Abstract: Objective To detect the serum levels of free protein binding uremic toxins including hippuric acid (HA), indoxyl sulfate (IS), p-cresyl sulfate (PCS), and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF) in patients with chronic kidney disease (CKD) and to calculate their albumin binding rates in order to understand the relationship between the serum concentrations of free protein binding uremic toxins, the protein binding rate, and renal function change. Method Free and total concentrations of serum HA, IS, PCS, and CMPF were measured in 10 healthy volunteers and 40 CKD patients. CKD patients were divided into CKD 3rd stage group, CKD 4th stage group, CKD 5th stage group, and maintenance hemodialysis (MHD) group. Free HA, IS, PCS and CMPF in serum were separated by ultrafiltration and measured by high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). Serum albumin binding rates were then calculated. Results The serum levels (μg/ml) of free HA, IS, PCS and CMPF were 2.13 (1.36～3.53), 0.11 (0.07～017), 0.43 (0.14～0.59), and 0.26 (0.24～0.28), respectively, in healthy controls, 2.88 (1.76～ 5.63), 0.23 (0.05～0.48), 0.96 (0.56～2.08), and 0.30 (0.25～0.31), respectively, in CKD 3rd stage group, 8.78 (3.32～17.88), 0.55 (0.26～0.91), 1.21 (0.53～4.33), and 0.32 (0.29～0.50), respectively, in CKD 4th stage group, 24.05 (3.56～62.55), 3.16 (0.96～8.10), 5.42 (0.62～12.83), and 0.28 (0.25～0.35), respectively, in CKD 5th stage group, and 101.95 (23.23～125.25), 6.29 (2.06～12.98), 7.53 (2.66～12.98), and 0.30 (0.28～0.33), respectively, in MHD group. The protein binding rate was >60% for HA, and >90% for IS, PCS and CMPF. Conclusion Serum levels of free HA, IS, and PCS elevated in late CKD stages, and their protein- binding rates reduced as well. However, the serum level of free CMPF and its protein- binding rate changed insignificantly. HA, IS, PCS and CMPF bind to serum albumin tightly.

Key words: Chronic kidney disease, Protein-bound uremic toxin, Free concentration, Protein binding rate