Latent proteinase activity of gamma-glutamyl transpeptidase light subunit

gamma-Glutamyl transpeptidase, which is composed of two unequal subunits, exhibits proteinase activity when treated with agents such as urea and sodium dodecyl sulfate. The heavy subunit is preferentially and rapidly degraded. The enzyme also degraded bovine serum albumin in the presence of urea; however, several other proteins and model proteinase substrates were not cleaved. Treatment of the enzyme with 6-diazo-5-oxo-L-norleucine, a gamma-glutamyl analog, results in parallel loss of transpeptidase and proteinase activities indicating that the site at which gamma-glutamylation of the enzyme occurs (presumably a hydroxyl group on the light subunit) is also involved in proteinase activity. The purified light subunit, but not the heavy subunit, exhibits proteinase activity even in the absence of urea. Results suggest that dissociation of the enzyme unmasks the proteinase activity of the light subunit involving the site at which gamma-glutamylation of the enzyme occurs, and that the heavy subunit may impose transpeptidase reaction specificity by contributing the binding domains for gamma-glutamyl substrates.