Purification and characterization of a trypsin-like protease in the submandibular gland of rats.

A trypsin-like protease (named RSP-V) was purified to homogeneity from rat submandibular glands by isoelectric focusing and high-performance liquid chromatography. The purified enzyme had an isoelectric point of 5.3 and an apparent molecular weight of 25,000, and consisted of two subunits with molecular weights of 19,500 and 6,000. RSP-V hydrolyzed BAEE, BAPA, and TAME, but not ATEE or BTPA. It had an optimum pH at around 10.0. RSP-V was strongly inhibited by soybean trypsin inhibitor, aprotinin, leupeptin, antipain, and benzamidine, but not by ovomucoid trypsin inhibitor, p-CMB, or iodoacetic acid. This enzyme partly resembled, but was not identical with, tonin. It was also different from kallikrein, salivain, and glandulain in rat submandibular gland. Although the physiological role of RSP-V has not yet been clarified, this enzyme inactivated dopa decarboxylase alone among catecholamine-synthesizing enzymes.