Methylamine‐induced conformational change of α2‐macroglobulin and its zinc(II) binding capacity

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Abstract
Methylamine induces a conformational change of α2‐macroglobulin which is very similar to that obtained by proteinase reaction and binding. This was shown by small‐angle X‐ray scattering at 21 °C in 0.03 M Hepes buffer of pH 8.0 containing 0.15 M NaCl and 0.3 mM EDTA. When α2‐macroglobulin reacts with methylamine the side maximum virtually disappears from the X‐ray scattering curve and the radius of gyration decreases from 7.8 nm to 7.2 nm. The X‐ray data of α2‐macroglobulin are consistent with an open shape model similar to that deduced via electron micrographs [Schramm, H. J. and Schramm, W. (1982) Hoppe‐Seyler's Z. Physiol. Chem. 363, 803–812]; one projection of the model resembles the letter H; the four subunits are mainly represented as elliptical cylinders which are connected via a central, quite flat cylinder. Zinc(II) ions cause aggregation of α2‐macroglobulin even at such a low total zinc concentration as 12.5 μM; for 25 μM zinc(II) concentration, the average molecular mass indicates that the aggregation goes beyond the dimeric stage. Monomeric species of α2‐macroglobulin appear to have the capacity specifically to bind 8.0 zinc(II) ions per molecule, which corresponds to two zinc(II) ions per subunit.