Conjugation With Glutathione and Mercapturic Acid Formation

PHASE II REACTION

CONJUGATION WITH GLUTATHIONE AND MERCAPTURIC ACID FORMATION

Glutathione ( γ-glutamyl cysteinyl glycine or GSH) is a tripeptide with a strongly nucleophilic character due to the presence of a -SH (thiol) group in its structure.

Thus, it has great affinity for electrophilic substrates, a number of which are potentially toxic compounds. It is important to note that a highly electrophilic metabolite has a tendency to react with tissue nucleophilic groups such as -OH, -NH₂ and -SH and precipitate toxicities such as tissue necrosis, carcinogenesis, mutagenesis, teratogenesis, etc. Conjugation with glutathione protects the tissue from such reactive moieties and thus, the reaction is an important detoxication route.

GSH conjugation differs from other conjugation reactions in that the process does not require initial activation of the coenzyme or the substrate since the GSH, which is a nucleophile itself, is highly reactive towards an electrophilic substrate. The interaction between the substrate and the GSH is catalysed by enzyme glutathione-S-transferase to form S-substituted glutathione conjugate. This conjugate is not excreted as such in the urine but undergoes cleavage, first by the enzyme -glutamyl transpeptidase to release the free glutamyl residue and cysteinyl glycine derivative; the latter is cleaved by enzyme cysteinyl glycinase to produce free glycine and the cysteine conjugate. Finally, N-acetylation of this conjugate by the enzyme N-acetylase yields S-substituted-N-acetyl cysteine products called as mercapturic acids.

GSH conjugation occurs by one of the two mechanisms:

1. Nucleophilic substitution at an electron deficient carbon or heteroatom such as alkyl halides, alkyl nitrates (e.g. glyceryl trinitrate), sulphates, sulphonates, organophosphates, epoxides, lactones, etc.

RX + GSH → R–S–G + H⁺ + X^–

2. Nucleophilic addition at the electron deficient double bond such as the unsaturated carbonyl compounds, e.g. ethacrynic acid.

R–CH=CH–R' + GSH → RCH–S–G–CH₂–R'

where R = electron withdrawing group.

The hepatotoxicity from paracetamol overdose is due to depletion of GSH.