The Inhibition of Proteinases by Human α1‐Antitrypsin

The inhibition of human plasmin by α1‐antitrypsin was time dependent and the rate of inhibition rapidly decreased with decreasing temperature. The effect of substrates on the inhibitions of trypsin and chymotrypsin was investigated. Theoretically the final degree of inhibition should be the same, whether the substrate is added to the proteinase‐inhibitor system before or after the formation of the complex. The inhibition of chymotrypsin by α1‐antitrypsin and of trypsin by commercial soybean trypsin inhibitor was in line with the prediction, with either casein or synthetic esters as substrates. However, in the trypsin‐α1‐antitrypsin system the degree of inhibition depended on the order in which the reactants were mixed. With both casein and p‐tosyl‐l‐arginine methyl ester as substrates, inhibition was high when trypsin was preincubated with the inhibitor and the substrate was added later, and low or absent when the proteinase was added to the mixture of inhibitor and substrate. In the inhibition mixtures of plasmin, trypsin and chymotrypsin with α1‐antitrypsin, distinct changes were detected by electrophoresis in polyacrylamide gel. The unreacted inhibitor diminished or disappeared and reaction products were formed at rates corresponding to rates of inhibition.