Gas Chromatographic-Mass Spectrometric Analysis for Measurement of P-Cresol and Its Conjugated Metabolites in Uremic and Normal Serum (Technical Briefs‪)‬

p-Cresol (4-methylphenol; 108 Da) is a protein-bound solute retained within the body in renal failure (1). p-Cresol is of interest because it has several toxic effects in vitro (2-6) and clinical correlates have been demonstrated (7, 8). In the absence of external exposure (9), p-cresol originates uniquely from bacterial tyrosine fermentation in the large intestine (10). During passage through the colonic mucosa and liver, it is detoxified by conjugation processes (sulfation and glucuronidation) (11-13). Thus, one might expect to find p-cresylsulfate and p-cresylglucuronide in the serum, but reports on conjugated p-cresol in renal failure patients are scarce (14-16). Most techniques to deproteinize serum samples (e.g., heat and acidification) may also partially hydrolyze sulfate esters and glucuronide bonds. Hence, the "total" (i.e., proteinbound and unbound) and "unbound" p-cresol reported in most studies probably reflect both unconjugated and (part of the) conjugated forms of the solute (17-21). We determined the extent of desulfation and deglucuronidation by deproteinization with heat and acid followed by gas chromatographic-mass spectrometric (GC-MS) analysis (19) with p-nitrophenylglucuronide and p-nitrophenylsulfate as model substrates. We also calculated exact amounts of unconjugated p-cresol, p-cresylsulfate, and p-cresylglucuronide in serum of hemodialysis patients and healthy controls. Percentage desulfation and deglucuronidation by different methods (see below) was determined for random serum samples. Further analyses were performed on 9 serum pools from hemodialysis patients [n = 86; 49 male; mean (SD) age, 69.8 (1.5) years] and 5 serum pools from healthy controls [n = 29; 10 male; 31.0 (1.4) years; creatinine clearance, 87.1 (1.4) mL * [min.sup.-1] * [(1.73 [m.sup.2]).sup.-1]. Serum pools were stored at -80 [degrees]C until analysis. The ethics committee of the University Hospital Leuven approved the study, and informed consent was obtained from all participants. p-Nitrophenylglucuronide, p-nitrophenylsulfate, and 2,6-dimethylphenol (all 98% purity) were from Sigma-Aldrich; p-cresol and p-nitrophenol were from Supelco; and [beta]-glucuronidase (Escherichia coli K12) was from Roche. Other materials were of analytical grade.