Pyrrolidone carboxylate 7-glutamyl cyclotransferase

It is known that peptides and proteins exhibit a higher absorbance at wavelengths below 240 nm than the component amino acids (40, 41) and that the absorbance at 205 nm of peptides and proteins is approximately proportional to the number of peptide bonds. Pyrrolidone carboxylic acid, which has an internal peptide bond, exhibits this type of absorbance, which may be used under appropriate conditions for its quantitative determination. Thus, Orlowski et al. (13) followed the activity of y-glutamyl cyclotransferase by this procedure after incubation of the enzyme with y-glutamyl amino acids, the reaction mixture was passed through a small column of Dowex 50 (H+) in order to remove interfering substances, and the absorbance of the eluate at 205 nm was then determined. 

Fig. 1. The y-glutamyl cycle. I, y-Glutamylcysteine synthetase II, glutathione synthetase III, y-glutamyl transpeptidase IV, y-glutamyl cyclotransferase V. peptidase. PCA = pyrrolidone carboxylic acid AA = amino acid.

---