The cellular binding and uptake was studied for α1-inhibitor3, a monomeric 200 kDa proteinase inhibitor present in rat plasma. After intravenous injection in the rat the inhibitor disappeared from the circulation with a half-time of 2.5 min when complexed with chymotrypsin, whereas the half-time for uncomplexed inhibitor was more than 60 min. 6 min after the injection of labelled complex, 83% was in the liver and 2.5% in the spleen. In vitro experiments at 4°C with isolated hepatocytes and peritoneal macrophages showed binding to the previously described receptors which bind and internalize the tetrameric rat and human α2-macroglobulin-proteinase complexes. The binding affinities were similar for the two types of complexes and binding was followed by uptake and degradation of the labelled complex when the cells were warmed to 37°C. The binding of uncomplexed α1-inhibitor3 was low and did not increase following treatment with methylamine in spite of cleavage of the internal thiol ester. α1-Inhibitor3-methylamine was changed to the receptor binding form when treated with chymotrypsin which caused the cleavage of at least one peptide bond in the bait region.
