Minireview Vol. 266, No. 21. Issue of July 25, pp. 13469-13472, 1991 THE JOURNAL OF BIOLOGICAL CHEMISTRY

0 1991 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.

Cytochrome P-450 MULTIPLICITY OF ISOFORMS, SUBSTRATES, AND CATALYTIC AND REGULATORY MECHANISMS*

Todd D. Porter and Minor J. Coon

From the Department of Biological Chemistry, Medical School, University of Michigan, A n n Arbor, Michigan 481 09

The carbon monoxide-binding pigment of liver microsomes (1, 2 ) was shown over 25 years ago to be a hemoprotein of the b type (3) and, as judged by the photochemical action spectrum, to be involved in the oxidation of drugs and steroids (4). The solubi- lization and resolution of the components of this enzyme system from microsomal membranes and the reconstitution of an active complex containing cytochrome P-450, NADPH-cytochrome P- 450 reductase, and phosphatidylcholine (5 , 6) permitted the pu- rification and thorough characterization of these constituents. What is now known to be the cytochrome P-450 gene superfamily encodes numerous enzymes that are remarkable in the variety of chemical reactions catalyzed and in the number of substrates attacked (7-9). Indeed, it is no exaggeration to state that P-450 is the most versatile biological catalyst known. Considering the rapid progress that has been made in recent years in the charac- terization of over 150 isoforms, it may seem surprising that P- 450, a name first used to describe a red pigment having a reduced CO-difference spectrum with a major band at an unusually long wavelength (about 450 nm) (3), has not been replaced by a terminology based on function. Even the term cytochrome is unsuitable, since in most instances P-450 acts as an oxygenase rather than simply as an electron carrier. Since many of the individual P-450s catalyze multiple reactions, the usual method of naming enzymes is inadequate for this group of heme proteins, and a systematic nomenclature has been devised based on struc- tural homology (lo).’ It may be noted in this connection that chloroperoxidase and P-450 exhibit some physicochemical and catalytic similarities (11) but have no antigenic determinants in common (12). The aims of this review are to summarize briefly our current knowledge of the function, structure, mechanism of action, and regulation of this interesting group of catalysts and t o describe recent progress in several areas where much still remains to be learned.

Reactions Catalyzed Our knowledge of the scope of P-450-catalyzed reactions is still

incomplete, as this cytochrome is widespread in nature and many isoforms have yet to be fully characterized or even identified.

National Institutes of Health and Grant “06221 from the National Institute * Research in this laboratory was supported by Grant DK-10339 from the

on Flcohol Abuse and Alcoholism.

evolving as more P-450s are characterized but may be described in general as The systematic nomenclature based on structural homology (10) is still

follows. Those P-450 proteins with 40% or greater sequence identity are

greater than 55% identity are then included in the same subfamily (designated included in the same family (designated by an Arabic number), and those with

by a capital letter). Presently there are 27 families, of which 10 exist in all mammals. The individual genes are arbitrarily assigned numbers. For example, the major phenobarbital-inducible cytochrome in rabbit liver microsomes, orig- inally called P-450LM?, or form 2, is assigned to family 2 and subfamily B, and the gene and the enzyme are designated CYP2B4 and CYP2B4, respectively; the enzyme may also he called P-450 2B4. Other examples from rahhit liver are 3-methylcholanthrene-inducible form 4 (now 1A2), ethanol-inducible form 3a (now 2E1). antihiotic-inducible form 3c (now 3A6), and constitutive form 3b

identical or highly similar P-450s are easily recognizable, regardless of the (now 2C3). The main advantage of the unified nomenclature is that structurally

source (species, tissue, or organelle), the inducer, or the catalytic activity examined.

Animals, plants, and microorganisms contain P-450, and in mam- mals the enzyme system has been found in all tissues examined. P-450 is found predominantly in the endoplasmic reticulum and mitochondria, and in greatest abundance in the liver. As shown in Table I, the substrates for cytochrome P-450 encompass a host of xenobiotics, including substances that occur biologically but are foreign to animals, such as antibiotics and unusual compounds in plants, as well as synthetic organic chemicals, and a variety of steroids and other physiologically occurring lipids (7-9, 13). The number of man-made “environmental chemicals” has been esti- mated as greater than 200,000, most of which are thought to be potential substrates for P-450; many may also serve as inducers or inhibitors of various isoforms. Given the possibilities for syn- thetic modification of new and existing drugs and xenobiotics, one cannot put an upper limit on the number of compounds acted on by this enzyme family.

