Part 4: Glycine, the simplest amino acid, is considered conditionally essential in man. Inadequate amounts can impair those functions that are glycine-dependent such as: production of collagen, elastin, porphyrins for hemoglobin and cytochrome P450s, bile salts, purine bases, and glutathione; for detoxification of benzoic acid; and methyl donation via the glycine cleavage system. That’s a lot.
The functional need for glycine can be reviewed in several urinary markers including pyroglutamate, benzoate, hippurate, and 2-methyl-hippurate.
Pyroglutamate
Under normal conditions, only a small fraction of pyroglutamate, a product of glutathione metabolism (see figure below) is excreted. However, during times of increased glutathione synthesis, urinary excretion of pyroglutamate increases. Pyroglutamate excretion is also increased when glycine is limited. So, urinary pyroglutamate is a marker of glutathione production and/or glycine availability.
High amounts of pyroglutamate have been found in vegetarians and in those on low-protein diets which might be accounted for by differences in dietary nitrogen and glycine synthesis.
Detoxification – glycine conjugation
Another functional marker of glycine status is the detoxification of benzoic acid or benzoate, which gets converted into hippurate via glycine conjugation. Benzoic acid is a component of fruits such as prunes and cranberries, and is used as a preservative. An oral dose of 4 to 10 g of sodium-benzoate can lead to glycine depletion. This same process can be seen in 2-methylhippurate, which is made via glycine conjugation of 2-methylbenzoate.
Put them all together, pyroglutamate, benzoate, hippurate, and 2-methyl-hippurate, and you get a functional picture of the need for glycine.
Next week, Christie Egeston will discuss the importance of the Krebs cycle.
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