Theobromine, theophylline, paraxanthine and 1,3,7-trimethyluric acid were identified as caffeine metabolites after incubation of [2-14C]caffeine with rat liver microsomes and separation by high-pressure liquid chromatography (HPLC). A 9-fold induction of caffeine metabolism was observed in 3-methylcholanthrene (MC)-induced microsomes and this induction ranged from 4 to 11 times for individual metabolites. Induction of aryl hydrocarbon hydroxylase (AHH) was about 5-fold, comparable to that of theophylline formation. Primary caffeine metabolism is an enzymic process catalyzed by the microsomal mixed-function oxidases.
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