INDUCTION OF HEPATIC CYP1A2 BY THE ORAL-ADMINISTRATION OF CAFFEINE TORATS - LACK OF ASSOCIATION WITH THE AH LOCUS

Caffeine was administered to male Wistar albino rats for two weeks at three concentrations, namely 0.1, 0.2 and 0.3%, and hepatic cytochrome P450-dependent mixed-function oxidases determined. Caffeine administr ation gave rise to a marked, dose-dependent increase in the O-deethyla tion of ethoxyresorufin and, to a lesser extent, in the O-depentylatio n of pentoxyresorufin. Erythromycin N-demethylase, p-nitrophenol hydro xylase and lauric acid hydroxylase activities, as well as total cytoch rome P450 content were unaffected by this treatment. Immunoblot analys is revealed that caffeine gave rise to a dose-dependent increase in th e hepatic CYP1A2, and at the highest dose only, CYP2B apoprotein level s. Apoprotein levels of CYP3A and CYP2E1 were not modulated by the tre atment with caffeine at all dose levels studied. Caffeine could not di splace [H-3]TCDD from the rat hepatic cytosolic Ah receptor. Computer analysis showed that caffeine is essentially a planar molecule with an area/depth(2) ratio 4.8, characteristic of CYP1A substrates/inducers. Molecular modelling revealed that the caffeine molecule could orienta te itself within the putative CYP1A2 active site so as to facilitate d emethylation of the N-1, N-3 and N-7 positions. However, at physiologi cal pH, the N-9 nitrogen atom is likely to be partially protonated, al lowing it to participate in an electrostatic interaction with the nega tively-charged glutamate 318-residue, favouring N-3 demethylation, the major pathway of metabolism in both humans and animals. In conclusion caffeine, being essentially planar, is an inducer of CYP1A2 in rat li ver.