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...

Cytochrome P450 (P450)-dependent metabolism of all-transretinoic acid (atRA) is important for the expression of its biological activity. Because the human P450s involved in the formation of the principal atRA metabolites have been only partially identified, the purpose of this study was to identify the human P450s involved in atRA metabolism. The use of phenotyped human liver microsomes (n ϭ 16) allowed the identification of the following P450s: 2B6, 2C8, 3A4/5, and 2A6 were involved in the formation of 4-OH-RA and 4-oxo-RA; 2B6, 2C8, and 2A6 correlated with the formation of 18-OH-RA; and 2A6, 2B6, and 3A4/5 activities correlated with 5,6-epoxy-RA formation (30-min incubation, 10 M atRA, HPLC separation, UV detection 340 nm). The use of 15 cDNA-expressed human P450s from lymphoblast microsomes, showed the formation of 4-OH-RA by CYP3A7 Ͼ CYP3A5 Ͼ CYP2C18 Ͼ CYP2C8 Ͼ CYP3A4 Ͼ CYP2C9, whereas the 18-OH-RA formation involved CYPs 4A11 Ͼ 3A7 Ͼ 1A1 Ͼ 2C9 Ͼ 2C8 Ͼ 3A5 Ͼ 3A4 Ͼ2C18. Kinetic studies identified 3A7 as the most active P450 in the formation of three of the metabolites: for 4-OH-retinoic acid, 3A7 showed a V max /K m of 127.7, followed by 3A5 (, and 4A11 (V max /K m ϭ 1.9); for 4-oxo-RA, 3A7 showed a V max /K m of 13.4, followed by a 10-fold lower activity for both 2C18 and 4A11 (V max /K m ϭ 1.2); and for 18-OH-RA, 3A7 showed a V max /K m of 10.5 compared with a V max /K m of 2.1 for 4A11 and 2.0 for 2C8. 5,6-Epoxy-RA was only detected at high substrate concentrations in this system (Ͼ10 M), and P450s 2C8, 2C9, and 1A1 were the most active in its formation. The use of embryonic kidney cells (293) stably transfected with human P450 cDNA confirmed the major involvement of P450s 3A7, 1A1, and 2C8 in the oxidation of atRA, and to a lesser extent, 1A2, 2C9, and 3A4. In conclusion, several human P450s involved in atRA metabolism have been identified, the expression of which was shown to direct atRA metabolism toward the formation of specific metabolites. The role of these human P450s in the biological and anticancer effects of atRA remains to be elucidated.Retinoids (vitamin A and its derivatives, which include atRA) play a central role in embryogenesis, vertebrate development, differentiation, and homeostasis (Gudas et al., 1994), and have been used in the prevention and the treatment of certain types of cancer (Hong and Itri, 1994). In humans, retinoids are obtained in the diet in the form of carotenoids (provitamin A), or preformed retinoids. Retinol (vitamin A) is the major retinoid absorbed after complex metabolic reactions in the intestines, and is stored in esterified form in the liver. After ester hydrolysis, retinol is then transported by a plasma protein (retinol binding protein) to the tissues where it can exert its activity. A small fraction of the plasma retinoids is available in the form of atRA bound to albumin, which is rapidly taken up by tissues (Blaner and Olson, 1994). The conversion of retinol to retinal by the P450s is considered to be the rate-limiting step for the biosynthesis of atR...