Binding of iophenoxate and iopanoate to human serum albumin.

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.