CLONING OF A SERINE PROTEINASE-INHIBITOR FROM BOVINE BRAIN - EXPRESSION IN THE BRAIN AND CHARACTERIZATION OF ITS TARGET PROTEINASES

A cDNA encoding of the serine proteinase inhibitor (serpin), B-43, was cloned from the cDNA library of the bovine brain. It encoded 378 amin o acids, and the MW of the protein was estimated to be 42.6 kDa, which is consistent with that of the native B-43 purified from the bovine b rain. The homology search revealed that B-43 belongs to the ovalbumin branch of the serpin superfamily. Among them, B-43 was most homologous to human placental thrombin inhibitor (PI-6) and its murine counterpa rt, with the amino acid identity of 76% and 71%, respectively. Norther n blot analysis showed that the size of the transcript was 1.4 kb, and that the expression of B-43 in the bovine brain varied depending on t he brain regions, i.e. a lower level of expression was observed in the cerebral cortex and the hippocampus compared to the level of expressi on that was observed in the medulla oblongata. [S-35]-labeled B-43 pro tein was synthesized in vitro by using a rabbit reticulocyte lysate sy stem, which formed complexes with proteinases such as thrombin, trypsi n, alpha-chymotrypsin, and 7S nerve growth factor (NGF), but not with urokinase or plasmin. These results, together with the immunohistochem ical localization of B-43 in astrocytes and in some neurons which was observed in the previous study suggest that B-43 may be involved in th e regulation of serine proteinases present in the brain or extravasate d from the blood.