Isolation and characterization of mouse countertrypin, a new trypsin inhibitor belonging to the mammalian fetuin family.

A novel trypsin inhibitor, tentatively named countertrypin, was isolated from mouse plasma in an apparently homogeneous state. Countertrypin is a 53-kDa glycoprotein having about 30% carbohydrate, and did not cross-react immunologically with either mouse alpha 1-antiproteinase (also called alpha 1-proteinase inhibitor or alpha 1-antitrypsin) or contrapsin. Countertrypin had no inhibitory activity against chymotrypsin, pancreatic elastase, neutrophil elastase, thrombin, plasmin, plasma kallikrein, pancreatic kallikrein, clotting factor Xa, or papain. This inhibitory spectrum does not correspond to any of the known plasma proteinase inhibitors that have been well characterized in human or other mammals. NH2-terminal amino acid sequence analysis of the intact molecule and three peptides obtained by CNBr digestion revealed that a total of 93 amino acid residues could be aligned with stretches in human alpha 2-HS glycoprotein, bovine fetuin, and rat pp63 (rat fetuin). Human alpha 2-HS glycoprotein and bovine fetuin prepared without use of ethanol inhibited trypsin and pancreatic and neutrophil elastases. These results indicate that mouse countertrypin is a new member of the mammalian fetuin family, which possibly has the trypsin-inhibiting activity in common.