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

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Abstract
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.