Validity of an ELISA for N-acetyltransferase-2 (NAT2) phenotyping

Determination of caffeine and its metabolites in urine by capillary electrophoresis-mass spectrometry

The caffeine content of foods and beverages varies considerably, interfering with our ability to obtain valid interpretations in many human studies with regard to the mechanism of action(s) of caffeine and/or its metabolites. The rate of metabolism of caffeine and other xanthine drugs also varies greatly from one individual to another. Therefore, it is extremely important to develop accurate, reliable analytical methods to quantify caffeine and its metabolites in simple and complex matrixes. A simple method is described for the separation and characterization of caffeine and its major metabolites employing capillary electrophoresis (CE) coupled to ultraviolet-absorption and mass spectrometry (MS) detection. After optimization of the electrophoresis separation conditions, a reliable separation of caffeine and 11 of its major metabolites was achieved in 50 mM ammonium carbonate buffer, pH 11.0. The volatile aqueous electrolyte system used with a normal electroosmotic flow polarity also provided an optimal separation condition for the characterization of the analytes by MS. The CE method achieved baseline resolution for all 12 compounds in less than 30 min. The CE-MS method is suitable for use as a routine procedure for the rapid separation and characterization of caffeine and its metabolites. The usefulness of this method was demonstrated by the extraction, separation, and identification of caffeine and its 11 metabolites from normal urine samples. The urine specimens were first acidified to obtain optimum binding efficiency to the sorbents of the off-line, solid-phase extraction procedure employed here, and an acidified eluent solvent was employed for the desorption step to maximize the recovery of the bound analytes.

Identification of N1-methyl-2-pyridone-5-carboxamide and N1-methyl-4-pyridone-5-carboxamide as components in urine extracts of individuals consuming coffee

Caffeine metabolites were extracted from urine samples collected 4 h after consumption of a cup of coffee and were separated by high-performance liquid chromatography (HPLC) on a C18 (5 microm) reverse-phase column using an acetonitrile (5%), acetic acid (0.05%) solution as the mobile phase. The elution profiles indicated the constant presence of a major and a minor components eluting between the caffeine metabolites 5-acetamido-6-formyl-3-methyluracil (AFMU) and 7-methylxanthine (7X) in an approximate nine. A procedure was developed for the isolation of the major component in an apparent pure form, and the yield was 10-20 mg from 400 ml of urine. The minor component was isolated in an apparent pure form by this procedure as well, and the yield was 0.5 mg from 200 ml of urine. The average ratio of the two components in urine, UV absorption and 1H-NMR spectra of the two components, and 13C-NMR spectrum, mass spectrum and elemental analysis of the major component identified the major and minor components as N(1)-methyl-2-pyridone-5-carboxamide and N(1)-methyl-4-pyridone-5-carboxamide, respectively, two major metabolites of the vitamin niacin present in a significant amount in coffee beans. The two metabolites were present in the same average amount in urine extracts of individuals irregardless of coffee consumption. The findings are briefly discussed in relation to the nutritional sources of niacin and to current procedures for measuring amounts of the two metabolites in urine samples.

Stability of 5-acetamido-6-formylamino-3-methyluracil in buffers and urine

The caffeine metabolite 5-acetamido-6-formylamino-3-methyluracil (AFMU) and its product of spontaneous deformylation 5-acetamido-6-amino-3-methyluracil (AAMU) were synthesized. Their ultraviolet absorption spectra differed significantly from each other and wavelengths of absorption maximum and molar extinction coefficients varied with pH. The changes of the absorption spectrum parameters of AFMU and AAMU with pH indicated that they ionized with pK(a) of 5.7 and 8.3, respectively. The spontaneous deformylation of AFMU in solutions of different pH and urine were investigated spectrophotometrically and by high-performance liquid chromatography. The data showed the following: (a) AFMU transformed uniquely to AAMU; (b) deformylation obeyed first-order kinetics under the different conditions tested; (c) the half-life of AFMU varied between 7.8 and 36 h between pH 9.0 and 2.0 at 24 degrees C, with a maximum of 150 h at pH 3.0; (d) AFMU deformylated below pH 2.0 and above pH 10.0 with a half-life of less than 4.6 h; (e) half-lives of AFMU in urine were 57 and 12.5 h at 24 and 37 degrees C, respectively, comparable to those in buffers at equivalent pH and temperature. The results are discussed in relation to the mechanism of deformylation and the use of caffeine as a probe drug for NAT2 phenotyping.