Identification of a defence mechanism in vivo against the leakage of enterokinase into the blood.

1. The serum proteinase inhibitors alpha 1-antitrypsin, alpha 2-macroglobulin, inter-alpha-trypsin inhibitor and C1-esterase inhibitor were found not to affect the catalytic activity of human enterokinase, whereas bovine trypsin activity was modified essentially as expected. Enterokinase was also not inhibited by Trasylol (trypsin inhibitor from bovine lung) or bovine pancreatic trypsin inhibitor. No other component in human or mouse serum complexing with enterokinase was identified. 2. Human enterokinase administered intravenously into mice was rapidly cleared from the circulation with a half-life of 2.5 min. This removal was not the result of the difference in species, since partially purified mouse enterokinase was cleared at the same rate as the human enzyme. Clearance was mediated by recognition of the carbohydrate portion of enterokinase and not through specific recognition of its catalytic site. Immunofluorescent staining showed that the enzyme accumulated in the liver. Attempts to block the clearance by the simultaneous infusion of competing glycoproteins suggested that enterokinase was taken up by hepatocytes. Of the glycoproteins tested only two, human lactoferrin (terminal fucosyl alpha 1 leads to 3 N-acetylglucosamine) and bovine asialo-fetuin (terminal galactosyl beta 1 leads to 4 N-acetylglucosamine) were weakly competitive. Two inhibitors of endocytosis, Intralipid and Triton WR1339, failed to delay the removal of enterokinase. It is proposed that enterokinase is cleared from the circulation by an as yet uncharacterized hepatocyte receptor.