Effect of N-acetylcysteine on plasma cysteine and glutathione following paracetamol administration

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Abstract
The effect of oral N-acetyl-L-cysteine (NAC) on plasma sulphhydryls has been studied in healthy volunteers. Following NAC 30 mg·kg−1, total NAC in plasma (i.e. free NAC and NAC as disulphides) reached a median peak concentration of 67 nmol·ml−1 within 45 to 60 min, and disappeared with an apparent half-life of 1.3 h. Only a fraction of total NAC (AUC 163 nmol·ml−1·h) was in the form of free NAC (AUC 12 nmol·ml−1·h, peak concentration 9 nmol·ml−1). Free cysteine was markedly increased (peak increment 49 nmol·ml−1; AUC 80 nmol·ml−1·h). Total cysteine and free and total glutathione in plasma were unchanged. Following the administration of 2 g paracetamol plasma cysteine and glutathione decreased (median decrement in AUC over 3 h was 5.1 nmol·ml−1·h and 3.8 nmol·ml−1·h, respectively). In contrast, the administration of 2 g NAC together with paracetamol resulted in an increase in the AUC of cysteine (+29.2 nmol·ml−1·h) and glutathione (+4.6 nmol·ml−1·h). The data show that NAC leads to a marked increase in circulating cysteine, in part by reacting with cystine and thereby forming mixed disulphides with cysteine and releasing free cysteine as shown in vitro. NAC had no effect on plasma glutathione in the absence of increased stress on the glutathione pools. However, NAC supports glutathione synthesis when the demand for glutathione is increased, as during the metabolism of paracetamol.