Protein‐chemical analysis of pertussis toxin reveals homology between the subunits S2 and S3, between S1 and the A chains of enterotoxins of Vibrio cholerae and Escherichia coli and identifies S2 as the haptoglobin‐binding subunit

The purified toxin of Bordetella pertussis was dissociated in 5 M urea in the presence of immobilized haptoglobin. The toxin was dissociated in free S1, free S5 and the free complexes S2‐S4 and S3‐S4, with S2‐S4 as the only haptoglobin‐binding moiety, identifying S2 as the haptoglobin‐binding protein. Partial NH2‐terminal amino acid sequences were obtained from the dissimilar toxin subunits, after separation by SDS‐polyacrylamide gel electrophoresis followed by electroblotting onto polybrene‐coated glass‐fiber sheets. The sequences reveal extensive homology of the N‐terminal portions of the constitutive subunits S2 and S3 and between S1 and the enterotoxin A chains of Vibrio cholerae and Escherichia coli.