Cytochrome P450 in the brain; a review.

After many frustrating decades of unsuccessful attempts to characterize the isoforms of P450 in the brain, several scientific breakthroughs in the 80s and 90s have resulted in major advances in our understanding of cytochromes P450 (CYP) in brain. We now know that classical CYP inducers, e.g. phenobarbital and pregnenolone 16alpha-carbonitrile, which regulate drug-metabolizing enzymes in the liver, are specific ligands for ligand-activated transcription factors, and that the brain content of many of these transcription factors is low. This explains why these inducers have little effect on brain CYP content. The most effective inducers of brain P450 are some of the CNS active drugs and solvents. The level of CYPs in brain, approximately 0.5-2% of that in liver, is too low to significantly influence the overall pharmacokinetics of drugs and hormones in the body. Instead CYPs appear to have specific functions in brain, e.g. regulation of the levels of endogenous GABAA receptor agonists maintenance of brain cholesterol homeostasis and elimination of retinoids The novel CYPs which catalyse these reactions have recently been characterized. They are abundantly expressed in the brain confirming what has been previously found, i.e. that the major hepatic, adrenal and gonadal CYP isozymes contribute very little to the overall content of CYP in brain. It is not clear what fraction of brain CYP has been characterized, although a complete characterization of constitutive and induced CYPs in brain is essential for understanding the role of these enzymes in brain physiology as well as in age-related and xenobiotic-induced neurotoxicity.