Glutathione paracetamol

Treatment of paracetamol overdose is based on replenishment of antioxidant thiols to supplement the role of glutathione the most commonly used antidote is N-acetyl cysteine, but is only effective if given within a particular time window after ingestion. 

Glutathione is also implicated in the removal of toxic metabolites from the analgesic paracetamol (USA acetaminophen). Oxidative metabolism of paracetamol produces an A-hydroxy derivative, and this readily loses water to generate a reactive and toxic quinone imine, which interacts with proteins to cause cell damage. 

Figure 7.10 Metabolism of paracetamol. With therapeutic doses, paracetamol is metabolised to the glucuronide and sulphate conjugates. With higher doses these pathways become saturated and metabolism proceeds via die P-450-mediated route, with the formation of the toxic metabolite benzoquinone. This is normally metabolised by conjugation with glutathione. When glutathione is depleted benzoquinone is free to interact with cellular macromolecules, leading to cellular damage.

As well as microsomal enzymes showing diurnal variation, other factors important in toxicology also show differences over time. For example, the level of glutathione varies significantly. Studies have shown that the hepatic GSH level in mice is significantly lower at 8 p.m. versus 8 a.m. Correspondingly, the susceptibility to paracetamol toxicity is much greater when administered at 8 p.m. versus 8 a.m. (Table 5.18). 

Table 5.18 Variation in Liver Glutathione Levels with Time and Effect on Paracetamol Toxicity...

However, the reverse is the case with the hepato toxicity of paracetamol, which is increased after a low-protein diet. This may be due to the reduced levels of glutathione in rats fed low-protein diets, which offsets the reduced amount of cytochromes P-450 caused by protein deficiency. 

The converse is true of drugs requiring metabolic activation for toxicity. For example, paracetamol is less hepatotoxic to newborn than to adult mice, as less is metabolically activated in the neonate. This is due to the lower levels of cytochromes P-450 in neonatal liver (Fig. 5.30). Also involved in this is the hepatic level of glutathione, which is required for detoxication. Although levels of this tripeptide are reduced at birth, development is sufficiently in advance of cytochrome P-450 levels to ensure adequate detoxication (Fig. 5.30). The same effect has been observed with the hepatotoxin bromobenzene. (For further details of paracetamol and bromobenzene see chap. 7.) Similarly, carbon tetrachloride is not hepatotoxic in newborn rats as metabolic activation is required for this toxic effect, and the metabolic capability is low in the neonatal rat. 

Figure 7.18 (A/B) Relationship between hepatic glutathione, covalent binding of radio-labeled paracetamol to hepatic protein, and urinary excretion of paracetamol mercapturic acid after different doses of paracetamol. Source. From Ref. 9.

Figure 7.19 Proposed metabolic activation of paracetamol to a toxic, reactive intermediate /V-acetyl-p-benzoquinone imine (NAPQI). This can react with glutathione (GSH) to form a conjugate or with tissue proteins. Alternatively, NAPQI can be reduced back to paracetamol by glutathione, forming oxidized glutathione (GSSG).

Side-effects are rare and may include hematological reactions, leucopenia, agranulocytosis and other hypersensitivity reactions. Paracetamol has a narrow therapeutic dose range and overdosage induces severe liver and renal damage (Lewis and Paloucek, 1991) via accumulation of a toxic metabolite, N- acetyl-benzoquinoneimine (NABQI). Acetylcysteine or methionine, which increase glutathione conjugation of the metabolite, are used as the antidote. 

Lauterburg BH, Velez ME. Glutathione deficiency in alcoholics risk factor for paracetamol hepatotoxicity. Gut 1988 29 1153-1157. 

Paracetamol is a popular and safe analgesic if used properly but an overdose is insidiously dangerous. The patient often seems to recover only to die later from liver failure. The problem is that paracetamol is metabolized into an oxidized compound that destroys glutathione. 

Glutathione detoxifies this oxidizing agent by a most unusual mechanism. The unstable hydroxylamine loses water to give a reactive quinone inline that Is attacked by glutathione on the aromatic ring. The adduct is stable and safe but, for every molecule of paracetamol, one molecule of glutathione is consumed. 

Acetylcysteine is given as a glutathione precursor in paracetamol overdose and in many other liver failures, including septic shock. 

Children under six years of age seem to be less susceptible to paracetamol toxicity, possibly owing to a more efficient detoxification pathway or greater glutathione content, or simply a greater liver size in relation to body mass [8]. 

Hepatotoxicity from paracetamol overdose can occur with single doses as low as 10-15 g. The risk factors for hepatotoxicity in excessive doses include induction of cytochrome P450 enzymes malnutrition or fasting, due to reduced glutathione stores and reduced glucuronidation chronic alcohol use and age over five years [8, 13]. 

Because this patient has taken a paracetamol overdose, further administration of paracetamol must be avoided. Glutathione stores will be severely depleted, hence detoxification of the toxic metabolite will be reduced, leading to even greater hepatocyte damage. 

Whatever the actual pathway, the administration of compounds containing sulphydryl groups has been shown to be effective in paracetamol intoxication, presumably because they are able to bind to the electrophilic species in the same way as glutathione. Another example of drug toxicity related to the extent and nature of metabolism is that associated... 

Disposition in the Body. Readily absorbed after oral administration, but subject to extensive first-pass metabolism. It is metabolised mainly in the liver by O-dealkylation to paracetamol and acetaldehyde, followed by conjugation of the paracetamol with sulphate or glucuronic acid (see under Paracetamol) other reactions are deacetylation to phenetidine (/7-ethoxyaniline), N-,2-, and a-hydroxylation forming mainly 2-hydroxyphenet-idine and also 2-hydroxyphenacetin, sulphate conjugation of de-acetylated metabolites, and glutathione conjugation to form... 

As discussed in Chapter 2, after a drug (or other chemical) is taken by mouth, it first reaches the stomach and then the small intestine. It is most likely to be absorbed from the small intestine, where it enters the bloodstream which will transport it directly to the liver (see Figure 3, p. 15). In the liver paracetamol is changed into two other substances, which are relatively harmless but another metabolite, which is produced in only small amounts, is potentially poisonous. This metabolite is normally safely removed by further metabolism, which employs the substance glutathione (see box, p. 24). After a large overdose, however, this process of detoxication is overwhelmed and the poisonous substance attacks the liver. 

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