Isolation of a thiol-activated T-kininogenase from the rat submandibular gland

T-kininogenase (T-kgnase) activity has been investigated in tissues of the rat and submandibular glands of the rat, mouse and guinea pig. Both rat and mouse submandibular homogenates showed high T-kgnase activity. The enzyme has been purified 360-fold from rat submandibular gland homogenate supernatant fluid. The enzyme has an apparent molecular mass of 28 kDa and a pH optimum of 8.0 toward T-kininogen. It cleaved T-kininogen in catalytic quantities to release T-kinin (Ile-Ser-bradykinin) and small quantities of bradykinin and an unknown kinin. The activity of the enzyme was increased 10-fold in the presence of thiol groups (dithiothreitol) and inhibited by leupeptin (90%) and to a lesser extent by aprotinin (49%), TLCK (46%) and soybean trypsin inhibitor (27%). Pepstatin and PMSF did not inhibit the enzyme. Studies on substrate specificity, pH optimum and agents which inhibit T-kgnase activity demonstrate that this enzyme is different from plasma and tissue kallikreins, cathepsin D, esterase A and esterase B (other known kininogenases). It is the first thiol-activated kininogenase to be reported.