Conformational differences between mouse contrapsin and alpha-1-antitrypsin as studied by ultraviolet absorption and circular dichroism spectroscopy.

We have compared the conformation of mouse contrapsin , a novel trypsin inhibitor ( Takahara , H. and Sinohara , H. (1982) J. biol. Chem., 257, 2438-2446), and alpha-1-antitrypsin by UV absorption and CD spectroscopy. The two mouse inhibitors share a similar secondary structure, i.e., 24-29% alpha-helix, 31% beta-sheet, and 10-11% beta-turn. The corresponding values for human alpha-1-antitrypsin are 48, 25, and 9%, respectively. These results suggest that peptide backbone structures of the two mouse inhibitors resemble each other, but differ slightly from that of human alpha-1-antitrypsin. On the other hand, evidence is accumulated that the microenvironments of aromatic side chains are considerably different in mouse contrapsin and alpha-1-antitrypsin: (i) An unusual fine vibrational structure was seen in the UV absorption spectra of murine and human alpha-1- antitrypsins , but not in that of contrapsin . (ii) CD bands in contrapsin which may be assigned to phenylalanyl residues were several-fold greater in intensity than those in the two alpha-1- antitrypins . (iii) The above-mentioned bands in contrapsin were more susceptible to pH changes than those in alpha-1-antitrypsin. (iv) CD bands in contrapsin which may contain unresolved contributions from tyrosyl and tryptophanyl residues also markedly differed from those in the two alpha-1- antitrypsins . These differences were, however, largely diminished in the presence of 50 mM sodium dodecyl sulfate, suggesting that mouse contrapsin and alpha-1-antitrypsin have a similar structure in the presence of this perturbant .