INDUCTION OF HPRT(-) MUTANTS IN CHINESE-HAMSTER V79 CELLS AFTER HEAVY-ION EXPOSURE

The induction of resistance to 6-thioguanine by heavy ion exposure was investigated with various accelerated ions (oxygen-uranium) up to lin ear energy transfer (LET) values of about 15 000 keV/mu m. Survival cu rves are exponential with fluence; mutation induction shows a linear d ependence. Cross-sections (sigma(i): inactivation, sigma(m): mutation) were derived from the respective slopes. Generally, sigma(i) rises ov er the whole LET range, but separates into different declining curves for single ions with LET values above 200 keV/mu m. Similar behaviour is seen for sigma(m). The new SIS facility at GSI, Darmstadt, makes it possible to study the effects of ions with the same LET but very diff erent energies and track structures. Experiments using nickel and oxyg en ions (up to 400 MeV/u) showed that inactivation cross-sections do n ot depend very much on track structure, i.e. similar values are found with different ions at the same LET. This is not the case for mutation induction, where very energetic ions display considerably smaller ind uction cross-sections compared with low-energy ions of identical LET. Preliminary analyses using the polymerase chain reaction (PCR) demonst rate that even heavy ions cause ''small alterations'' (small deletions or base changes). The proportion of the total deletions seems to incr ease with LET.