Mechanism of Action of N-Acetylcysteine in the Protection Against the Hepatotoxicity of Acetaminophen in Rats In Vivo

Abstract

N-Acetylcysteine is the drug of choice for the treatment of an acetaminophen overdose. It is thought to provide cysteine for glutathione synthesis and possibly to form an adduct directly with the toxic metabolite of acetaminophen, N-acetyl-p-benzoquinoneimine. However, these hypothese have not been tested in vivo, and other mechanisms of action such as reduction of the quinoneimine might be responsible for the clinical efficacy of N-acetylcysteine. After the administration to rats of acetaminophen (1 g/kg) intraduodenally (i.d.) and of [(35)S]-N-acetylcysteine (1.2 g/kg i.d.), the specific activity of the N-acetylcysteine adduct of acetaminophen (mercapturic acid) isolated from urine and assayed by high pressure liquid chromatography averaged 76+/-6% of the specific activity of the glutathione-acetaminophen adduct excreted in bile, indicating that virtually all N-acetylcysteine-acetaminophen originated from the metabolism of the glutathione-acetaminophen adduct rather than from a direct reaction with the toxic metabolite. N-Acetylcysteine promptly reversed the acetaminophen-induced depletion of glutathione by increasing glutathione synthesis from 0.54 to 2.69 mumol/g per h. Exogenous N-acetylcysteine did not increase the formation of the N-acetylcysteine and glutathione adducts of acetaminophen in fed rats. However, when rats were fasted before the administration of acetaminophen, thereby increasing the stress on the glutathione pool, exogenous N-acetylcysteine significantly increased the formation of the acetaminophen-glutathione adduct from 57 to 105 nmol/min per 100 g. Although the excretion of acetaminophen sulfate increased from 85+/-15 to 211+/-17 mumol/100 g per 24 h after N-acetylcysteine, kinetic simulations showed that increased sulfation does not significantly decrease formation of the toxic metabolite. Reduction of the benzoquinoneimine by N-acetylcysteine should result in the formation of N-acetylcysteine disulfides and glutathione disulfide via thiol-disulfide exchange. Acetaminophen alone depleted intracellular glutathione, and led to a progressive decrease in the biliary excretion of glutathione and glutathione disulfide. N-Acetylcysteine alone did not affect the biliary excretion of glutathione disulfide. However, when administered after acetaminophen. N-acetylcysteine produced a marked increase in the biliary excretion of glutathione disulfide from 1.2+/-0.3 nmol/min per 100 g in control animals to 5.7+/-0.8 nmol/min per 100 g. Animals treated with acetaminophen and N-acetylcysteine excreted 2.7+/-0.8 nmol/min per 100 g of N-acetylcysteine disulfides (measured by high performance liquid chromatography) compared to 0.4+/-0.1 nmol/min per 100 g in rats treated with N-acetylcysteine alone. In conclusion, exogenous N-acetylcysteine does not form significant amounts of conjugate with the reactive metabolite of acetaminophen in the rat in vivo but increases glutathione synthesis, thus providing more substrate for the detoxification of the reactive metabolite in the early phase of an acetaminophen into...

Autoprotection in Acetaminophen Intoxication in Rats: The Role of Liver Regeneration

“…NAPQI forms adducts with several cellular proteins, thereby inactivating them (Nelson 1990). The importance of this mechanism is demonstrated by the beneficial effect of treatment with N-acetylcysteine which serves to restore the amount of hepatocellular glutathione (Massey & Racz 1981;Lauterburg et al 1983;Pratt & Ioannides 1985;Corcoran & Wong 1986). It is also known that other factors may modulate the toxic effects of acetaminophen on the liver and other organs, such as alcohol consumption (Sinclair et al 1998) intake of other drugs (Jørgensen et al 1988), dietary habits (Whitcomb & Block 1994) inducible nitric oxide synthase (iNOS) (Gardner et al 1998).…”

Aging: Drugs to Eliminate Methylglyoxal, a Reactive Glucose Metabolite, and Advanced Glycation Endproducts

“…There are good reasons for using NAC as an anti-MG compound: NAC can increase GSH levels [181], which is an efficient MG scavenger and antioxidant [70,179,180], NAC is a cysteine containing thiol compound and MG binds with high affinity to cysteine [180,181], and NAC is already used clinically for other conditions such as acetaminophen overdose [179,181]. More studies employing NAC as an anti-MG drug should provide interesting results and may help to establish the potential of NAC in this regard.…”

Combined Butalbital/Acetaminophen/Caffeine Overdose: Case Files of the Robert Wood Johnson Medical School Toxicology Service

“…Second, increased circulating catecholamines stimulate β 2 adrenoreceptors in vascular smooth muscle, resulting in vasodilation and hypotension [3,24,25]. By carrying additional methyl groups, caffeine has greater CNS penetration relative to other methylxanthines and has been able to cause prolonged seizures [24,62]. Under normal conditions, adenosine receptor A 1 stimulation is responsible in terminating seizure activity.…”

“…Likewise, stimulation of adenosine receptor A 2 enhances blood flow to the brain during seizures via cerebral vasodilatation. During methylxanthine intoxication, the increased metabolic demand from seizures may not be met by an increase in cerebral blood flow, due to vasoconstriction, allowing hypoxia to contribute to prolonged seizures [24,62]. Furthermore, caffeine has been found to be a competitive antagonist of benzodiazepines at the benzodiazepine receptor and may contribute to the seizures' intractability [3].…”

“…Its main efficacy comes in its ability to stimulate glutathione synthesis [62]. By repleting glutathione, NAC works to prevent the binding of N-acetyl-p-benzoquinoneimine to hepatocytes with subsequent necrosis.…”