Interaction of Porcine α2-Macroglobulin with Chemically Modified Proteinases

Abstract

The interactions of porcine α₂-macroglobulin (α₂M) with native proteinases, their zymogens and the chemically-modified enzymes were compared. The α₂M did not bind to chymo-trypsinogen, or to most of the chemically modified derivatives of α-chymotrypsin, trypsinogen, DIP- and PMS-trypsins, but it could interact with anhydrotrypsin, PMS-subtilisin, and O-acetylated neutral subtilopeptidase. Anhydrotrypsin appeared to bind very tightly to α₂M, as does netive trypsin, whereas the binding of PMS-subtilisin to α₂M was weaker than that of the native enzyme, judging from exchange experiments with labeled enzyme and from competitive enzyme assay. There are, however, some differences in the mode of interaction with α₂M between native and anhydrotrypsins. (1) The shape and the magnitude of ultraviolet difference spectra caused by the interaction with α₂M were significantly different. (2) The interaction of α₂M with active proteinase led to the formation of new amino-terminal amino acids, while that with anhydrotrypsin did not. (3) In vivo experiments showed that radioactivity of ³H-labeled trypsin-α₂M complex was rapidly cleared from the plasma of rats, whereas the anhydrotrypsin-α₂M complex was cleared very slowly. These results suggest that the proteolytic activity of the enzyme is not obligatory for the first phase of α₂M-proteinase interaction (formation of a Michaelis-type complex), but only the proteolytically modified complex is cleared rapidly from the blood circulation system.