Determination of the binding affinities of 125I-labeled cholecystographic agents to human serum albumin by ultrafiltration techniques is complicated by the appearance of radiochemical impurities resulting from radiolysis of the parent compound. With labeled compounds purified daily by two extractions through chloroform, iophenoxic acid has an extremely high binding affinity. The dissociation constant (K) is 0.013 micronM for iophenoxate, compared to 0.15 micronM for iopanoate, its close analogue. However, at the weaker sites, iophenoxic acid is less strongly bound than iopanoate. The exceptionally high affinity of iophenoxate for a single site of serum albumin appears to underlie its unusual persistence in plasma. Binding in vivo is reversible and not covalent in nature. The choleretic compounds cinchophen and taurocholate have differential effects on the biliary excretion of iophenoxate and iopanoate. This cannot be attributed to selective inhibition of binding to plasma protein.

Crystallization↗

PubMedDate not supplied

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.

Binding Sites↗