cells were cross-resistant to TGua and beta-2'-deoxythioguanosine. TGuo-30-2 cells exhibited a marked decrease in the capacity to accumulate intracellular TGua nucleotides after treatment with TGua. The decrease in accumulation was not caused by a defect in transport, a lack or alteration of hypoxanthine-guanine phosphoribosyltransferase activity, or enhanced degradation of TGua nucleotides but appeared to be due to the maintenance of a lowered level of 5-phosphoribosyl 1-pyrophosphate (PRPP) in the resistant variant, which corresponded to 20% of the parental concentration. Despite the decrease in PRPP levels, incorporation of glycine into purine nucleotides was greater in TGuo-30-2 than in parental cells. Measurement of PRPP amidotransferase activity using cell homogenates revealed altered kinetics for the enzyme from TGuo-30-2 cells, which included significant loss of sensitivity to feedback inhibition by 6-thioguanosine 5'-phosphate and greater catalytic activity at low concentrations of PRPP.