Circular dichroism tests on the effect of alkali on conformation of lectins

The structural properties of eight lectins (agglutinins) were studied by the circular dichroism (CD) probe. The lectin from Maclura pomifera (osage orange seed) exhibited in the far-ultraviolet spectral zone a positive CD band centered at 201 nm, whereas the other lectins had positive bands at 195--198 nm. Structural transitions were effected by alkali, pH 9.6--11.9. Native M. pomifera lectin had a positive CD band also at 236 nm which shifted to the red and enhanced by alkali. Significant individual differences in resistance to the denaturing effect of alkali were observed even between the lectins the CD spectra of which were similar in neutral solutions. The CD spectrum of the lectin from Wistaria floribunda was similar to the CD spectrum of the Dolichos biflorus lectin, but the former was much less sensitive to alkali than the latter. The CD spectra of the native lectins from soybean (Glycine max) and the erythroagglutinin from kidney bean (Phaseolus vulgaris) also were similar, yet the former was denatured at pH 10.6--11.6, whereas the CD of the latter was affected very little. The tertiary structures of the lectins from D. biflorus, the Sophora japonica and the fucose binding lectin from Tetragonolobus purpureas were more sensitive to alkali than those of the other lectins. While the CD bands at 270--300 nm, related to the tyrosine and tryptophan chromophores are generally diminished on denaturation, in the D. biflorus and S. japonica lectins these bands were enhanced at pH 9.6--10.6. In more strongly alkaline solutions, a new CD band developed at 248--252 nm. The main polypeptide chain was more or less completely disorganized at the very high pH values.