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.