Lung protection by thiol-containing antioxidants

It is becoming increasingly clear that certain types of pulmonary injury may be closely related to oxidant-antioxidant imbalance in the lung, resulting from the production of reactive oxygen species within the lung during endogenous metabolism and xenobiotic insult. We have investigated the role of glutathione in pneumoprotection from such reactive species and, in particular, methods of manipulating the resident antioxidant capacity of lung glutathione. One such approach has been the use of the thiol-containing drug N-acetylcysteine. We have shown that N-acetylcysteine is able to both support intracellular glutathione biosynthesis and act as a 'scavenger' of reactive electrophilic species through the chemical reactivity of its thiol group. N-acetylcysteine reduces hydrogen peroxide to water, with the commensurate formation of N-acetylcysteine disulphide both when the peroxide was supplied directly or generated enzymatically. This basal reduction of hydrogen peroxide by N-acetylcysteine was greatly enhanced by the inclusion of catalytic amounts of the selenium-containing heterocycle, Ebselen, in the incubations. Thus, Ebselen mimics the activity of glutathione peroxidase but, unlike the enzyme, is able to use N-acetylcysteine as a co-substrate. Thus, the combination of N-acetylcysteine and Ebselen may provide a useful therapeutic tool in conditions of pulmonary toxicity associated with oxidant insult.