Cell fusion-induced mouse neuroblastomas HPRT revertants with variant enzyme and elevated HPRT protein levels

Fusion of 6-thioguanine-resistant mouse neuroblastoma to HeLa whole and minicells generated neuroblastoma HPRT revertants in addition to true cell hybrids. All revertants contained HPRT with decreased electrophoretic mobility and heat stability relative to wild-type mouse enzyme. Revertant HPRT expression was dependent on continuous HAT selection. Quantitative immunoadsorption experiments showed that revertants with low HPRT specific activity had wild-type levels of HPRT protein while a revertant with high apparent activity (NBR4) contained elevated levels of variant protein. HPRT extracted from NBR4 had decreased affinity of both hypoxanthine and PRPP relative to wild type. Evidence is presented that HPRT elevation is dependent on the reversion process itself.