Competition for binding to multiple sites of human serum albumin for cholecystographic agents and sulfobromophthalein.

The binding of two cholecystographic agents, iophenoxate and iopanoate, to human serum albumin was studied with 11 putative competitors; the results were qualitatively consistent with competitive binding to common sites. A more precise analysis of competition was achieved with four pairs of compounds for which the free and bound concentration of each was determined. The results were analyzed by a computer program and the dissociation constants calculated for both binder and competitor at specified sites on albumin. With numbering based on the rank order of dissociation constants for iophenoxate, the highest binding of the four compounds occurs at different sites: iophenoxate at site I; iopanoate at site II, sulfobromophthalein at site III; and bromphenol blue at site II. For a given compound, there is close agreement in the calculated affinities at different sites regardless of the competitor.