title: Inhibition of alpha 2-macroglobulin-bound trypsin by soybean trypsin inhibitor.
abstract: Soybean trypsin inhibitor, a protein of Mr = 20,000, has been used to assess the degree of inaccessibility of porcine trypsin within the alpha 2-macroglobulin-trypsin complex. The interaction between alpha 2-macroglobulin-bound trypsin and the inhibitor was demonstrated by affinity chromatography and trypsin inhibition. Whereas the free trypsin-inhibitor association is very fast (k = 1.2 X 10(7) M-1 s-1), the reaction between complexed trypsin and inhibitor takes 10 h to reach equilibrium. In addition, alpha 2-macroglobulin reduces, by several orders of magnitude, the affinity of trypsin for the inhibitor. Only one of the two trypsin molecules of the ternary (trypsin)2-alpha 2-macroglobulin complex is readily accessible to soybean inhibitor. It is postulated that the recently discovered proximity of the alpha 2-macroglobulin binding sites (Pochon, F., Favaudon, V., Tourbez-Perrin, M., and Bieth, J. (1981) J. Biol. Chem. 256, 547-550) accounts for this behavior. In the light of these results it is concluded that the proteinase binding sites are localized on the alpha 2-macroglobulin surface and that the two subunits of this protein are either not identical or not symmetrically arranged.
mesh_terms: D000818:Animals; D007700:Kinetics; D008433:Mathematics; D010179:Pancreas / Q000201:enzymology; D011485:Protein Binding; D013552:Swine; D014357:Trypsin / Q000378:metabolism*; D014360:Trypsin Inhibitor, Kunitz Soybean / Q000494:pharmacology*; D014361:Trypsin Inhibitors / Q000494:pharmacology*; D000511:alpha-Macroglobulins / Q000378:metabolism*
chemical_list: D014361:Trypsin Inhibitors; D000511:alpha-Macroglobulins; D014360:Trypsin Inhibitor, Kunitz Soybean; D014357:Trypsin