Effect of pH on the binding of Vicia graminea lectin to erythrocytes. Dependence on the chemical character of red-cell receptors.

Binding of the radioactive Vicia graminea lectin to human blood-group M and N erythrocytes and to horse erythrocytes was studied at pH 6-10. Binding of the lectin to untreated human erythrocytes and to those treated with Vibrio cholerae neuraminidase increased severalfold from pH 6 to pH 8 and was maintained at the maximal level up to pH 9/9.5. On the other hand, interaction of V. graminea lectin with native or desialylated horse erythrocytes was not significantly affected by pH and small differences in the binding were opposite to those found with human erythrocytes: the binding decreased when pH increased from 6 to 9.5. Binding of the lectin to all erythrocytes tested at pH 10 was lowered to about 80% of the maximal values. The differences in pH dependence of V. graminea lectin binding to human and horse erythrocytes most probably resulted from the presence of amino groups in human red-cell receptors and their absence from receptors of horse erythrocytes. The earlier data on the enhancing effect of amino group modification on the interaction of human red-cell glycopeptides with V. graminea lectin support the conclusion that an increase in the lectin binding to human erythrocytes at pH 6-8 is confined to the decreased protonization of the receptor amino groups. V. graminea lectin was irreversibly inactivated at pH 3 and was inactivated by EDTA at pH 7.4 and reactivated by Ca2+ or Mn2+. This suggested that the lectin is a metaloprotein, requiring bivalent cations for the full binding activity. Some quantitative differences between the binding properties of V. graminea lectin, prepared from different batches of seeds, are reported.