Y-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. 

In contrast to y-glutamyl transpeptidase, which is almost entirely bound to particulates, y-glutamyl cyclotransferase is present in the supernatant fractions of liver and other tissues. Studies on the distribution of the enzyme shows that it is present in most animal tissues but that there are some species differences (Table IV). Relatively high enzymic activity is found in kidney, liver, brain, and skin. In man, the highest activity is found in the brain. The unusually high activity in... 

Table V gives data on the specificity of the purified y-glutamyl cyclotransferases obtained from human and sheep brain the two enzymes exhibited similar specificity. The model substrate, L-y-glutamyl-L-y-glu-tamyl-p-nitroanilide was the most active substrate the corresponding l,d-, d,l-, and D,D-isomers were inactive. The enzyme preparations also...

Specificity of the y-Glutamyl Cyclotransferases Isolated from Human and Sheep Brain"... 

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.

T3. Taniguchi, N and Meister, A., y-Glutamyl cyclotransferase from rat kidney Sulfhydryl groups and isolation of a stable form of the enzyme. J. Biol. Chem. 258, 1799-1806 (1978). 

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