Glutathione and Urea
Glutathione, an important tripeptide with a reactive sulfhydryl group, is also found in the aqueous humor. Its concentration in primates ranges from 1 to 10 µmol/L. Blood contains a high concentration of glutathione; however, virtually all glutathione resides within the erythrocytes, and plasma has a low concentration of only 5 µmol/L or less. Glutathione stabilizes the oxidation-reduction (redox) state of the aqueous by reconverting ascorbate to its functional form after oxidation, as well as by removing excess hydrogen peroxide. Glutathione also serves as a substrate in the enzymatic conjugation by cytosolic enzymes; this process is involved in the cellular detoxification of electrophilic compounds. These enzymes (glutathione S-transferases) are important in protecting tissues from oxidative damage and oxidative stress and are highly concentrated in the ocular ciliary epithelium.
The concentration of urea in the aqueous is between 80% and 90% of that in plasma. This compound is distributed passively across nearly all biological membrane systems, and its high aqueous-to-plasma ratio indicates that this small molecule (molecular weight, 60) readily crosses the epithelial barrier. Urea is effective in hyperosmotic infusion treatment for glaucoma. However, mannitol (molecular weight, 182) is preferred to urea because urea crosses the epithelial barrier more easily.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.