RT Journal Article
SR Electronic
T1 Evidence for the involvement of several cytochromes P-450 in the first steps of caffeine metabolism by human liver microsomes.
JF Drug Metabolism and Disposition
JO Drug Metab Dispos
FD American Society for Pharmacology and Experimental Therapeutics
SP 561
OP 567
VO 19
IS 3
A1 Berthou, F
A1 Flinois, J P
A1 Ratanasavanh, D
A1 Beaune, P
A1 Riche, C
A1 Guillouzo, A
YR 1991
UL http://dmd.aspetjournals.org/content/19/3/561.abstract
AB Caffeine biotransformation and four monooxygenase activities involving cytochrome P-450IA2, namely ethoxy- and methoxyresorufin O-dealkylases, phenacetin O-deethylase, and acetanilide 4-hydroxylation were studied in 25 human liver microsomes. All these activities were highly significantly intercorrelated (r greater than 0.72, p less than 0.001) and correlated with the level of immunoreactive P-450IA2 content (r greater than 0.65; p less than 0.001). P-450IA content was measured by immunoblotting with anti-rat P-450 beta-naphthoflavone-B, an antibody that detects only a single band corresponding to P-450IA2. The formation rate of two caffeine metabolites, namely paraxathine and theobromine, was correlated with the four monooxygenase activities measured and P-450IA2-specific content (r greater than 0.75). However, inhibition studies of caffeine metabolism by phenacetin, a specific substrate of P-450IA2, clearly indicated that only the N-3 demethylation of caffeine was supported by this enzyme. These in vitro data demonstrate that P-450IA2 is predominantly responsible for the major metabolic pathway of caffeine and that the formation of other demethylated metabolites is mediated, at least partly, by other P-450 enzymes.