Binding of Hydrophobic Ligands to Plant Lectins: Titration with Arylaminonaphthalenesulfonates’

Binding of the hydrophobic ligands l&anilinonaphthalenesulfonic acid (ANS) and 2,6-toluidinylnaphthalenesulfonic acid (TNS) to a variety of plant lectins was studied by lectin-induced alteration of the fluorescence spectra of the two ligands. With one exception, all legume lectins examined bound ANS, with affinity constants ranging from lo3 to lo4 M-‘. Similar ANS binding was noted for some nonlegume lectins. Ti-tration of the five isolectins from Phaseolus vulgar & with ANS indicated positive co-operative binding of ANS to the two isolectins E4 and E3L1. Titrations with TNS re-vealed high-affinity sites for this ligand in a number of lectins. Addition of haptenic sugars did not inhibit binding of ANS, suggesting that the hydrophobic binding sites of lectins are independent of the carbohydrate binding sites. Lectins are widely used as tools for studying the nature and distribution of carbohydrate on cell surfaces and tissues. In order to interpret the results of these studies in a meaningful manner it is im-portant to have a thorough understanding of the properties of these carbohydrate binding proteins. Binding of ligands other than carbohydrates has been observed for a few lectins. Hydrophobic binding prop-erties of concanavalin A (Con A)3 have been described (l-3). Indirect evidence for the involvement of hydrophobic interactions in the binding of Phaseolus vulgaris lectin to erythrocyte stroma has also been pre