Ricin D-Saccharide Interaction as Studied by Ultraviolet Difference Spectroscopy1

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Abstract
The interaction of ricin D with specific saccharides was investigated by ultraviolet difference spectroscopy. Upon binding to saccharides, ricin D displayed ultraviolet difference spectra with maxima at 280 nm and 288 nm. Such difference spectra suggest that the environment of a tyrosine residue(s) located at or near the sacchande binding site is changed by the binding of saccharide. In addition to the two positive peaks, a small trough was observed around 300 am in the complexes with galac tose-containing saccharides but not in the complex with N-acetylgalactosamine or galactosamine, suggesting the participation of tryptophan in the binding with galactose-containing saccharides. The magnitude of the difference maxima increased with increasing concentration of saccharides until the binding site was saturated. From the variation of the maximum at 288 nm as a function of saccharide concen tration, the association constants were obtained for the binding of saccharides to ricin D at various temperatures and pH's. The saccharide binding of ricin D decreased with increasing temperature and with decreasing pH below pH 6.0. It was suggested that difference maximum at 288 nm observed in the ricin D-saccharide interaction reflects the binding of saccharides to the high-affinity saccharide-binding site of ricin D.