Expression Cloning of an Oxysterol 7α-Hydroxylase Selective for 24-Hydroxycholesterol

The synthesis of 7α-hydroxylated bile acids from oxysterols requires an oxysterol 7α-hydroxylase encoded by theCyp7b1 locus. As expected, mice deficient in this enzyme have elevated plasma and tissue levels of 25- and 27-hydroxycholesterol; however, levels of another major oxysterol, 24-hydroxycholesterol, are not increased in these mice, suggesting the presence of another oxysterol 7α-hydroxylase. Here, we describe the cloning and characterization of murine and human cDNAs and genes that encode a second oxysterol 7α-hydroxylase. The genes contain 12 exons and are located on chromosome 6 in the human (CYP39A1locus) and in a syntenic position on chromosome 17 in the mouse (Cyp39a1 locus). CYP39A1 is a microsomal cytochrome P450 enzyme that has preference for 24-hydroxycholesterol and is expressed in the liver. The levels of hepatic CYP39A1 mRNA do not change in response to dietary cholesterol, bile acids, or a bile acid-binding resin, unlike those encoding other sterol 7α-hydroxylases. Hepatic CYP39A1 expression is sexually dimorphic (female > male), which is opposite that of CYP7B1 (male > female). We conclude that oxysterol 7α-hydroxylases with different substrate specificities exist in mice and humans and that sexually dimorphic expression patterns of these enzymes in the mouse may underlie differences in bile acid metabolism between the sexes. The synthesis of 7α-hydroxylated bile acids from oxysterols requires an oxysterol 7α-hydroxylase encoded by theCyp7b1 locus. As expected, mice deficient in this enzyme have elevated plasma and tissue levels of 25- and 27-hydroxycholesterol; however, levels of another major oxysterol, 24-hydroxycholesterol, are not increased in these mice, suggesting the presence of another oxysterol 7α-hydroxylase. Here, we describe the cloning and characterization of murine and human cDNAs and genes that encode a second oxysterol 7α-hydroxylase. The genes contain 12 exons and are located on chromosome 6 in the human (CYP39A1locus) and in a syntenic position on chromosome 17 in the mouse (Cyp39a1 locus). CYP39A1 is a microsomal cytochrome P450 enzyme that has preference for 24-hydroxycholesterol and is expressed in the liver. The levels of hepatic CYP39A1 mRNA do not change in response to dietary cholesterol, bile acids, or a bile acid-binding resin, unlike those encoding other sterol 7α-hydroxylases. Hepatic CYP39A1 expression is sexually dimorphic (female > male), which is opposite that of CYP7B1 (male > female). We conclude that oxysterol 7α-hydroxylases with different substrate specificities exist in mice and humans and that sexually dimorphic expression patterns of these enzymes in the mouse may underlie differences in bile acid metabolism between the sexes. kilobase pair(s) Two metabolic pathways that differ in their initial steps produce 7α-hydroxylated bile acids (1.Schwarz M. Lund E.G. Russell D.W. Curr. Opin. Lipidol. 1998; 9: 1-6Crossref Scopus (86) Google Scholar). In one pathway, cholesterol (5-cholesten-3β-ol) is first converted into 7α-hydroxycholesterol (cholest-5-ene-3β,7α-diol) by the enzyme cholesterol 7α-hydroxylase, which is encoded by the Cyp7a1 gene in mice. In the other pathway, cholesterol is first converted into one of several oxysterols prior to being 7α-hydroxylated by oxysterol 7α-hydroxylase, which is encoded by the Cyp7b1 gene. The 7α-hydroxylated intermediates produced by these different initiating steps are subsequently converted into primary bile acids by a series of shared enzymes in the liver (2.Russell D.W. Setchell K.D.R. Biochemistry. 