DECREASE IN GAMMA-GLUTAMYL-TRANSFERASE ACTIVITY IN RAT TYPE-II CELLS EXPOSED IN-VITRO TO HYPEROXIA - EFFECTS OF THE 21-AMINOSTEROID U-74389G

Although the effect of hyperoxia on antioxidant enzymes is well known, the effect of subtoxic levels of hyperoxia on gamma-glutamyltransfera se (gamma-GT), involved in the degradation, and uptake of extracellula r GSH for intracellular GSH synthesis, is unknown. The aim of the stud y was to investigate (1) the effects of in vitro hyperoxia on gamma-GT activity of type II cells and (2) the effects of the lazaroid U-74389 G and N-acetylcysteine (NAG) on the hyperoxia-induced changes in gamma -GT and antioxidant enzyme activities. At 48 h after isolation, rat ty pe II cells were exposed for 2 days to air, 60% O-2 or 85% O-2, with o r without 30 mu M U-74389G or 100 mu M NAc. After the exposure, the ce lls were harvested and assayed for superoxide dismutase (SOD), glutath ione peroxidase (GPx), gamma-GT activity, and GSH levels. In another s eries of experiments 85% O-2-exposed cells, with or without U-74389G, were used or Northern blotting of gamma-GT mRNA. Exposure to GO qb O-2 decreased gamma-GT and GSH by -47 and -34%, respectively, while SOD a nd GPx activities remained unchanged. After 85% O-2-exposure gamma-GT decreased by -55%, SOD and GPx increased try +55 end +87%, respectivel y, while GSH decreased by -35%. NAC treatment decreased gamma-GT activ ity by -42% in the air-exposed cells. After 60% O-2, U-74389G led to s ignificantly higher gamma-GT (+117%) and GSH (+26%) while NAC only led to higher GSH (+28%) compared to the oxygen-exposed cells not treated with NAC or U-74389G. After 85% O-2 U-74389G increased gamma-GT, SOD, and GSH by +72, +58, and +68%, respectively, while NAC only increased SOD (+49%) and GSH (+26%) compared to the oxygen-exposed cells not tr eated with NAC or U-74389G. The 85% O-2 exposure, with or without U-74 389G, had no effect on gamma-GT mRNA levels. The results show that hyp eroxia decreases rat type II cell gamma-GT activity in vitro. This eff ect was not related to an altered regulation at mRNA level and it was not associated with the hyperoxia-induced decrease in intracellular GS H, since restoration of the GSH levels by NAC did not restore gamma-GT activity. The lazaroid U-74389G with vitamin E-like properties effect ively prevented the decrease in gamma-GT and GSH, so that direct inact ivation of the membrane-bound gamma-GT by hyperoxia is the most likely mechanism.