Albumin is the main plasma binding protein for indoxyl sulfate and p-cresyl sulfate.

Indoxyl sulfate and p-cresyl sulfate are two uremic retention solutes implicated in the uremic syndrome. Removal during dialysis is limited, mainly due to protein binding. Binding characteristics to healthy albumin have recently been characterized. Whether uremia alters the binding characteristics of albumin is currently unknown. Moreover, protein binding values previously determined with ultrafiltration are in sharp contrast to recently reported values based on microcalorimetry. In the present study, indoxyl sulfate and p-cresyl sulfate binding were therefore quantified using both equilibrium dialysis and ultrafiltration. Deming regression demonstrated good agreement between equilibrium dialysis and ultrafiltration. Free serum concentrations of indoxyl sulfate (+26.6%) and p-cresyl sulfate (+19.7%) were slightly higher at body temperature compared with at room temperature. To investigate binding kinetics, the plasma of healthy individuals or hemodialysis patients was titrated with albumin solutions. Theoretical models of protein binding were fitted to observed titration curves. Binding coefficients of both toxins were highest in purified albumin, and were reduced from healthy to uremic plasma. In conclusion, the ultrafiltration-HPLC technique reliably measures free serum concentrations of indoxyl sulfate and p-cresyl sulfate. Albumin is the main binding protein, both in health and in advanced stages of chronic kidney disease. Modeling suggests that albumin contains two binding sites for both toxins, a single high affinity binding site and a second low affinity binding site. The high affinity binding site accounts for at least 90% of overall binding. Competition for this binding site could be used to augment free solute concentrations during dialysis, thus improving epuration.