Biochemistry and Metabolism:

Biochemistry and Metabolism:

Reduced Glutathione (GSH) is a linear tripeptide of L-glutamine, L-cysteine, and glycine. Technically N-L-gamma-glutamyl-cysteinyl glycine or L-Glutathione, the molecule has a sulfhydryl (SH) group on the cysteinyl portion, which accounts for its strong electron-donating character.

As electrons are lost, the molecule becomes oxidized, and two such molecules become linked (dimerized) by a disulfide bridge to form Glutathione disulfide or oxidized Glutathione (GSSG). This linkage is reversible upon re-reduction.

Glutathione is under tight homeostatic control both intracellularly and extracellularly. A dynamic balance is maintained between GSH synthesis, it’s recycling from GSSG/oxidized Glutathione, and its utilization.

Glutathione synthesis involves two closely linked, enzymatically-controlled reactions that utilize ATP. First, cysteine and glutamate are combined by gamma-glutamyl cysteinyl synthetase. Second, GSH synthetase combines gamma-glutamylcysteine with glycine to generate Glutathione. As Glutathione levels rise, they self-limit further GSH synthesis; otherwise, cysteine availability is usually rate-limiting. Fasting, protein-energy malnutrition, or other dietary amino acid deficiencies limit Glutathione synthesis.

Glutathione recycling is catalyzed by Glutathione disulfide reductase, which uses reducing equivalents from NADPH to reconvert GSSG to 2GSH. The reducing power of ascorbate helps conserve systemic Glutathione.

Glutathione is used as a cofactor by (1) multiple peroxidase enzymes, to detoxify peroxides generated from oxygen radical attack on biological molecules; (2) transhydrogenases, to reduce oxidized centers on DNA, proteins, and other biomolecules; and (3) Glutathione S-transferases (GST) to conjugate Gluathione with endogenous substances (e.g., estrogens), exogenous electrophiles (e.g., arene oxides, unsaturated carbonyls, organic halides), and diverse xenobiotics. Low GST activity may increase risk for disease—but paradoxically, some Glutathione conjugates can also be toxic.

Direct attack by free radicals and other oxidative agents can also deplete Glutathione. The homeostatic Glutathione redox cycle attempts to keep Glutathione repleted as it is being consumed. Amounts available from foods are limited (less that 150 mg/day), and oxidative depletion can outpace synthesis.

The liver is the largest Glutathione reservoir. The parenchymal cells synthesize GSH for P450 conjugation and numerous other metabolic requirements—then export GSH as a systemic source of SH-reducing power. Glutathione is carried in the bile to the intestinal luminal compartment. Epithelial tissues of the kidney tubules, intestinal lining and lung have substantial P450 activity—and modest capacity to export Glutathione.

Glutathione equivalents circulate in the blood predominantly as cystine, the oxidized and more stable form of cysteine. Cells import cystine from the blood, reconvert it to cysteine (likely using ascorbate as cofactor), and from it synthesize GSH. Conversely, inside the cell, Glutathione helps re-reduce oxidized forms of other antioxidants—such as ascorbate and alpha-tocopherol.