No abstract

1 Caffeine is widely used as an in vivo probe for CYP1A2; the distribution/activity of this enzyme is reported to be reflected by metabolic ratios. 2 Several metabolic ratios using different combinations of urinary metabolites have been used to measure CYP1A2, with varying conclusions on its distribution. 3 A mathematical comparison of five metabolic ratios claiming to reflect CYP1A2 activity was made using data from 237 healthy volunteers. 4 All five metabolic ratios were symmetrically distributed. The five ratios however, measured at least three different parameters, with no one ratio correlating exactly with any other. 5 Data in the literature claiming to measure CYP1A2 using caffeine may reflect other parameters. 6 The complex metabolism of caffeine together with different parameters controlling the renal clearance of each metabolite, makes the use of urinary metabolic ratios an inaccurate probe for assessing the distribution of CYPlA2 activity in populations.

Patients with familial adenomatous polyposis (FAP) and age and sex matched controls were tested for cytochrome P4501A2 (CYP1A2), N-acetyltransferase, and xanthine oxidase activities using caffeine urinary metabolites as a discriminator. FAP patients showed significant underactivity of N-acetyltransferase (which inactivates some carcinogens) and significant overactivity of CYP1A2 (which activates some carcinogens). Xanthine oxidase activity, which can generate free radicals and cause cellular damage, was significantly increased in the FAP patients. All but one of the FAP patients had undergone colectomy. A separate group of six patients was therefore assessed before and at an average time of eight weeks after colectomy. No effect on enzyme activity was seen. The differences in enzyme activities detected in this study could produce an excess of active carcinogenic metabolites in the bile of FAP patients and contribute to the high risk for intestinal cancer in FAP. (Gut 1995; 36: 251-254 Of the FAP patients, six (21%) were smokers, while 11 (20%) of the controls were smokers. All patients were white with normal liver and kidney function. All but one of the FAP patients had undergone colectomy between 1 and 38 years before sampling. All controls had intact colons.To assess the effect of colectomy on enzyme function, five FAP patients and one patient with ulcerative colitis (average age 24 years, range 17-46; 5 males, 1 female) received analysis of N-acetyltransferase, CYP1A2, and xanthine oxidase activity immediately before colectomy and at a mean time of eight weeks (range 6 to 12) after colectomy.Urine samples were collected at two to six hours after the oral administration of a 300 mg tablet of caffeine (FAP patients) or after a caffeine containing beverage (controls), caffeine intake having been shown not to affect the metabolic profiles obtained.6 Patients were not fasted and were not subject to any dietary restrictions. Ethics committee approval was obtained for this study. Urine samples were stored at -20°C and were later analysed as a single batch.7The caffeine metabolites 1-methyluracil (1-MU), 1-methylxanthine (1-MX), 5-acetylamino-6-amino-3-methyluracil (AAMUwhich is the metabolite of 5-acetylamino-6-formylamino-3-methyluracil (AFMU)) were assayed in the urine samples by a modification of the method of Grant et al.

Caffeine has been used to determine acetylator phenotype for some 15 years but the interpretation of metabolic ratios with this substance raises theoretical and methodological issues. N-acetyltransferase type 2 (NAT2) status was assessed in 23 young healthy subjects using both caffeine overnight and spot urine samples, and sulphadimidine. Frequency distribution analysis of the metabolic ratios of NAT2, indicated two distinct groups for sulphadimidine, and for caffeine spot but not overnight samples. Spearman's rank correlation values were low indicating differences between the data sets for sulphadimidine and spot caffeine samples. Correlation between the two urine collection periods for caffeine was poor. The complex metabolism of caffeine may compromise its value as a probe for determining acetylator phenotype until the effects of important variables are better understood.