CYP3A isoform? are responsible for the biotransformation of a wide variety of exogenous chemicals and endogenous steroids in human tissues. Two members of the CYP3A subfamily display developmentally regulated expression in the liver; CYP3A7 is expressed in the fetal liver, whereas CYP3A4 is the major cyrochrome P-450 isoform present in the adult liver. To gain insight into the descriptive ontogenesis of CYP3A isoforms during the neonatal period, we have developed several approaches to explore a neonatal liver bank. Although CYP3A4 and CYP3A7 are structurally closely related, they differ in their capacity to carry out monooxygenase reactions. We have cloned CYP3A4 and CYP3A7 and established stable transfectants in Ad293 cells to investigate their substrate specificities. The 16a hydroxylation of dehydroepiandrosterone is catalyzed by both proteins, but CYP3A7 has a higher affinity and maximal velocity than CYP3A4. Conversely, the conversion of testosterone into its 6p derivative is essentially supported by CYP3A4. We used these two probes to determine the ontogenic evolution at the protein level; CYP3A7 was very active in the fetal liver and its activity was maximal during the first week following birth before to progressively decline and reached a very low level in adult livers. Conversely, the activity of CYP3A4 was extremely weak in the fetus and began to raise after birth to reach 30-40% of the adult activity after one month.CYP3A4 RNA accumulation displays a similar pattern of evolution; when probed with an oligonucleotide, its concentration increased rapidly after birth to reach a plateau as soon as the first week of age.These data supports the assumption that CYP3A4 expression is transcriptionally activated during the first week after birth and is accompanied by a simultaneous decrease of CYP3A7 expression, in such a way that the overall CYP3A protein content and the level of pentoxyresorufin dealkylase catalyzed by the two proteins remain nearly constant.Keywords: ontogenesis ; human liver; CYP3A7, CYP3A4 ; dehydroepiandrosterone.The cytochrome P-450 (P-450) gene superfamily encodes a group of hemoproteins that mostly catalyze the oxidative metabolism of hydrophobic endogenous compounds such as steroids, fatty acids, prostaglandins and exogenous chemicals including drugs, carcinogens and environmental pollutants. These substrates can be converted in presence of NADPH and molecular oxygen either to inert polar metabolites, further eliminated in a water-soluble form, or to cytotoxic or carcinogenic derivatives. One feature of the P-450 system is its capacity to act on thousands of compounds and to carry out a multiplicity of reactions, generically termed monooxygenations. To cope with the large number of substrates and the wide diversity of their chemical structures, several isoforms of P-450 coexist [l, 21 and exhibit overlapping substrate specificities (for review, see [3]). In the human liver, one can consider that the major P-450 isoforms are members of the CYP2C and CYP3A subfamilies. They are constitu...

Cytochromes P-450 are responsible for the biotransformation of drugs and other hydrophobic molecules by the liver. Several isoforms coexist which display an asynchronous onset during the perinatal period suggesting the involvement of multiple mechanisms of regulation. In this paper, we have shown that the CYP2E1 protein and its associated activity could not be detected in the fetal liver and rise during the first few hours following birth independently of the gestational age (between 25 -40 weeks). During this period, the CYP2E1 RNA content remains fairly low: the stabilization of the low amount of existing CYP2E1 protein by endogenous ketone bodies could explain the early neonatal rise of the protein level. From 1 month to 1 year, the protein content gradually increases and is accompanied by the accumulation of CYP2E1 RNA, suggesting a transcriptional activation of the gene during the late neonatal period.We examined the methylation status of CpG residues in the 5' flanking region, first exon and first intron of CYP2El gene cleaved with HpIIIMspI. Genomic DNA from fetal liver shows several hypermethylated spots in the first-exon -first-intron region, which progressively disappear in neonatal samples. We conclude that during the neonatal period, the accumulation of hepatic CYP2E1 RNA is correlated with the degree of methylation at the 5' end of the CYP2EI gene.Keywords: human liver; CYP2EI ; hypermethylation ; neonatal period; regulation.The cytochrome P-450 (P-450) gene superfamily (CUP) encodes a group of hemoproteins that mostly catalyze the oxidative metabolism (or monooxygenation) of hydrophobic endogenous compounds like steroids, fatty acids, prostaglandins and exogenous chemicals including drugs, carcinogens and environmental pollutants [I]. These substrates can be converted either to inert polar metabolites further eliminated in a water-soluble form or to cytotoxic or carcinogenic derivatives [2]. CYP2El is a member of this superfamily mainly expressed in the human liver and to a minor extent in the lung [3]; traces of CYP2E1 have also been detected in the small intestine [4] and in the brain [S]. CYP2E1 catalyzes the metabolic activation of chemicals presenting a wide structural diversity, including aromatic compounds, benzene, N-nitrosodimethylamine, halogenated alkanes and others low-molecular-mass compounds which have toxicological implications in humans [6-81. It has also been implicated in the metabolism of drugs and halogenated anesthetics, like acetaminophen, isoniazid, halothane and enflurane 19, 101.Additionally, CYP2E1 is involved in the metabolism of alcohols, aldehydes, ketones and plays a key role in gluconeogenesis from endogenous ketone bodies released in situations involving hormonal or metabolic changes associated with energy deprivation [ 1 1 -131.A general feature of the monooxygenase system is its ability to be activated by exogenous compounds. In this respect, the Enzymes. Cytochrome P-450 (EC 1.14.14.1); glucose-6-phosphate dehydrogenase (EC 1.1.1.49). CYP2E1 content is increased b...