Intracellular activity of HPRTCape Town: Purine uptake and growth of cultured cells in selective media

The low activity of the human variant HPRT_{Cape Town} is associated with substrate inhibition by hypoxanthine and guaninein vitro. The intracellular activity of this variant was investigated by studying the relative uptake or radiolabelled purine nucleotide precursors and the growth in selective media of EBvirus-transformed lymphoblasts prepared from the proband (TK). These cells incorporated less than 10% of the [¹⁴C]hypoxanthine and [¹⁴C]guanine of the control cells; while their purinede novo synthesis was accelerated 8-fold. In selective media the HPRT_CapeTown cells grew in a similar manner to HPRT-ve cells. These results indicate that if substrate inhibition is responsible for the low intracellular activity of HPRT_{Cape Town}, the concentration of either hypoxanthine or guanine in the vicinity of the active site of the enzyme must be greater than theK_i(app) for these substrates, 118 and 28 µmol L^–1 respectively. Evidence is presented that the intracellular concentration of guanine, but not hypoxanthine, is well in excess of theK_i(app) in cultured lymphoblasts.