Involvement of a tyrosyl residue in the interaction of peanut lectin with lactose.

The role of a tyrosyl residue in the binding of Arachis hypogaea (peanut) agglutinin, AHA, to lactose has been studied using two techniques, titration of the phenolic hydroxyl group of the tyrosine residue and chemical modification of the tyrosine with iodine. More than three tyrosyl residues per mol of AHA were masked when AHA was titrated in the presence of lactose. Lactose also protected some tyrosyl residues of AHA from the modification with iodine. Upon interaction with lactose, AHA iodinated in the presence of lactose gave a UV-difference spectrum with similar peaks to those of native AHA, while AHA iodinated in the absence of lactose gave a spectrum without such peaks. Though not only native AHA but also iodinated AHA was completely adsorbed on a column of lactamyl-Sepharose 6B, equilibrium dialysis showed that the binding constant and the number of binding sites of native AHA and iodinated AHA with lactose were 4.3 x 10(3) and 3.0 x 10(3) M-1, and 3.2 and 1.8, respectively. These results suggest that about two of four sugar binding sites have tyrosyl residues which induce the UV-difference spectra upon binding with lactose, and that the iodination of these tyrosyl residues results in a decrease of the number of binding sites on AHA.