1992; 31: 4737-4749Crossref PubMed Scopus (659) Google Scholar). Cholesterol 7α-hydroxylase shows a marked preference for cholesterol as a substrate and is only weakly active against other sterols (3.Ogishima T. Deguchi S. Okuda K. J. Biol. Chem. 1987; 262: 7646-7650Abstract Full Text PDF PubMed Google Scholar), whereas oxysterol 7α-hydroxylase prefers 25-hydroxycholesterol (cholest-5-ene-3β,25-diol) and 27-hydroxycholesterol (cholest-5-ene-3β,27-diol) (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, 5.Martin K.O. Reiss A.B. Lathe R. Javitt N.B. J. Lipid Res. 1997; 38: 1053-1058Abstract Full Text PDF PubMed Google Scholar). In agreement with the latter preference, Cyp7b1 knockout mice accumulate these two oxysterols in their plasma and tissues (6.Li-Hawkins J. Lund E.G. Turley S.D. Russell D.W. J. Biol. Chem. 2000; 275: 16536-16542Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). The levels of the other major oxysterol, 24-hydroxycholesterol (cholest-5-ene-3β,24-diol), are near normal in these animals (6.Li-Hawkins J. Lund E.G. Turley S.D. Russell D.W. J. Biol. Chem. 2000; 275: 16536-16542Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). In contrast, a human with a complete absence of oxysterol 7α-hydroxylase activity accumulated 24-hydroxycholesterol as well as 25- and 27-hydroxycholesterol in his plasma (7.Setchell K.D.R. Schwarz M. O'Connell N.C. Lund E.G. Davis D.L. Lathe R. Thompson H.R. Tyson R.W. Sokol R.J. Russell D.W. J. Clin. Invest. 1998; 102: 1690-1703Crossref PubMed Scopus (294) Google Scholar). These observations suggest that the human enzyme, unlike the mouse enzyme, may act on all three oxysterol substrates. The observed differences in the oxysterol accumulation patterns in mice and humans that express no oxysterol 7α-hydroxylase are not due to differences in the biosynthesis of oxysterols, as cholesterol 24-hydroxylase, cholesterol 25-hydroxylase, and sterol 27-hydroxylase are present in both species (8.Lund E.G. Guileyardo J.M. Russell D.W. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7238-7243Crossref PubMed Scopus (528) Google Scholar, 9.Lund E.G. Kerr T.A. Sakai J. Li W.-P. Russell D.W. J. Biol. Chem. 1998; 273: 34316-34348Abstract Full Text Full Text PDF PubMed Scopus (253) Google Scholar, 10.Rosen H. Reshef A. Maeda N. Lippoldt A. Shpizen S. Triger L. Eggertsen G. Björkhem I. Leitersdorf E. J. Biol. Chem. 1998; 273: 14805-14812Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar). The sequence of cholesterol 24-hydroxylase is the most highly conserved between mice and humans (95% identity), and in both, expression of the enzyme is limited to the brain (8.Lund E.G. Guileyardo J.M. Russell D.W. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7238-7243Crossref PubMed Scopus (528) Google Scholar). This preservation of expression and form suggests that 24-hydroxycholesterol and 24-hydroxylase play important physiological roles in mammals, perhaps participating in the turnover of cholesterol in the central nervous system (11.Lutjohann D. Breuer O. Ahlborg G. Nennesmo I. Siden A. Diczfalusy U. Björkhem I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 9799-9804Crossref PubMed Scopus (569) Google Scholar). To determine why Cyp7b1 −/− mice do not accumulate 24-hydroxycholesterol, we examined oxysterol 7α-hydroxylase activity in the livers of knockout animals. A hepatic activity was detected that produced 7α-hydroxylated 24-hydroxycholesterol. Here, we report these results and the isolation by expression cloning (9.Lund E.G. Kerr T.A. Sakai J. Li W.-P. Russell D.W. J. Biol. Chem. 1998; 273: 34316-34348Abstract Full Text Full Text PDF PubMed Scopus (253) Google Scholar) of murine and human cDNAs encoding this new enzyme activity. Total liver protein homogenates were prepared, and oxysterol 7α-hydroxylase activities were measured as described (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). The concentration of oxysterol substrate was 1 μm in the reaction. 25-3HHydroxycholesterol (81.5 Ci/mmol) was purchased from NEN Life Science Products. 22R-3HHydroxycholesterol (15 Ci/mmol), 24,25-3Hepoxycholesterol (76 Ci/mmol), and 27-3Hhydroxycholesterol (80 Ci/mmol) were synthesized by NEN Life Science Products. 