In order to develop a non-invasive phenotyping assay method useful in the assessment of human drug metabolizing enzyme activities, we studied the metabolism of caffeine using the rat liver microsomes and the isolated perfused rat livers and
compared
with the caffeine metabolites present in human urine. Metabolic oxidation of caffeine, 1, 3, 7-trimethylxanthine, catalyzed by various isoforms of cytochrome P450-containing mixed function oxidases is known to produce both N-demethylated and
8-hydroxylated metabolites. To determine which isoform of P450 is involved in the production of which specific caffeine metabolite, rats were pretreated with various inducers which are known to induce specific isoforms of P450 the liver
microsomes.
Among the rat liver microsomes obtained after treatment with various P450 inducers[eg., acetone(P450 2E1), dexamethasone(P450 3A1), 3-methylcholantherene(P450 1A2), and phenobarbital (P450 2B1)], only the liver mcrosomes isolated from
3-methylcholanthrene treated rats had increased activities (2.1 to 5.8 fold) for N1-demethylation(producing 3,7-dimethylxanthine, 37X), N3-demethylation(1, 7-dimethylxanthine, 17X) an 8-hydroxylation(1, 3, 7-trimethyluric acid, 137U). This result
indicated that the P450 1A2 isoform, inducible with 3-methylcholanthrene pretreatment, plays an importnat role in the initial demethylation of caffeine. The metabolic profile and the amount of caffeine metabolites obtained by using the isolated
perfused
rat liver resembled more closely with those obtained by analyzing the urinary caffeine metabolites present in human urine.
Upon analyzing the human urines collected from 60 healthy medical student volunteers of Inha medical School, who are composed of both male and female as well as smokers and non-smokers, we determined the activity of P450 1A2 in each individual by
comparing the molar ratio of(17X+17U)/(137X). the frequency distribution of P450 1A2 activity showed that they were distributed in a bimodal fashion, and that 855 of volunteers could be classified as extensive metabolizers and 15% as poor
metabolzers.
Furthermore, the P450 1A2 activity were, in general, higher for the males and cigarette smokers than the females and non-smokers, respectively. Based on these results, we suggested to those people with high P450 1A2 activity that they should
refrain
from smoking and consuming extensively brolied fish and meat to reduce the chance of suffering from bladder or colorectal cancer. We also advised those people who have low P450 1A2 activity that they should use lower doses of medicines like
acetaminophen, known to be metabolized and deactivated by P450 1A2, to reduce the chances of overdose related toxic side effects.
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