Fluorescence-polarization studies on binding of 4-methylumbelliferyl β-d-galactopyranoside to Ricinus communis (castor-bean) agglutinin

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Abstract
The binding of R. communis (castor-bean) agglutinin 1 to saccharides was studied by equilibrium dialysis and fluorescence polarization by using the fluorescently labeled sugar 4-methylumbelliferyl .beta.-D-galactopyranoside. No appreciable change in ligand fluorescence took place on its binding to Ricinus agglutinin 1; on the other hand, the fluorescence of 4-methylumbelliferyl .beta.-D-galactopyranoside was considerably polarized on its binding to the lectin. The association constants obtained by Scatchard analysis of equilibrium-dialysis and fluorescence-polarization data do not differ much from each other, and at 25.degree. C, Ka = 2.4 (.+-. 0.2) .times. 104 M-1. These values agree reasonably well with that reported in the literature for Ricinus agglutinin 1. The number of binding sites obtained by the different experimental procedures is 1.94 .+-. 0.1 per molecule of 120,000 daltons and is equal to the reported value of 2. The consistency in the values of Ka and number of binding sites indicate the absence of additional subsites on Ricinus agglutinin 1 for its specific sugars. In addition, the excellent agreement between the binding parameters obtained by equilibrium dialysis and fluoresence polarization indicate the potential of ligand-fluorescence-polarization measurements in the investigation of lectin-sugar interactions.

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