24-3HHydroxycholesterol (50 Ci/mmol) was synthesized by American Radiolabeled Chemicals (St. Louis, MO). To measure oxysterol 7α-hydroxylase enzyme activity in cultured cells, 60-mm dishes of Chinese hamster ovary cells expressing the middle T antigen of the polyoma virus (CHOP cells (12.Heffernan M. Dennis J.W. Nucleic Acids Res. 1991; 19: 85-92Crossref PubMed Scopus (94) Google Scholar)) (450,000 cells/dish) were transfected with 4.5 μg of the indicated expression vector using LipofectAMINETM (Life Technologies, Inc.) as a transfection reagent. Sixteen h later, 3H-labeled oxysterol substrate dissolved in ethanol was added to a final concentration of 1 μm to the media, and the incubation was continued for an additional 4–24 h. Media were harvested and extracted with 7 ml of chloroform/methanol (2:1, v/v); the organic phases were dried; and sterols were analyzed by thin-layer chromatography (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). A cDNA library was constructed from poly(A)+ mRNA isolated from the livers of threeCyp7b1 −/− male mice in the pCMV5 expression vector (13.Andersson S. Davis D.L. Dahlbäck H. Jörnvall H. Russell D.W. J. Biol. Chem. 1989; 264: 8222-8229Abstract Full Text PDF PubMed Google Scholar). Individual dishes of CHOP cells (450,000 cells/60-mm dish) were transfected with 4.5 μg of cDNA and 0.5 μg of pVA1 using LipofectAMINETM. The pVA1 plasmid contains the adenovirus VA1 gene, whose RNA product stimulates the expression of transfected genes (9.Lund E.G. Kerr T.A. Sakai J. Li W.-P. Russell D.W. J. Biol. Chem. 1998; 273: 34316-34348Abstract Full Text Full Text PDF PubMed Scopus (253) Google Scholar). After 3 h of incubation, the transfection mixture was replaced with 2.5 ml of Dulbecco's minimal essential medium/Ham's F-12 medium (1:1) containing 5% (v/v) fetal calf serum, 100 units/ml penicillin, 100 μg/ml streptomycin, and 1–2 μCi of 24-3Hhydroxycholesterol. After 60 h of incubation at 37 °C, media were harvested, and sterols were analyzed by thin-layer chromatography. 210 primary pools containing ∼100,000 individual cDNAs were screened in the first round. Positive pools were divided into progressively smaller subpools and analyzed by transfection to identify the target cDNA. Gas chromatography-mass spectrometry analyses were performed on a Varian 3400 gas chromatograph equipped with an HP-5MS capillary column (30 m × 0.25 mm, 0.25-mm phase thickness) connected to a Finnigan SSQ700 mass spectrometer. The gas chromatography temperature program was 180 °C for 1 min, followed by a linear gradient of 10 °C/min to 300 °C and a final elution at 300 °C for 15 min. The injector port and transfer line temperatures were maintained at 280 °C, and the injector was operated in the splitless mode. Helium was used as carrier gas at an injector valve pressure of 6 pounds/square inch. The mass spectrometer was operated in the electron ionization mode with electron energy set at 70 eV and an ion source temperature of 150 °C. CHOP cells were transfected with either CYP39A1 or CYP46 expression vectors as described above and incubated with sterols as indicated in the legend to Fig. 3. The cell medium was extracted with chloroform/methanol (2:1, v/v); the aqueous phase was aspirated; and the remaining organic phase was taken to dryness under a stream of nitrogen. Solids were redissolved in 1 ml of toluene and purified on Isolute Silica columns (14.Dzeletovic S. Breuer O. Lund E. Diczfalusy U. Anal. Biochem. 1995; 225: 73-80Crossref PubMed Scopus (477) Google Scholar), with the modifications described in the accompanying paper (6.Li-Hawkins J. Lund E.G. Turley S.D. Russell D.W. J. Biol. Chem. 2000; 275: 16536-16542Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). After to were analyzed by gas chromatography-mass spectrometry with the from to at a of one a human sequence and and an expressed sequence that shared sequence with the murine CYP39A1 cDNA. The expressed sequence was used as a to a human cDNA library cDNAs to the of the CYP39A1 mRNA were isolated in this To a a was as on the sequence of exons the human two were to the of the CYP39A1 gene. The were as and A was performed on human liver cDNA using an system The human CYP39A1 cDNA was into a plasmid vector prior to sequence and expression Total RNA and poly(A)+ RNA were as described (6.Li-Hawkins J. Lund E.G. Turley S.D. Russell D.W. J. Biol. Chem. 2000; 275: 16536-16542Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). and human RNA were purchased from were performed in either or containing m m and (v/v) acid from to of the sequence of the murine CYP39A1 protein was synthesized by as a The was used to two 3 the initial μg of was with complete and the several containing the of antigen in were at of were and the of the was in that from cells transfected with a murine CYP39A1 