?-Glutamylamine cyclotransferase

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Abstract
γ-Glutamylamine cyclotransferase, an enzyme found in a number of animal tissues and cells, catalyzes the conversion of ε-(L-γ-glutamyl)-L-lysine to free lysine and 5-oxo-L-proline as well as the release of free amines and the formation of 5-oxo-L-proline from a variety of other L-γ-glutamylamines. Among its substrates are both the mono- and di-γ-glutamyl derivatives of putrescine, spermidine and spermine, and a derivative of ε-(L-γ-glutamyl)-L-lysine in which both the α-amino group and the carboxyl group of the lysine moiety are blocked. The enzyme does not act on most γ-glutamyl-α-amino acids, nor is it active toward the ε-lysyl derivatives of L-aspartic acid or D-glutamic acid. Derivatives of ε-(L-γ-glutamyl)-L-lysine in which the α-amino or the α-carboxyl function of the glutamyl moiety is blocked also do not serve as substrates. The specificity of γ-glutamylamine cyclotransferase is in accordance with the proposal that it functions biologically in the latter stages of the catabolism of products of the action of transglutaminases. Some suggestions as to the manner in which γ-glutamylamine cyclotransferase serves this function are made based on present knowledge of protein degradation.
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