Most of the reactions begin with the transfer of electrons from NAD(P)H to either NADPH-cytochrome P-450 reductase in the microsomal system or a ferredoxin reductase and a nonheme iron protein in the mitochondrial and bacterial systems, and then to cytochrome P-450; this leads to the reductive activation of mo- lecular oxygen followed by the insertion of one oxygen atom into the substrate. The reactions that have been demonstrated include hydroxylation, epoxidation, peroxygenation, deamination, desul- furation, and dehalogenation, as well as reduction. Most of the substrates are lipophilic, and there is no evidence that charge interactions contribute to binding by the cytochrome. Some of the transformations are essential for life, as with the conversion of cholesterol to corticoid and sex hormones, and others, partic- ularly with xenobiotics, lead to the formation of more polar compounds that are more readily excreted directly or after con- jugation with water-soluble agents such as glucuronic acid and glutathione (14). This is usually a detoxication process, but in some instances foreign compounds are converted to products with much greater cytotoxicity, mutagenicity, or carcinogenicity. Some of the P-450s, such as those involved in steroid transformations, are fairly selective in their choice of substrates, whereas other P- 450s, particularly those in liver microsomes, have unusually broad and overlapping substrate specificity.

Mechanism of Oxygen and Peroxide Activation The active site of P-450 contains iron protoporphyrin IX bound

in part by hydrophobic forces. The fifth ligand is a thiolate anion provided by a cysteine residue, a feature that contributes to the unusual spectral and catalytic properties of P-450, and the sixth coordination position may be occupied by an exchangeable water molecule. Upon reduction of the iron, 0, (or, in a competitive fashion, CO) can be bound in the sixth position.

The scheme in Fig. 1, which is modified from an earlier version (15), is in accord with findings in a number of laboratories and with the known stoichiometry of the hydroxylation reaction, where RH represents the substrate.

RH + 0 2 + NADPH + H’ + ROH + HpO + NADP+

The first step in the reaction cycle is substrate binding, which perturbs the spin state equilibrium of the cytochrome and facili- tates uptake of the first electron. Substrates that undergo reduc- tion rather than oxygenation, such as epoxides, N-oxides, nitro and azo compounds, and lipid hydroperoxides, accept two elec- trons in a stepwise fashion as shown, to give RH(H),. To initiate the oxidative reactions, 0, is bound to the ferrous P-450 with coordination to iron trans to thiolate. This intermediate can also be written as the resonance form, Fe”’(O;), with substrate still present. Transfer of the second electron then occurs, with the possible involvement of cytochrome bs as an additional electron donor in mammalian microsomal systems (16, 17). The next step

13469

13470 Minireview: Cytochrome P-450 TABLE I

Substrates for cytochrome P-450

Xenob1otics Physiologically occurring comwunds

Drugs, including antibiotics Steroids Carcinogens Eicosanoids Antioxidants Fatty acids Solvents Lipid hydroperoxides Anesthetics Retinoids Dyes Acetone, acetol Pesticides Petroleum products Alcohols Odorants

ROH RH

I XOH XOOH 1 & RHCFe-O)3+

I

I

" * -,"

1

'

FIG. 1. Scheme for mechanism of action of P-450. Fe represents the heme iron atom at the active site, RH the substrate,RH(H), a reduction product, ROH a monooxygenation product, and XOOH a peroxy compound that can serve as an alternative oxygen donor.

is not well understood but involves splitting of the oxygen-oxygen bond with the uptake of two protons at some stage and the generation of an "activated oxygen," perhaps an iron-oxene spe- cies, and the release of HzO. Several resonance forms are possible for the active oxygen intermediate, considering the redox possi- bilities with the sulfur, iron, and oxygen atoms. Oxygen insertion into the substrate is believed to involve hydrogen abstraction from the substrate and recombination of the resulting transient hydroxyl and carbon radicals to give the product (18). Dissocia- tion of ROH then restores the P-450 to the starting ferric state. Also shown is the way in which a peroxy compound may substi- tute for O2 and reducing equivalents in what is termed the peroxide shunt. Homolytic cleavage is envisioned with the for- mation of an iron-bound hydroxyl radical and an alkoxy radical (XO') capable of hydrogen abstraction from the substrate (15). Thus, oxygenation by O2 and by peroxy compounds has some common mechanistic features. Although it is not clear what role peroxy compounds play in viuo, they have proved useful in gen- erating "activated oxygen" intermediates for spectral (19) and EPR analysis (20).

Much remains to be learned about the factors controlling regio- and stereospecificity in P-450-catalyzed reactions, as well as the dentity of the powerful oxidant that is necessary for oxygen insertion into those substrates without activating groups at or near the position attacked, as in fatty acid o-hydroxylation. Some particularly interesting variations on the scheme shown are the proposal of a cage radical mechanism for the rearrangement of a prostaglandin endoperoxide to a prostacyclin and a thromboxane (21), of radical intermediates in dehydrogenation reactions (22), and of aminium radical intermediates in amine oxidations (23). The unusual catalytic properties of P-450 contribute to the reg- ulation of its own activities, as with competitive inhibition by alternate substrates, mechanism-based inactivation, and the ef- fects of a variety of effectors and other lipophilic substances (24).

