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Effect of Urea on the Circular Dichroism of Lysozyme

From the ¹ From the Department of Pathology, University of Colorado School of Medicine, Denver, Colorado 80220

The effect of the protein denaturant urea upon the circular dichroism spectrum of lysozyme in aqueous solution has been determined. The near-ultraviolet spectrum of native lysozyme in water contained a prominent negative plateau centered at 258 nm. In the spectrum of lysozyme in 8 m urea this plateau was replaced by a broad shoulder with a value for the molar ellipticity at 258 nm 50% less negative. The shape and intensity of the negative plateau were, however, nearly identical for lysozyme in water or 8 m acetamide, an amide of poor denaturing ability. Thus the extent of change in this plateau correlates directly with change in the stability of lysozyme's disulfides toward reduction in aqueous urea or acetamide (Warren, J. R., and Gordon, J. A. (1970) J. Biol. Chem. 245, 4097). Alteration in the near-ultraviolet spectrum of lysozyme in urea solutions was unaccompanied by equivalent change in the far-ultraviolet spectrum of the protein. In contrast, both the near- and far-ultraviolet circular dichroism spectra of bovine serum albumin were altered upon exposure of the protein to increasingly concentrated urea solutions.

These circular dichroic data are consistent with the following conclusions: (a) the negative plateau in the near-ultraviolet spectrum of native lysozyme arises primarily from the intramolecular disulfide bonds of the protein; (b) upon interaction of urea with lysozyme a conformational rearrangement occurs around the disulfide bond or bonds with, unlike serum albumin, little or no disruption of helical structure; (c) lysozyme is not resistant to the denaturing action of urea, although unfolding appears to be restricted to nonhelical portions of the lysozyme molecule.

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