Effects of liquid holding by L-ethionine on the SCE frequency induced by nitrogen mustard in human lymphocyte cultures.

The highest SCE frequency induced by HN2 in PHA-stimulated human lymphocyte cultures is obtained when cells are treated immediately before the S phase, about 27-30 h after PHA stimulation. When HN2 treatment occurs during the G1 phase, the induced SCE frequency is inversely related to the duration between treatment and the onset of the S phase, suggesting that removal of the SCE-inducing lesion occurs during G1. High concentrations of L-ethionine (15-30 mM) inhibit cell-cycle progression in G1. The SCE frequency induced by treatment with HN2 (0.5 muM) during liquid holding is always lower than the SCE frequency induced by treatment with HN2 after release of the cells from liquid holding. This result indicates that SCE-inducing lesions caused by HN2 are removed during the cell-cycle delay. Cultures treated with HN2 after release from 2-12 h of liquid holding in late G1 show a higher SCE frequency than non-arrested, HN2-treated cultures. This effect is likely due to an enrichment of cells in S after release from liquid holding.