Ophthalmic acid (OPH), also known as ophthalmate (chemically L -γ-glutamyl- L -α-aminobutyrylglycine), is a tripeptide analog of glutathione. However, instead of the cysteine essential for many of glutathione's diverse functions, it contains L -2-aminobutyrate, a non-proteinogenic amino acid lacking the nucleophilic thiol group. Because of this, it has been widely, and incorrectly, considered an accidental byproduct of glutathione synthesis. In 2024, an article published by the federation of European biochemistry societies compiled evidence to put forward the major hypothesis that OPH serves as a glutathione regulating tripeptide, affecting both cellular and organelle influx and efflux of GSH, as well as modulating GSH-dependent reactions and signaling.

Biosynthesis

OPH is created using the precursor 2-aminobutyric acid through consecutive reactions of the same enzymes that create GSH, namely Glutamate–cysteine ligase and glutathione synthetase. Major regulators of OPH biosynthesis are local (relative) concentrations of cysteine and 2-aminobutyric acid, as well as their γ-glutamyl intermediate products.

Discovery and occurrence

OPH was first discovered and isolated from calf lens in 1956, and has since been found to be a ubiquitous metabolite. It is produced by: Distribution within (higher) organisms also appears to be ubiquitous as it has been found in the: In plants, it is found in:

Ophthalmic acid is not a biomarker of oxidative stress

OPH has mostly appeared in metabolomics studies correlating changes in its abundance with oxidative stress, following a study from 2006 on acetaminophen overdose in mice. However, this practice should generally be avoided, as there are major issues: