Evidence for the involvement of several cytochromes P-450 in the first steps of caffeine metabolism by human liver microsomes.
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.
Berthou
F
F
Laboratoires de Biochimie et Pharmacologie, Faculté de Médecine, Brest, France.
Flinois
J P
JP
Ratanasavanh
D
D
Beaune
P
P
Riche
C
C
Guillouzo
A
A
D000328:Adult; D001711:Biotransformation; D015153:Blotting, Western; D002110:Caffeine / Q000378:metabolism*; D019388:Cytochrome P-450 CYP1A2; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003640:Dealkylation; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000378:metabolism; D010615:Phenacetin / Q000494:pharmacology; D013806:Theophylline / Q000378:metabolism
D065607:Cytochrome P-450 Enzyme Inhibitors; D002110:Caffeine; D003577:Cytochrome P-450 Enzyme System; D013806:Theophylline; D006899:Mixed Function Oxygenases; D010088:Oxidoreductases; D019388:Cytochrome P-450 CYP1A2; D010615:Phenacetin; C021183:1,7-dimethylxanthine