gamma-Glutamyl cyclotransferase from rat kidney. Sulfhydryl groups and isolation of a stable form of the enzyme.

gamma-Glutamyl cyclotransferase, highly purified from rat kidney, contains several readily accessible sulfhydryl groups whose modification appears to be associated with the appearance of multiple enzyme forms as determined by isoelectric focusing and ion exchange chromatography. The enzyme was obtained in a 1000-fold purified and apparently homogeneous form by a procedures involving treatment with dithiothreitol followed by chromatography on thiol-Sepharose. The enzyme was also isolated in a highly active, apparently homogeneous, and stable form after reduction and treatment with iodoacetamide. The amino acid compositions and other properties of the two forms of the enzyme were very similar. Studies on the activity of the enzyme toward a variety of gamma-glutamyl amino acids and di-gamma-glutamyl amino acids showed that the enzyme is much more active toward certain di-gamma-glutamyl amino acids than toward the corresponding gamma-glutamyl amino acids; thus, the preferred substrates have the general structure gamma-Glu-gamma-Glu-NH-R in which the nature of the R moiety has relatively little effect on activity.