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Isotachophoretic analysis of the pH dependent carrier properties of bovine serum albumin for rifamycin AF/013

The binding sites of bovine serum albumin for rifamycin AF/013 were examined by means of analytical capillary isotachophoresis. At pH 7.5–7.9, there was only one binding site on non‐defatted bovine serum albumin, while under the same conditions up to 20 binding sites could be detected on fatty acid‐free albumin. In both cases the interaction became nonspecific above pH 8.0, resulting in a large number of rifamycin AF/013 molecules bound to albumin. Co‐binding with oleic acid in vitro resulted in competition with all but the first high‐affinity binding site for rifamycin AF/013, corroborating the findings made with non‐defatted albumin. Considering albumin as a model, the results might explain the conflicting reports about the number of binding sites of DNA‐dependent RNA polymerases for rifamy cin AF/013. Within a sharp pH‐limit the highly specific binding becomes a nonspecific one. Moreover, the results suggest that the carrier function of bovine serum albumin might drastically change in vivo, depending on the pH of physiological fluids.