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Part 18 extends the general amino acids overview in Part 2, by reviewing specifically glutamine and glutamate, and the tripeptide antioxidant, glutathione.
Glutamine supplementation has been well studied in both clinical and exercise situations, particularly in terms of its effects on immune function. Sports drinks containing glutamine as a free amino acid or part of a dipeptide are widely available but the low levels recommended are unlikely to help improve immune or muscle function.
The first product of glutamine metabolism, catalysed by the enzyme glutaminase, is the excitatory neurotransmitter glutamate. The latter has sometimes been used for supplementation. This may seem rather surprising, since its appearance in plasma at a high concentration correlates neurotoxicity and sometimes with clinical problems.
Glutathione, for which glutamine is a precursor via glutamate, is a powerful antioxidant and, in its reduced form, is a good marker of antioxidant capacity, while an increase in its oxidised form is a good marker of oxidative stress.
Glutathione and glutamate
P Newsholme and M Krause
Glutathione (γ-glutamyl-cysteinyl-glycine; GSH) is the predominant low molecular weight thiol (0.5–10 mmol/l) in mammalian cells. Most GSH (85–90%) is cytosolic, with the remainder located in organelles (including mitochondria, nuclear matrix and peroxisomes).1 This tripeptide is a key antioxidant within cells, critical to regulating the reactive oxygen species (ROS) concentration.2 Reduced glutathione (GSH) may be used to remove damaging ROS such as H2O2 and convert it to harmless H2O, generating oxidised glutathione (GSSG) via glutathione peroxidase (figure 1). Disulphide formation and gluathionylation are reversible forms of protein covalent modification dependent on glutathione and can provide mechanisms for regulation of metabolic, signalling and transcriptional processes,3 including skeletal muscle adaptation to exercise and training.4 The cellular redox state is crucial for molecular signalling, and glutathione is a key regulator/sensor for redox status; thus strategies aiming at …
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