expression The final the CYP39A1 protein expressed in cells and the protein present in microsomal from murine liver. we measured the levels of oxysterols in the plasma −/− mice. The of oxysterol accumulation in these animals was 25-hydroxycholesterol > 27-hydroxycholesterol 24-hydroxycholesterol (6.Li-Hawkins J. Lund E.G. Turley S.D. Russell D.W. J. Biol. Chem. 2000; 275: 16536-16542Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). To determine the levels of 24-hydroxycholesterol were due to the presence of another enzyme that this oxysterol, tissue homogenates were from the livers of −/− mice and incubated with 24-3Hhydroxycholesterol. were extracted from the mixture and by thin-layer chromatography. A product was that the 24-hydroxycholesterol substrate the homogenates from mice The position to which the on the was with a of this product from Cyp7b1 −/− knockout mice an activity 3 results were from male liver were that the of activity was that in As a the were incubated in the presence and absence of with the substrate This oxysterol was converted into at three from male or mice were in a with 10 and with male mice levels of mice. of of the that (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, 5.Martin K.O. Reiss A.B. Lathe R. Javitt N.B. J. Lipid Res. 1997; 38: 1053-1058Abstract Full Text PDF PubMed Google Scholar). The synthesis of these was not homogenates −/− knockout mice were used 12 and and in this the product was in homogenates from the mice. These were with the presence of an oxysterol in both and knockout mice that a preference for 24-hydroxycholesterol. To cDNAs that encoded this 24-hydroxycholesterol 7α-hydroxylase, a cDNA library was in a expression vector from hepatic mRNA from three Cyp7b1 −/− mice. 210 pools of cDNAs were transfected into CHOP cells, which were subsequently for oxysterol 7α-hydroxylase enzyme activity. cells not the substrate the cDNA several expressed an activity that converted 24-hydroxycholesterol into two which were as 7α-hydroxylated 24-hydroxycholesterol and an additional produced by the of an in the CHOP indicated that the 7α-hydroxylated 24-hydroxycholesterol product first in the transfected cells and that this initial product was subsequently converted into not of the primary product by the transfected cells was the as that in tissue homogenates and The active of cDNAs was and to a cDNA that encoded the enzyme activity 3 and Gas mass spectrometry was used to determine the of the primary sterol product of the enzyme In these or cells were incubated with the indicated sterol and in the media were analyzed by gas chromatography-mass cells not 24-hydroxycholesterol In contrast, cells transfected with the purified cDNA isolated by expression cloning converted 24-hydroxycholesterol into a primary product a in which from the gas chromatography column the The ionization mass of a was with the presence of three on the sterol 3 Two of these three were present on the substrate 3 and and the enzyme expressed in the transfected cells added the chromatography analyses of a that the was located on 7 Fig. we not have to an 7α-hydroxylated 24-hydroxycholesterol with which to the mass of this To this was taken of that cholesterol 24-hydroxylase converted 7α-hydroxycholesterol into a product E.G. Guileyardo J.M. Russell D.W. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7238-7243Crossref PubMed Scopus (528) Google and not We that this product of the cholesterol 24-hydroxylase enzyme have elution on a gas chromatography column and a ionization as a from the The of Fig. 3 that several sterols were produced cholesterol cells were incubated with of these in the gas in Fig. 3 at the position as The mass of was to that of a 3 these that the cDNA isolated in the of Fig. encoded a 7α-hydroxylase that on 24-hydroxycholesterol. of the expression plasmid encoding this activity indicated a cDNA sequence of the a sequence of a of and a sequence of The acid sequence of the enzyme acids and of a microsomal cytochrome a and an located the The sequence was the CYP39A1 L. T. R. O. R.W. D.W. 1996; PubMed Scopus Google Scholar). a human expressed sequence that sequence with the murine cDNA and an human sequence and which to the gene. of the