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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)(3)(4)(5)(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)(12)(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)(15)(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., protein-bound and unbound) and “unbound” p -cresol reported in most studies probably reflect both unconjugated and (part of the) conjugated forms of the solute (17)(18)(19)(20)(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−1 · (1.73 m2)−1]. Serum pools were …

Gas Chromatographic–Mass Spectrometric Analysis for Measurement of p-Cresol and Its Conjugated Metabolites in Uremic and Normal Serum