TRYPSIN-LIKE ENDOPEPTIDASE(S) NATURALLY ENTRAPPED IN HUMAN BLOOD ALPHA(2)-MACROGLOBULIN

Normal human blood alpha(2)-macroglobulin was found to contain a signi ficant level of endopeptidase activity. Substrate specificity studies using various peptide 4-methylcoumaryl-7-amide (MCA) substrates and so me peptides revealed that the activity is due to trypsin-like endopept idase(s) trapped in the alpha(2)-macroglobulin molecule. Among the MCA substrate tested, t-butyloxycarbonyl-Leu-Lys-Arg-MCA was hydrolyzed m ost rapidly. The substrate specificity did not resemble those of throm bin, tissue kallikrein, plasmin and the alpha(2)-macroglobulin-associa ted trypsin-like endopeptidase previously isolated from porcine gastri c mucosa. The activity was strongly inhibited by diisopropylfluorophos phate and bovine pancreatic trypsin inhibitor. The content of the acti vity was estimated to be approximately 0.07 to 0.1 mu g bovine trypsin equivalent per mg of alpha(2)-macroglobulin. The possibility has been suggested that the entrapped endopeptidase(s) may be responsible part ly for degradation of physiologically active peptides in the blood cir culation.