A Mechanistic Study of the Protective Effects of S-Adenosyl-L-Methionine Against Hepatotoxicity of Acetaminophen

Hepatic toxicity is known to be associated with excessive doses of the over-the-counter analgesic, acetaminophen (APAP). APAP overdose is the leading cause of drug-induced liver failure in the United States. APAP hepatotoxicity is dependent on the biotransformation of APAP by cytochrome P450 to the toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI). APAP, when taken in excessive doses, can lead to severe liver damage with the potential to progress to liver failure. Despite substantial efforts in past studies, the mechanism by which APAP induces such damaging effects is not completely understood. Recent discoveries suggest that glutathione (GSH) depletion, protein alkylation and reactive metabolite formation may all play a part in APAP’s hepatotoxic effects. The present study hypothesized that S-Adenosyl-L-methionine (SAMe) diminished APAP hepatic toxicity by maintaining GSH status, reducing oxidative stress and altering formation of the toxic metabolite. The model selected for this study was a mouse model which is representative of the human response. This study was directed at investigating the mechanism by which APAP elicits its iii damaging effects on the liver and attenuating that damage with a nutraceutical, SAMe. Male C57Bl/6 mice were given an intraperitoneal injection (i.p.) of water, 250-500 mg/kg of APAP and/or 500 mg/kg of SAMe. Livers were removed 1-4 hours post-injection and were measured for toxicities using GSH levels, plasma alanine aminotransferase (ALT) levels, liver:body weight ratios, lipid peroxidation, histological examinations, protein levels, protein carbonyl levels and oxidized protein levels. Data showed that APAP had toxic effects at all of the measurable levels. These data reiterated that APAP was able to elicit damaging effects not only by GSH depletion, but also by other mechanisms. Pre- and post- treatment with SAMe attenuated the APAP-induced liver damage successfully in all measurable parameters compared to the animals dosed with APAP only. A further look into the mechanistic properties of SAMe was established after the initial findings. These studies included: (1) incorporating a known limiting reagent, vitamin B6, in SAMe’s pathway to the synthesis of GSH, (2) measuring and comparing metabolites produced when given APAP only compared to a dose of SAMe before or after APAP and (4) directly comparing the therapeutic effects of SAMe to the current antidote to APAP overdose, N-acetylcysteine (NAC). It was found that not only was SAMe able to reduce and/or eliminate the effects of APAP overdose, but when given on the same mmol basis, it was able to provide better protection than NAC. These results strengthen the potential use of SAMe as an antidote for APAP overdose cases