murine cDNA with human cDNAs isolated by a liver cDNA library with the expressed sequence and of these cDNAs with the human sequence produced the acid sequence of the human enzyme A human CYP39A1 cDNA expressed in cells produced an enzyme activity that converted 24-hydroxycholesterol into a 7α-hydroxylated product not with the in the a of the of the gene A of the human gene is in Fig. of indicated that the human gene is located on chromosome to the whereas the murine gene is located in a syntenic position on chromosome to the not The of the human gene with that for the sequence in the The mouse and human CYP39A1 acid were and These were most to the oxysterol 7α-hydroxylase encoded by the Cyp7b1 gene to a the cholesterol 7α-hydroxylase encoded by the Cyp7a1 gene The sequence between CYP39A1 and CYP7B1 and the that are both oxysterol 7α-hydroxylases their substrate To this the murine CYP7B1 and CYP39A1 cDNAs were expressed in cultured cells, and the of the transfected cells to oxysterols was The of that cells of the different oxysterols in these In contrast, cells transfected with the CYP7B1 cDNA all with a of 25-hydroxycholesterol > 27-hydroxycholesterol > > > 24-hydroxycholesterol expressing the CYP39A1 cDNA 24-hydroxycholesterol or no activity the other oxysterols These suggest that the CYP7B1 oxysterol 7α-hydroxylase oxysterol substrate whereas the CYP39A1 oxysterol 7α-hydroxylase was to 24-hydroxycholesterol. The tissue and of the murine CYP39A1 mRNA were examined by RNA of tissues a species of that was present only in the liver 6 of mRNA from 12 human tissues using a CYP39A1 cDNA a mRNA with a expression not mice with cholesterol, cholesterol or not change the levels of hepatic CYP39A1 mRNA to those in normal animals 6 not change the levels of hepatic CYP39A1 mRNA in mice the CYP7B1 oxysterol 7α-hydroxylase 6 A final series of the sexually dimorphic expression of the CYP39A1 oxysterol 7α-hydroxylase at As by the in Fig. 6 the of this mRNA in mice was in male liver. with an against the protein an increased protein in 6 middle as enzyme activity 6 These were −/− mice, that of the CYP7B1 oxysterol 7α-hydroxylase was not for by increased expression of the CYP39A1 oxysterol 7α-hydroxylase This the cloning of murine and human cDNAs encoding an oxysterol 7α-hydroxylase with for 24-hydroxycholesterol. These enzymes contain acids, sequence and a new of cytochrome P450 The human CYP39A1 gene contains 12 exons and of on chromosome 6 in the of the human antigen whereas the mouse is located in a position on chromosome The expression of CYP39A1 to limited to the liver and is sexually dimorphic in the with levels of CYP39A1 mRNA and enzyme activity in on that 24-hydroxycholesterol is converted by the liver into sterols that are most bile acids I. D. Breuer O. A. A. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar, I. D. Diczfalusy U. L. Ahlborg G. J. J. Lipid Res. 1998; Full Text Full Text PDF PubMed Google Scholar, T.A. PubMed Scopus Google Scholar) and the of from this oxysterol in transfected cells expressing a CYP39A1 cDNA Fig. we that the physiological of this enzyme is to bile The to three the of sterol 7α-hydroxylases that in the synthesis of bile are P450 of the Cholesterol 7α-hydroxylase S. Russell D.W. J. Biol. Chem. Full Text PDF PubMed Google Scholar) and the CYP39A1 oxysterol 7α-hydroxylase are expressed only in the whereas the expression of the CYP7B1 oxysterol 7α-hydroxylase is and between species (1.Schwarz M. Lund E.G. Russell D.W. Curr. Opin. Lipidol. 1998; 9: 1-6Crossref Scopus (86) Google Scholar, G. M. M. Lathe R. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, D.W. H. I. S. M. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, J. O. J. 1995; PubMed Scopus Google Scholar, K.O. Javitt N.B. J. Lipid Res. 1999; Full Text Full Text PDF PubMed Google Scholar). These three sterol 7α-hydroxylases sequence and are at a of their gene and suggests that may have by two different The human cholesterol 7α-hydroxylase and CYP7B1 oxysterol 7α-hydroxylase genes have and are on chromosome (7.Setchell K.D.R. Schwarz M. O'Connell N.C. Lund E.G. Davis D.L. Lathe R. Thompson H.R. Tyson R.W. Sokol R.J. Russell D.W. J. Clin. Invest. 