Caffeine as a probe for human cytochromes p450: Validation using cDNA-expression, immunoinhibition and microsomal kinetic and inhibitor techniques

Wichittra Tassaneeyakul, Zahurin Mohamed, Donald J. Birkett, Michael E. McManus, Maurice E. Veronese, Robert H. Tukey, Linda C. Quattrochi, Frank J. Gonzalez, John O. Miners

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107 Citations (Scopus)

Abstract

The molecular basis for the use of caffeine (CA; 1,3,7-trimethylxanthine) as a probe for specific human cytochromes P450 has been investigated. The CA 1-, 3- and 7-demethylations (to form theobromine, paraxanthine and theophylline, respectively) all followed biphasic kinetics in human liver microsomes. Mean apparent Km values for the high- and low-affinity components of the demethylations ranged from 0.13-0.31 mM and 19.2-30.0 mM, respectively. cDNA-expressed CYP1A2 catalysed all three CA demethylations, and the apparent Km for CA 3-demethylation (the major metabolic pathway in humans) by the expressed enzyme was similar to the Km for the high-affinity liver microsomal CA 3-demethylase. IC50 values for inhibition of the CA demethylations by a-naphthoflavone were similar for both expressed CYP1A2 and the high-affinity microsomal demethylases. Moreover, CA was a competitive inhibitor of expressed CYP1A2 catalysed phenacetin 0-deethylation, with the apparent Km (0.080 mM) closely matching the apparent Km (0.082 mM) for CA 3-demethylation by the expressed enzyme. Expressed CYP1A1 was additionally shown to catalyse the 3-demethylation of CA, although activity was lower than that observed for CYP1A2. While these data indicate that CYP1A2 is responsible for the high-affinity component of human liver CA 3-demethylation, two limitations associated with the use of CA as an in vitro probe for CYP1A2 activity have been identified: (i) CA 3-demethylation reflects hepatic CYP1A2 activity only at appropriately low substrate concentrations; and (ii) CA is a non-specific CYP1A substrate and CYP1A1 may therefore contribute to CA 3-demethylase activity in tissues in which it is expressed. An anti-CYP3A antibody essentially abolished the 8-hydroxylation of CA to form trimethyluric acid, suggesting formation of this metabolite may potentially serve as a marker of CYP3A isozyme(s) activity.

Original languageEnglish
Pages (from-to)173-183
Number of pages11
JournalPharmacogenetics
Volume2
Issue number4
DOIs
Publication statusPublished - Aug 1992
Externally publishedYes

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