ABA purification and function. ABA was purified from culture supernatants of Vero cells infected with rAd-ABA. (a) The purified ABA exhibited a single dominant band at 66 kDa on a SDS-PAGE gel. (b) Dot blotting showed binding of ABA to TcdA and TcdB. Serially diluted TcdA and TcdB proteins were spotted on nitrocellulose paper. The paper was then blocked with milk before adding ABA at 1 μg ml⁻¹ final concentration. The binding of ABA to both TcdA and TcdB was determined by an HRP-conjugated anti-E-tag antibody.