5.10 : Phase II Reactions: Glutathione Conjugation and Mercapturic Acid Formation

Glutathione, a tripeptide made up of glutamate, cysteine, and glycine, is a critical player in the detoxification of drugs and xenobiotics via a process known as glutathione conjugation or mercapturic acid formation. This phase II biotransformation reaction involves the covalent binding of glutathione to a drug or its metabolite, enhancing the compound's water solubility and enabling its excretion.

Several distinctive characteristics distinguish glutathione conjugation from other phase II reactions, such as glucuronidation and sulfation. For instance, glutathione's high cellular concentration ensures its immediate availability for conjugation reactions. Furthermore, glutathione conjugates are relatively stable, making them effective biomarkers for exposure to specific drugs or toxins. Glutathione S-transferases (GSTs), the enzymes that drive glutathione conjugation, facilitate the thiol group of glutathione's nucleophilic attack on the electrophilic centers of the drug or metabolite. Depending on the compound's chemical structure, this reaction can proceed via nucleophilic substitution or addition.

The mechanism of glutathione conjugation involves the creation of a covalent bond between glutathione's sulfur atom and the compound's electrophilic center. This reaction can occur with various functional groups, including epoxides, halides, and nitro. Acetaminophen serves as an example of a drug that undergoes glutathione conjugation, forming a reactive metabolite that can induce liver toxicity if not promptly conjugated with glutathione. In conclusion, glutathione conjugation is a vital phase II biotransformation reaction that assists in detoxifying and eliminating drugs and xenobiotics from the body. It involves the covalent attachment of glutathione to a compound, thereby increasing its water solubility and facilitating excretion. The GSTs catalyze this reaction, which transpires through nucleophilic substitution or addition, with drugs like acetaminophen as examples of compounds that undergo glutathione conjugation.