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The complex formed between trypsin and alpha(2)-macroglobulin retains the whole hydrolytic activity for benzoylarginine-p-nitroanilide (DL BAPNA). We have observed that the inhibition of this activity is dependent on the inhibitor used. While soya bean inhibitor (MW = 20 000) is ineffective, the basic pancreatic inhibitor (MW = 6 500) progressively inhibits the complex. In this latter case, we have observed the formation of a very weakly active ternary compound by two methods: kinetic and fluorescence polarization.Such differences suggest either that the contact site of trypsin for binding is not the same for each inhibitor or that the steric hindrance by macroglobulin prevents large inhibitors from reaching the active site. The pancreatic inhibitor can reach this site but its affinity is lower for the complex than for free trypsin.