Different timing of increases in activities of four X-chromosome linked enzymes during the cell cycle of synchronized human lymphoblasts

Mutagen-induced diploid human lymphoblast variants containing altered hypoxanthine guanine phosphoribosyl transferase

The human lymphoblast line MGL8 was treated with HAT and subsequently "mutagenized" with EMS (200 microgram/ml) to give 15% survival, and 6-thioguanine-resistant cells were selected by cloning in soft agarose containing the drug (1 microgram/ml). Eighteen sublines of independently derived resistant clones were isolated and studied in detail. One subline had a low residual HGPRT activity of about 1% of the parental cells. The HGPRT of this subline had a higher Km for PRPP, was more sensitive to heat, and was degraded faster by trypsin than the enzyme in extracts of MGL8 cells. This resistant subline and three others contained CRM levels of 1--38%, compared to the wild-type, so they probably represent true structural mutants of the HGPRT gene. All the variants maintained the karyotype of the parental line (46, XY, 6p-).