10. GSH Nutrition

What does this mean for horses? How do horses get their GSH, and how do they cope with oxidative stress?

At the beginning of life, nature provides mammals and birds with a special GSH precursor called gamma-glutamylcysteine (GGC). With GGC, the cells can produce GSH simply by adding one more amino acid. This allows them to bypass a major "rate-imiting" step in GSH production. Infant mammals get the GGC dipeptide from their mothers' milk. A much lower level of GGC is present in the raw white of birds' eggs. Since these are the only two food sources of GGC, after weaning or hatching animals must produce GSH internally from other nutrients in a regulated, self-limiting process.³⁷ Like all animals, weaned horses produce their GSH from amino acids they get from protein in their food.

GSH is made of three amino acids, so it is called a tripeptide. The aminos are glutamic acid, cysteine, and glycine. Glutamic acid, or glutamate, occurs in many feeds and is heavily used by the horse’s body to make other amino acids for protein building (transamination). Glycine is the simplest of the amino acids and, like glutamate, is usually available for GSH synthesis.

Cysteine, on the other hand, is more likely to be in short supply, especially in the required GGC form (gamma-glutamyl-cysteine). It is this rate-limited step in GSH synthesis that most researchers have tried to overcome in their attempts to increase GSH production.³⁸

Cysteine by itself is very unstable. It rapidly auto-oxidizes into cystine, a dipeptide that combines two molecules of cysteine in an unusually fragile disulfide bond. Because cystine and cysteine are unstable, bioactive food forms of these amino acids are rare. Most of the cysteine in the body is produced from a much more common amino acid, methionine. Unfortunately, increasing the dietary level of methionine does not substantially increase GSH production in the cells.³⁹

Glutathione's unique antioxidant and conjugating power comes from cysteine and its sulfhydryl unit (-SH), which enables GSH to protect a broad variety of tissues from free radical damage. The three-dimensional (tertiary) structure of GSH is also important to its flexibility and power as an antioxidant and detoxicant.⁴⁰