1998; 102: 1690-1703Crossref PubMed Scopus (294) Google Scholar, M. Russell D.W. 1992; PubMed Scopus Google Scholar), that may from an In contrast, the human CYP39A1 oxysterol 7α-hydroxylase gene contains 12 exons and is located on chromosome which suggests that this gene of the other the of these three enzymes in the may the marked of bile acids in different A of the CYP39A1 oxysterol 7α-hydroxylase is present in the and enzyme activity suggest that and other contain the CYP7B1 oxysterol 7α-hydroxylase (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). the sequence between the three sterol 7α-hydroxylases of bile acid the enzymes have different substrate Cholesterol 7α-hydroxylase cholesterol, whereas the CYP7B1 enzyme on 25- and 27-hydroxycholesterol (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, 5.Martin K.O. Reiss A.B. Lathe R. Javitt N.B. J. Lipid Res. 1997; 38: 1053-1058Abstract Full Text PDF PubMed Google Scholar), and the CYP39A1 enzyme prefers 24-hydroxycholesterol is important to that these are and not as the two oxysterol 7α-hydroxylases different oxysterols at in tissue homogenates or expressed in cultured cells the or exist in is by the accumulation of 25- and not 24-hydroxycholesterol, in mice the CYP7B1 oxysterol 7α-hydroxylase (6.Li-Hawkins J. Lund E.G. Turley S.D. Russell D.W. J. Biol. Chem. 2000; 275: 16536-16542Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). This that mice with a in the gene may accumulate 24-hydroxycholesterol, not the other two The accumulation of all three oxysterols in a human the CYP7B1 oxysterol 7α-hydroxylase (7.Setchell K.D.R. Schwarz M. O'Connell N.C. Lund E.G. Davis D.L. Lathe R. Thompson H.R. Tyson R.W. Sokol R.J. Russell D.W. J. Clin. Invest. 1998; 102: 1690-1703Crossref PubMed Scopus (294) Google Scholar) may a substrate of this enzyme, a in the expression of the CYP39A1 enzyme, or a of hepatic by in this that only a with this has and that the of other may The of 24-hydroxycholesterol in the mouse (8.Lund E.G. Guileyardo J.M. Russell D.W. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7238-7243Crossref PubMed Scopus (528) Google Scholar), I. D. Breuer O. A. A. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar), and human I. D. Diczfalusy U. L. Ahlborg G. J. J. Lipid Res. 1998; Full Text Full Text PDF PubMed Google Scholar) is in the The brain is the most tissue in unlike cholesterol with (11.Lutjohann D. Breuer O. Ahlborg G. Nennesmo I. Siden A. Diczfalusy U. Björkhem I. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 9799-9804Crossref PubMed Scopus (569) Google Scholar). The synthesis of 24-hydroxycholesterol is to a by which cholesterol is to the of the and central nervous system I. D. Diczfalusy U. L. Ahlborg G. J. J. Lipid Res. 1998; Full Text Full Text PDF PubMed Google Scholar). The presence of an oxysterol 7α-hydroxylase with for 24-hydroxycholesterol in the liver suggests that the major by which the cholesterol is the liver into the A metabolic for which is produced in the A. O. D. J. D. Diczfalusy U. Björkhem I. J. Lipid Res. 1999; Full Text Full Text PDF PubMed Google Scholar) and converted to bile acids in the liver by the CYP7B1 oxysterol 7α-hydroxylase. The that conserved synthesis and pathways exist for oxysterols their roles as of cholesterol The three sterol 7α-hydroxylase genes of bile acid metabolism are The cholesterol 7α-hydroxylase gene is to by bile acids and to by cholesterol in mice and S. Russell D.W. J. Biol. Chem. Full Text PDF PubMed Google Scholar). from the CYP7B1 oxysterol 7α-hydroxylase gene is only by bile acids (4.Schwarz M. Lund E.G. Lathe R. Björkhem I. Russell D.W. J. Biol. Chem. 1997; 272: 23995-24001Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar), and the levels of CYP39A1 oxysterol 7α-hydroxylase mRNA do not change in response to bile acids, cholesterol, or bile acid-binding 6 CYP39A1 is in mice. whereas is in male mice 1 and dimorphic expression patterns of the CYP7B1 and CYP39A1 oxysterol with their substrate may underlie differences in bile acid metabolism that exist between male and mice R. M. 1997; Full Text PDF Scopus Google Scholar, S.D. Schwarz M. J.M. 1998; PubMed Scopus Google Scholar). We M. and L. Davis for for for of cytochrome P450 and and for of the