Biological and biochemical characterisation of purine analogue resistant clones of V79 Chinese hamster cells

Purine analogue resistant clones have been selected from the closely related Chinese hamster lines V79A and V79S. Clones were of either spontaneous origin or induced by EMS or ultraviolet light. The majority of clones selected in 8-azaguanine showed stable cross resistance to 6-thioguanine. Clones derived from V79A and selected for 6-thioguanine resistance were cross resistant to 8-azaguanine: however a group of 6-thioguanine resistant mutants selected from V79S cells were 8-azaguanine sensitive. All clones except two were unable to grow in HAT medium. The two exceptions were 8-azaguanine resistant, showed partial sensitivity to 6-thioguanine, and also differed in other biochemical characteristics. HGPRT activity was measurable in extracts of all clones under standard conditions. In many clones, HGPRT activity increased as the hypoxanthine concentration was reduced. Whole cell uptake of [14C] hypoxanthine was low in all cases examined and was not modified by incubation in the presence of amethopterin. The heat sensitivity and electrophoretic mobility of HGPRT in extracts of some clones was compared to that in wild-type extracts. All clones tested except one, which was consistently HAT positive, showed enhanced heat sensitivity and reduced electrophoretic mobility. None of the mutants reverted spontaneously at detectable frequency but some could be induced to revert by EMS. The presence of measurable enzyme with altered properties in all clones suggests that these revertable drug resistant clones represent missense mutants.