ISOLATION AND CHARACTERIZATION OF MOUSE COUNTERTRYPIN, A NEW TRYPSIN-INHIBITOR BELONGING TO THE MAMMALIAN FETUIN FAMILY

A novel trypsin inhibitor, tentatively named countertrypin, was isolat ed from mouse plasma in an apparently homogeneous state. Countertrypin is a 53-kDa glycoprotein having about 30% carbohydrate, and did not c ross-react immunologically with either mouse alpha1-antiproteinase (al so called alpha1-proteinase inhibitor or alpha1-antitrypsin) or contra psin. Countertrypin had no inhibitory activity against chymotrypsin, p ancreatic elastase, neutrophil elastase, thrombin, plasmin, plasma kal likrein, pancreatic kallikrein, clotting factor Xa, or papain. This in hibitory spectrum does not correspond to any of the known plasma prote inase inhibitors that have been well characterized in human or other m ammals. NH2-terminal amino acid sequence analysis of the intact molecu le and three peptides obtained by CNBr digestion revealed that a total of 93 amino acid residues could be aligned with stretches in human al pha2-HS glycoprotein, bovine fetuin, and rat pp63 (rat fetuin). Human alpha2-HS glycoprotein and bovine fetuin prepared without use of ethan ol inhibited trypsin and pancreatic and neutrophil elastases. These re sults indicate that mouse countertrypin is a new member of the mammali an fetuin family, which possibly has the trypsin-inhibiting activity i n common.