Adenine binding sites of the lectin from lima beans (Phaseolus lunatus).

Abstract

A single high-affinity binding site for adenine and related compounds was identified in the lima bean lectin (LBL) component III tetramer. This site is identical with the high affinity site for 2,6-toludinyl-naphthalenesulfonate described previously (Roberts, D. D., and Goldstein, I. J. (1982) J. Biol. Chem. 257, 11274-11277). [14C]Adenine was bound with high affinity (Kd = 1.2 +/- 0.1 X 10(-5) M, T = 25 degrees C) and a high degree of specificity in that hypoxanthine and guanine were very poor ligands for this site. Specificity was also observed for free purine bases relative to nucleosides or nucleotides. A number of N6 derivatives of adenine with cytokinin activity were found to bind to LBL, with relative affinities decreasing in the order: N6 - benzyladenine greater than kinetin greater than zeatin greater than N6 - [delta 2-isopentenyl]adenine greater than dihydrozeatin greater than zeatin riboside. Evidence was also obtained for heterotropic interaction between the adenine binding site and a second class of hydrophobic sites present on each subunit of LBL. Binding of adenine and N6-benzyladenine to LBL was found to produce a 2.3- and 3.8-fold increase, respectively, in the affinity of the lectin subunit hydrophobic sites for 1,8-anilinonaphthalenesulfonate. 1,8-Anilinonaphthalenesulfonate, in turn, enhanced the affinity of LBL for adenine, demonstrating that binding of ligands to the two classes of hydrophobic sites is thermodynamically linked. Equilibrium dialysis also revealed high affinity binding sites for [14C]adenine on the lectins from Dolichos biflorus, Phaseolus vulgaris, and soybean (Glycine max).