γ-Glutamyl transpeptidase was found to be in relatively much higher concentrations in human retina and iris than in the lens. In the lens, the enzyme was localized to a major extent in the epithelium plus capsule fraction. The lens cortex and nucleus had very little enzyme activity. γ-Glutamyl transpeptidase was purified about 200-fold to a specific activity of 897 mU per mg protein from cultured human lens epithelium. The purification steps included heating at 37° for 2 hr, deoxycholate extraction, DE-52 column chromatography, Sephadex G-200 gel filtration and affinity chromatography. The enzyme preparation was found to have a Km of about 0·7 mm for γ-glutamyl p-nitroanilide and a pH optimum of 8·2. Magnesium had no significant effect on the enzyme activity, whereas, sodium and potassium inhibited the enzyme slightly. The competitive inhibition of γ-glutamyl transpeptidase by GSH and GSSG when γ-glutamyl p-nitroanilide was used as substrate indicates that the artificial substrate binds to the protein at the same site as the natural substrate, i.e. GSH. The enzyme was not inhibited by sulfhydryl blocking reagents up to a concentration of 1 mm indicating that the enzyme does not require the presence of sulfhydryl groups for its activity. Antibodies raised against an apparently homogeneous γ-glutamyl transpeptidase from human kidney precipitated the enzyme from ocular tissues and the enzyme purified from the cultured lens epithelial cells. This indicates that the enzyme of the ocular tissues and the kidney are of similar genetic origin.
ASJC Scopus subject areas
- Sensory Systems
- Cellular and Molecular Neuroscience