title: Urinary excretion of the uraemic toxin p-cresol in the rat: contribution of glucuronidation to its metabolization.
abstract: BACKGROUND
  Increasing evidence indicates that lipophilic and/or protein-bound substances such as p-cresol are responsible for adverse physiological alterations in uraemic patients. To better understand the evolution of p-cresol disposition in renal failure and dialysis patients, it is necessary to determine its kinetic characteristics and biotransformation pathways.
  
  
  METHODS
  We studied the biotransformation of p-cresol after intravenous injection of the compound in eight rats with normal renal function. Urine was collected in four 1 h intervals. To evaluate the presence of p-cresol metabolites, beta-glucuronidase was added to urine samples and the isolated unidentified chromatographic peak observed in previous experiments was submitted to tandem mass spectrometry (MS/MS) analysis.
  
  
  RESULTS
  Administration of p-cresol produced a p-cresol peak and an unknown peak, suggesting biotransformation of the compound. Addition of beta-glucuronidase to urine samples and incubation at 37 degrees C resulted in a marked decrease in the unidentified peak height (P<0.001) together with an increase in p-cresol peak height (P<0.001), suggesting that the unidentified peak was composed, at least in part, of p-cresylglucuronide. Mass spectrometry (MS) and MS/MS analysis of the isolated unidentified peak confirmed the presence of p-cresylglucuronide. Linear regression between the peak height of p-cresylglucuronide before enzyme treatment and the increase in p-cresol peak height after enzyme treatment in samples incubated with beta-glucuronidase allowed us to calculate the amount of p-cresylglucuronide as its p-cresol equivalents. This revealed that 64% of the injected p-cresol was excreted as glucuronide. There was no change in peak heights when sulphatase was added to the urine. When p-cresol and p-cresylglucuronide levels were combined, approximately 85% of all administered p-cresol was recovered in the urine. In addition, the combined urinary excretion of p-cresol and p-cresylglucuronide was more than four times greater than excretion of p-cresol by itself (P<0.01).
  
  
  CONCLUSIONS
  In rats with normal renal function, intravenous administration of p-cresol results in immediate and extensive metabolization of the compound into p-cresylglucuronide. The elimination of p-cresol from the body depends largely on the urinary excretion of this metabolite.
authors.affiliation: Renal Division, Heymans Institute, University Hospital, Gent, Belgium. gerrit.lesaffer@kahosl.be
authors.forename: Gerrit
authors.initials: G
authors.lastname: Lesaffer
authors.forename: Rita
authors.initials: R
authors.lastname: De Smet
authors.forename: Frans M
authors.initials: FM
authors.lastname: Belpaire
authors.forename: Bruno
authors.initials: B
authors.lastname: Van Vlem
authors.forename: Marijn
authors.initials: M
authors.lastname: Van Hulle
authors.forename: Rita
authors.initials: R
authors.lastname: Cornelis
authors.forename: Norbert
authors.initials: N
authors.lastname: Lameire
authors.forename: Raymond
authors.initials: R
authors.lastname: Vanholder
mesh_terms: D000818:Animals; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D003408:Cresols / Q000378:metabolism* / Q000493:pharmacokinetics* / Q000652:urine; D004195:Disease Models, Animal; D020719:Glucuronides / Q000378:metabolism* / Q000493:pharmacokinetics* / Q000652:urine; D008297:Male; D013058:Mass Spectrometry; D051381:Rats; D013997:Time Factors; D014118:Toxins, Biological / Q000378:metabolism* / Q000493:pharmacokinetics* / Q000652:urine; D014511:Uremia / Q000378:metabolism* / Q000652:urine*
chemical_list: D003408:Cresols; D020719:Glucuronides; D014118:Toxins, Biological; C032538:4-cresol