PT  - JOURNAL ARTICLE
AU  - Berthou, F
AU  - Flinois, J P
AU  - Ratanasavanh, D
AU  - Beaune, P
AU  - Riche, C
AU  - Guillouzo, A
TI  - Evidence for the involvement of several cytochromes P-450 in the first steps of caffeine metabolism by human liver microsomes.
DP  - 1991 May 01
TA  - Drug Metabolism and Disposition
PG  - 561--567
VI  - 19
IP  - 3
4099  - http://dmd.aspetjournals.org/content/19/3/561.short
4100  - http://dmd.aspetjournals.org/content/19/3/561.full
SO  - Drug Metab Dispos1991 May 01; 19
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.