Ultraviolet Difference Spectral Studies on Concanavalin A

The interaction of concanavalin A, the phytohemagglutinin of the jack bean, with a variety of glycosides has been studied by the technique of ultraviolet difference spectroscopy. Whereas methyl a-D-glUC0and manno-pyranoside gave rise to relatively low intensity difference spectra, p-nitrophenyl a-D-mannopyranoside and a-D-glucopyranoside yielded large difference spectra upon interaction with concanavalin A. Using this technique as a measure of concanavalin A activity, it was demonstrated that the protein specifically binds low molecular weight carbohydrates at much lower pH values (e . g. pH 2.4) than previously believed. Although polysaccharides are also bound at these low pH values, they are not precipitated by concanavalin A. Molecular weight studies in acid media indicate that the protein does not dissociate and it is suggested that electrostatic repulsion of the protein molecules due to their high net positive charge prevents protein-polysaccharide lattice formation and hence failure of the complex to precipitate.