Abstract
When used in overdoses, acetaminophen (APAP) is a common cause of morbidity and mortality in humans. At present, N-acetylcysteine (NAC) is the antidote of choice for acetaminophen overdoses. Prompt administration of NAC can prevent the deleterious actions of APAP in the liver. In view of the similarities in antioxidant effects demonstrated by NAC, hypotaurine (HYTAU) and taurine (TAU) in this and other our laboratories, the present study was undertaken to compare these compounds for the ability to attenuate plasma and liver biochemical changes associated with a toxic dose of APAP. For this purpose, fasted male Sprague-Dawley rats, 225-250 g in weight, were intraperitoneally treated with APAP (800 mg/kg), NAC, HYTAU or TAU (2.4 mM/kg) followed 30 min later by APAP, or 50% PEG 400 (the vehicle for APAP). At 6 hr after APAP administration, all animals were sacrificed by decapitation and their blood and livers collected. The plasma fractions were analyzed for indices of liver damage (alanine transaminase, aspartate transaminase, lactate dehydrogenase), levels of malondialdehyde (MDA), reduced (GSH) and oxidized (GSSG) glutathione, and activities of glutathione reductase (GR), glutathione S-transferase (GST) and γ-glutamylcisteinyl synthetase (GCS). Suitable liver homogenates were analyzed for the same biochemical parameters as the plasma but indices of liver damage. By itself, APAP increased MDA formation and had a significant lowering influence on the levels of GSH and GSSG, the GSH/GSSH ratio, and the activities of GR, GST and GCS both in the plasma and liver. In addition, APAP promoted the leakage of transaminases and lactate dehydrogenase from the liver into the plasma. Without exceptions, a pretreatment with a sulfur-containing compound led to a significant attenuation of the liver injury and the biochemical changes induced by APAP. Within a narrow range of potency differences, HYTAU appeared to be the most protective and TAU the least. The present results suggest that, irrespective of the differences in structural features and in vitro antioxidant properties that may exist among NAC, TAU and HYTAU, these compounds demonstrate equivalent patterns of protection and, to a certain extent, equipotent protective actions against the toxic actions of APAP in the liver when tested in equimolar doses and under the same conditions in an animal model.
Highlights
Therapeutic doses of the analgesic drug acetaminophen (APAP) are readily detoxified by hepatic phase II drugmetabolizing systems mediating glucuronidation and sulfation [1], with a small portion undergoing a cytochrome P-450-mediated bioactivation to the highly reactive electrophilic arylating intermediate N-acetyl-p-benzoquinoneimine (NAPQI) [2]
While NAC is regarded as the antidote of choice for APAP overdoses [18] and as an effective antioxidant [26,27,28], information on the protective actions of TAU in APA-induced acute liver injury appears to be limited to a report indicating that this amino acid can attenuate the leakage of intracellular enzymes from and DNA fragmentation, lipid peroxidation (LPO), apoptosis and necrosis in the hepatocytes of rats receiving a toxic dose of APAP [29]
This study has compared NAC, TAU and HYTAU for their ability to protect the liver against the oxidative stress and hepatocellular injury that follows a supratherapeutic dose of APAP
Summary
Therapeutic doses of the analgesic drug acetaminophen (APAP) are readily detoxified by hepatic phase II drugmetabolizing systems mediating glucuronidation and sulfation [1], with a small portion undergoing a cytochrome P-450-mediated bioactivation to the highly reactive electrophilic arylating intermediate N-acetyl-p-benzoquinoneimine (NAPQI) [2]. In the event of the intake of an overdose of APAP, the increased production of NAPQI rapidly overwhelms GST, eventually exhausts GSH, UDP-glucuronic acid and inorganic sulfate [3], inhibits GSH synthesis [3,4] and decreases cytosolic GST activity [5]. This APAP metabolite is a major cause of hepatocellular damage, centrilobular hepatic necrosis and even fatalities upon entering in adduct formation with liver macromolecules, especially proteins [6]. APAP may cause hepatotoxicity by mechanisms leading to the formation of reactive oxygen species (ROS), such as superoxide anion (O2-), hydrogen peroxide (H2O2) and hydroxyl radical (HO), reactive nitrogen species (RNS), such as nitric oxide and peroxynitrite (ONOO-), and peroxidation reaction products [6,7,8,10]
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