The role of histidyl residues in zinc-induced precipitation of α 2-macroglobulin-proteinase complexes

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When α 2 -macroglobulin ( α 2M ) is reacted with proteinases including trypsin, plasmin, α-thrombin, or with CH 3NH 2, each resulting α 2M derivative is precipitated by Zn 2+ in a similar manner. By contrast, unreacted α 2M is not precipitated over the same Zn 2+ concentration range. Zn 2+-induced precipitation of α 2M -CH 3NH 2 or α 2M -trypsin is prevented by acylation of the protein employing the histidine-specific reagent diethylpyrocarbonate (DEP). The Zn 2+-induced precipitation of α 2M -trypsin is prevented by acylation of the preformed α 2M -trypsin complex or by the reaction of acylated native α 2M with trypsin. Acylation of α 2M by treatment with DEP results in the modification of 13.5 histidyl residues per subunit of either native α 2M or α 2M -CH 3NH 2. Subsequent treatment with hydroxylamine reverses the modification of 10.5 histidyl residues per subunit in each protein preparation. These results indicate that histidyl residues are involved in the Zn 2+-induced precipitation of α 2M -proteinase or α 2M -CH 3NH 2 complexes, and that these residues are accessible to extensive protein-metal interactions only after α 2M has undergone a major conformational change. These appear to be the same histidyl residues which, upon acylation by DEP, are responsible for recognition of α 2M -proteinase complexes by the acyl-low-density-lipoprotein cell surface receptor ( S. V. Pizzo, P. A. Roche, S. R. Feldman, and S. L. Gonias (1986) Biochem. J. 238, 217–225).