CEREBRAL METABOLISM OF IMIPRAMINE AND A PURIFIED FLAVIN-CONTAINING MONOOXYGENASE FROM HUMAN BRAIN

Flavin-containing monooxygenase (FMO), previously reported both from h epatic and extrahepatic tissues, including brain, catalyze the oxidati on of certain xenobiotics and drugs that contain a nucleophilic hetero atom. Psychoactive drugs, including the antidepressant imipramine, are substrates for the brain FMO. Since FMO-mediated metabolism of these drugs might contribute to local pharmacodynamic modulation within the human brain, the metabolism of imipramine by human brain FMO was studi ed in further detail. In the present study, the FMO activity was deter mined in human brain microsomes by estimating the actual amount of imi pramine N-oxide formed. It was then compared with the corresponding ac tivity measured using substrate (imipramine)-stimulated rates of nicot inamide adenine dinucleotide phosphate (NADPH) oxidation, which teas s ignificantly higher than the activity estimated as the amount of N-oxi de assayed using high-pressure liquid chromatography (HPLC). The brain FMO activity was measurable only in the presence of detergents (sodiu m cholate or Lubrol PX) or in microsomes that were freeze-thawed sever al times. The activity was inhibited by an antibody to rabbit pulmonar y FMO, but an antiserum to the mt liver NADPH cytochrome P-450 reducta se had no effect indicating that cytochrome P-450 was not involved in the above metabolic pathway. The optimum pH for N-oxidation of imipram ine was found to be 8.5; thermolability experiments indicated that the FMO activity was completely lost only after the incubation of brain m icrosomes at 45 degrees C for 20 minutes. An FMO purified to apparent homogeneity from a human brain had a molecular weight of 71,000 Da. Th e purified enzyme cross-reacted with the antibody to rabbit pulmonary FMO and efficiently catalyzed the metabolism of imipramine to its N-ox ide. The human brain clearly contains an active FMO system, and it is conceivable that such enzymes are significantly involved in the focal metabolism and modulation of pharmacological and/or toxic effects of c ertain xenobiotics, including psychoactive drugs.