Ž . Brain Research 855 2000 235–243 www.elsevier.comrlocaterbres Research report Cytochrome P-450 activities in human and rat brain microsomes Pierre Voirol a , Michele Jonzier-Perey a , Franc ¸ois Porchet b , Marianne J. Reymond c , ` Robert C. Janzer d , Constantin Bouras e , Henry W. Strobel f , Markus Kosel a , Chin B. Eap a , Pierre Baumann a, ) a Unite de Biochimie et Psychopharmacologie Clinique, Departement UniÕersitaire de Psychiatrie Adulte, CH-1008 Prilly, Lausanne, Switzerland ´ ´ b SerÕice de Neurochirurgie, CHUV, Switzerland c DiÕ. Endocrinologie et Metabolisme, CHUV, Switzerland ´ d DiÕ. neuropathologie, CHUV, Lausanne, Switzerland e Departement de Psychiatrie, HUG, GeneÕa, Switzerland f Department of Biochemistry and Molcular Biology, UniÕersity of Texas Medical School, Houston, USA Accepted 9 November 1999 Abstract The role of cytochrome P450 in the metabolism of dextromethorphan, amitriptyline, midazolam, S-mephenytoin, citalopram, fluoxetine and sertraline was investigated in rat and human brain microsomes. Depending on the parameters, the limit of quantification using gas chromatography–mass spectrometry methods was between 1.6 and 20 pmol per incubation, which generally contained 1500 mg protein. Amitriptyline was shown to be demethylated to nortriptyline by both rat and human microsomes. Inhibition studies using ketoconazole, furafylline, sulfaphenazole, omeprazole and quinidine suggested that CYP3A4 is the isoform responsible for this reaction whereas CYP1A2, CYP2C9, CYP2C19 and CYP2D6 do not seem to be involved. This result was confirmed by using a monoclonal antibody against CYP3A4. Dextromethorphan was metabolized to dextrorphan in rat brain microsomes and was inhibited by quinidine and by a polyclonal antibody against CYP2D6. Only the addition of exogenous reductase allowed the measurement of this activity in human brain microsomes. Metabolites of the other substrates could not be detected, possibly due to an insufficiently sensitive method. It is concluded that cytochrome P450 activity in the brain is very low, but that psychotropic drugs could undergo a local cerebral metabolism which could have pharmacological andror toxicological consequences. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Rat brain; Human brain; Cytochrome P-450; Psychotropic drug; Metabolism; Microsome 1. Introduction Ž . Cytochrome P-450 P 450 is the most important xeno- biotic metabolizing enzyme system. It is also involved in the metabolism of endogenous compounds such as steroids and fatty acids. P 450 activity requires the presence of cytochrome P 450 reductase which transfers reducing equivalents from NADPH to P 450. Even if the liver is the organ that plays the essential role in drug metabolism, the extrahepatic presence and activity of P 450 is now a major area of interest. Brain has become one of the most studied organs because of the possible pharmacological and toxi- cological implications of cerebral drug metabolism. Sev- Ž eral isoforms of P 450 1A1, 1A2, 2B1, 2B2, 2D, 2E1, . 3A have been described in the brain, mostly in rodents ) Corresponding author. Fax: q41-21-643-6444; e-mail: pierre.baumann@inst.hospvd.ch w x w x 8,34,45 but also in humans 9,19,20,32,37 . These inves- tigations have been performed using immunohistochem- Ž . istry, polymerase chain reaction PCR and metabolism studies. The first two approaches aim to demonstrate the presence of the enzymes or its mRNA while metabolic studies show the activity of the enzymatic system. How- ever, discrepancies exist in the results. The differences in the principle and in the sensitivity of the methods and the antibodies used are thought to be partly responsible for the w x differences 22 . The levels and activities of P 450 found in cerebral tissues are very low and make metabolic studies difficult. However, metabolism of several substrates has been shown using brain microsomes. Recently, biotrans- formation of imipramine to desipramine and hydroxy-im- w x ipramine 41 , and of dextromethorphan to dextrorphan has been described in rat brain microsomes. The latter reaction w x is thought to be mediated by CYP2D1 26 . Its human equivalent form, CYP2D6, is known to be polymorphic 0006-8993r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0006-8993 99 02354-9