The interactions between iophenoxic acid, iopanoic acid, bilirubin and human serum albumin as studied by fluorescence and Sephadex gel filtration.

Iophenoxic acid increases the fluorescence of bilirubin bound to human serum albumin at drug/albumin molar ratios lower than 1, while iopanoic acid decreases it. The fluorescence enhancement results probably from a change in the fluorescence efficiency due to an iophenoxic acid-induced conformational change in the albumin, which in turn causes displacement of bilirubin from the protein. Iophenoxic acid does not affect the high-affinity bilirubin binding site of albumin. Therefore any enhancement in bilirubin fluorescence caused by the drug indicates that bilirubin is bound to the low-affinity binding sites of albumin. The use of iophenoxic acid in the determination of the extent of saturation of the high-affinity bilirubin binding site of albumin may be of value in the clinical management of infants with neonatal jaundice.