Relevance of NADPH depletion and mixed disulphide formation in rat lung to the mechanism of cell damage following paraquat administration

We have examined the possibility that the mechanism of paraquat toxicity in the lung involves both the formation of mixed disulphides (the amount of NPSH or GSH involved in protein disulphide formation) and the prolonged oxidation of NADPH, leading to NADPH depletion. We have compared the oxidation-reduction status of the lung, 2, 8 and 24 hr after dosing rats subcutaneously with 20 mg paraquat/kg (a dose which causes extensive lung damage 24 hr after dosing) or 20 mg diquat/kg (a chemically related bipyridyl which only causes very minimal lung damage 24 hr after dosing). Lung NADP⁺ levels were not affected 2, 8 or 24 hr after dosing with either bipyridyl. However, although NADPH levels were unchanged 2 hr after dosing with paraquat, and 2, 8 and 24 hr after dosing with diquat, there was a significant decrease in NADPH levels by 8 and 24 hr after dosing with paraquat. The changes in NADPH levels were coincident with the lung damage (characterized previously) caused by these treatments. In contrast with these effects on NADPH levels, there was an increase in NPSH and GSH levels in the lung by 8 and 24 hr after dosing with paraquat or diquat. Thus, there was no simple relationship between lung NADPH levels and lung sulphydryl levels.Lung mixed disulphide levels (the amounts of NPSH or GSH involved in disulphide formation) were increased 2, 8 and 24 hr after dosing with paraquat or diquat, although oxidized glutathione levels remained normal. Thus, an early and persistent biochemical effect of paraquat and diquat in the lung involves an increase in mixed disulphide levels, which is probably a consequence of the lungs' response to an increase in the oxidation of NADPH and GSH. As suggested previously, the increase in mixed disulphide levels appears to be a mechanism for regulating the normal redox state of the lung. However, despite this regulatory mechanism, NADPH depletion occurs 8 and 24 hr after dosing with paraquat, but not diquat, coincident with the development of lung damage.In conclusion, we suggest that mixed disulphide formation is not only a regulatory mechanism, but in some circumstances also may cause changes in essential biosynthetic and regulatory functions of the lung. This may ultimately lead to the drop in NADPH levels, which we propose is a critical biochemical event in the development of alveolar epithelial cell damage following the administration of paraquat.