Comparative studies on the binding specificities of pertussis toxin and different lectins to human erythrocytes

The binding specificity of pertussis toxin (PT) was compared with lectins with well-defined specificities by hemagglutination, hemagglutination inhibition and competitive binding assays. Neuraminidase-treated human erythrocytes were much less weakly agglutinated by PT than untreated ones. Hemagglutination of untreated human type A erythrocytes by PT was inhibited by fetuin, haptoglobulin and hog A + H. Mono- and disaccharides, and N-acetylneuraminic acid alone were ineffective at the highest concentrations used. On the other hand, hemagglutination by Ricinus communis agglutinin (RCA-1) was effectively inhibited by these substances. Similar results were also obtained in competitive binding assays with 123I-labeled PT and untreated type A erythrocytes. The binding of 125I-labeled PT to type A erythrocytes was not effectively inhibited by any of lectins with well-defined specificities. These results suggest that the combining site of PT may be specific for terminal sialic acid and/or sialic acid-linked carbohydrate portion(s) which can not be recognized by lectins reported previously.