**2. Glutathione-dependent enzymes**

Glutathione (GSH, γ-glutamylcysteinylglycine) is a low molecular weight sulfhydryl com‐ pound. It is a tripeptide with the sequence glutamic acid, cysteine, and glycine. GSH is a

crystalline solid with a melting point of 192–195°C and molecular weight of 307.33. It dissolves readily in water. There are two peptide bonds, two carboxylic acid groups (p*K*COOH = 3.53 and 2.12), one amino group (p*K*NH3+ = 8.66), and one thiol group (pHSH = 9.66). GSH is found intracellularly in all mammalian tissues and is the major nonprotein thiol compound present in the cell, with concentrations ranging from 0.1 to 10 mM. It is involved in a variety of metabolic processes, for instance, detoxication of xenobiotics, reduction of hydoperoxides, synthesis of leukotrienes and prostaglandins, maintenance of protein and membrane struc‐ tures, and regulation of numerous enzyme activities. This functional diversity is due to the properties of the thiol group that participates in redox transitions, thiol exchange reactions, thioether formation, and radical scavenging.

A variety of different enzymes utilize glutathione in a variety of biotransformations [1]. Glutathione reductase (GR) catalyses the reduction of GSSG (oxidized glutathione) using NADPH as a reductant. GR is important in maintaining the high cellular reduction potential.

Selenium-dependent glutathione peroxidase (GPOX) is another type of GSH-requiring enzyme that catalyses the reduction of peroxides using GSH as the reducing agent. There are the glutathione S-transferases (GSTs) that are also GSH dependent enzymes with many catalytic activities including the conjugation of GSH to xenobiotics [2,3].
