A comparison of the protective effects of N-acetyl-cysteine and S-carboxymethylcysteine against paracetamol-induced hepatotoxicity.

The protective effect of the sulphur-containing amino acids N-acetyl-cysteine and S-carboxymethylcysteine against paracetamol-induced hepatotoxicity was evaluated in the hamster by biochemical and histological methods. Of the animals receiving paracetamol alone 25% died within 24 h following administration. All surviving animals showed acute hepatocellular injury and marked loss of cytochrome P-450 and hepatic mixed-function oxidase activities. Simultaneous administration of N-acetylcysteine decreased the mortality rate, partly prevented the paracetamol-induced liver damage and partly restored enzyme activities. Simultaneous administration of S-carboxymethylcysteine with paracetamol afforded no protection. Kidneys from all animals were histologically normal. Human liver microsomes and liver microsomes from 3-methylcholanthrene-pretreated hamsters metabolished paracetamol to intermediate(s) that bind covalently to microsomal proteins. The rate of covalent binding was inhibited markedly by N-acetylcysteine and to a lesser extent by S-carboxylmethylcysteine.