Cytochrome P450 enzymes are a superfamily of constitutive and inducible haemoproteins, with a central role in the oxidative metabolism of a wide range of compounds. These enzymes have been shown to play important roles both in tumour development and mediation of chemotherapy. Although cytoclirome P450 was first identified in the liver, evidence has accumulated for the presence of cytochrome P450 in extrahepatic tissues including the brain. The presence of the xenobiotic metabolising cytochrome P450 enzymes in the brain is intriguing because of the possible ramifications on function from small changes in P450 levels. o This project has highlighted the presence of several cytochrome P450 enzymes belonging to families 1, 2 and 3 in normal and neoplastic human brain. o In addition, it is the first study to look at cytochrome P450 expression in a number of different human brains using a variety of molecular biology and biochemical techniques. This methodology has provided some indication as to the inter-individual differences in the levels and types of cytochrome P450 enzymes in human brain tissue. o All the P450s examined were identified in specific regions of brain with CYPlAl and CYP2C8 mRNA being the most fi'equently expressed forms. CYP2D6 mRNA was localised primarily to the substantia nigra of the mid brain. The distribution of individual P450s in brain is important in determining the response of the brain to xenobiotics. o This project has also established the presence of several cytochrome P450 enzymes in neoplastic human brain by immunohistochemistry. CYPlAl and CYPIBI were over-expressed in the majority of tumours at high intensity, and almost 50% of tumours exhibited strong CYP3A expression. P450s are important in the aetiology and treatment of various cancers, the over-expression of CYPIBI was demonstrated in the majority of astrocytomas investigated, with no concomitant expression observed in the adjacent normal tissue. Data are emerging that CYPIBI has the capacity to metabolise a variety of putative human carcinogens, including polycyclic aromatic hydrocarbons and heterocyclic amines. In addition, preliminary findings from our group suggest CYPIBI may metabolise key chemotherapeutic drugs. This observations highlights CYPIBI as a possible target for gene therapy and CYPIB1 activated anti-cancer drugs. The major mechanism of induction of members of the CYPl gene family is via the Ah receptor (AhR) complex. To obtain a clearer picture of the mechanisms involved in the regulation of the CYPIBI gene in the brain, five glioma cell lines were investigated, only one of the five (MOG-G-CCM) exhibited constitutive AhR mRNA expression. The lack of the AhR mRNA expression which is the main mechanism of induction of members of the CYPl gene family paralleled the lack of cytochrome P450 gene expression in four out of the five cell lines. A key finding of this study suggests that, although the glioma cell line (MOG-G- CCM) may constitutively express CYPIBI mRNA, the presence of an inducing agent is required for subsequent protein expression. This finding requires further examination to investigate the factors involved in the regulation of the CYPIBI gene. It may be possibly that CYPIBI mRNA is present in normal tissue, and that translation/production of the accompanying protein is usually repressed suggesting that the expression of CYPIBI protein observed in tumour tissue may be due to derepression.