2kll U1R 3Al

pfl I A l 2c12 Sh8 1 A 2 1h2 2D9 I A 1

1111 W l Ml aut

281 2E1 1lA1 282 2til 1161

2BZ lAln ZC12 3 M

ZC7 2HZ 17 m ll&l 2Cl1 3A112 PIA1

FIG. 2. Multiplicity in the regulation of cytochrome P-450 expres- sion. P-450 designations (shown at the bottom of the figure) are as described in Nebert et al. (10) and are based on selected examples from various mam- malian s cies. mRNA stabilization was inferred from transcriptional and messa e E e l studies, rather than measured directly, but may in some instances actualfy result from enhanced precursor processing, as shown for P-450 1A2 (41).

Regulation of Expression Consonant with the multiplicity of P-450 cytochromes is the

considerable diversity in the mechanisms of regulation of these enzymes, as depicted schematically in Fig. 2. Not surprisingly, the most common means of regulation is transcriptional. Post- transcriptional mechanisms include mRNA stabilization and pro- tein stabilization or degradation that may be mediated through changes in the phosphorylation state of the enzyme. Moreover, many P-450s are subject to tissue-specific patterns of expression, with resulting differences in isoform compositions and activities in various tissues. Several recent, in-depth reviews of these topics are available (25, 26).

The most extensively characterized P-450 with regard to reg- ulation is P-450 1Al (27), a member of the PAH2-inducible gene family. This is the only P-450 for which a receptor-mediated mechanism of induction has been clearly demonstrated, via the Ah or TCDD receptor (28). The 5'-flanking region of the gene for this P-450 contains several short sequence motifs, termed xenobiotic responsive elements, or XREs, that function as tran- scriptional enhancers when Ah receptor ligands, such as TCDD, are added to Ah-responsive cells in culture (29). These XREs bear some resemblance to the glucocorticoid-responsive element, an interesting finding in light of the apparent similarity between the Ah and glucocorticoid receptors (30,31). Evidence that these sequences bind a trans-acting factor, presumably the ligand- bound Ah receptor, has been provided by a variety of in vitro studies (29, 31-33). In addition to these enhancer elements, several transcription factor recognition sequences have been de- fined in the promoter region of CYPlAl(34), as well as a possible repressor binding site (35), and, surprisingly, a glucocorticoid- responsive element in the first intron (36).

In contrast to P-450 1A1, much less is known about the mechanism of regulation of P-450 1A2, a closely related isoform. Although the latter P-450 is strongly induced by PAHs, suggest- ing a role for the Ah receptor, the 1A2 gene lacks XREs in the proximal 5'-flanking region? Indeed, the 20-40-fold increase in the level of 1A2 mRNA found after treatment of cells or animals with various PAHs has been shown to be mediated for the most part post-transcriptionally (38-40), apparently through enhanced stability and intranuclear processing of the 1A2 mRNA precursor (41). Interestingly, the Ah receptor appears to be involved in this post-transcriptional induction.

mRNA stabilization as a mechanism of induction is found with several other P-450s, including the alcohol-inducible cytochrome, P-450 2E1. In fact, this isoform is subject to multiple modes of regulation. Although diabetes and fasting each produce up to a 10-fold elevation in 2E1 mRNA, the increase with diabetes results from mRNA stabilization (42), whereas the increase following

2,3,7,8-tetrachlorodibenzo- dioxin; XRE, xenobiotic-responsive element; GH, * The abbreviations used are: PAH, polycyclic aromatic hydrocarbon; TCDD,

growth hormone; ACTH, a&nocorticotropic hormone; CRE, CAMP-responslve element.

to Ah receptor ligands, suggesting a more remote location for these elements a Gene transfection studies have revealed a modest transcriptional response

(up to 3 kilobases upstream of the transcription start site) (37).

Minireview: Cytochrome P-450 13471

fasting (43, 44) results from an increase in gene transcription (45). Furthermore, chemical inducers of this cytochrome act predominantly to stabilize the protein, as has been demonstrated with acetone (46). The mechanism of this stabilization appears to be through ligand-mediated protection from phosphorylation, which otherwise leads to denaturation and degradation (47). The presence of phosphorylation sites on other P-450s and the ability of substrates to stabilize other P-450s suggest that this may be an important mechanism of P-450 regulation.

In rodents, but less so in man, the expression of a number of P-450s is sexually determined, by neonatal imprinting and by hormonal regulation in mature animals. These constitutively expressed P-450s generally are not responsive to xenobiotic in- duction. Neonatal castration has been shown to abolish the expression in adult animals of several male-specific steroid hy- droxylases and to diminish the expression of a female-specific P- 450 (48-52). Early administration of androgens to neonatally castrated male or female animals imprints the male pattern of P- 450 expression; this steroidal programming in immature animals is thought to be mediated through pituitary growth hormone (GH), with neonatal androgens imprinting a pulsatile pattern of secretion that is characteristic of males. Indeed, intermittent administration of GH to hypophysectomized animals results in the expression of male-specific P-450s, and a more continuous administration of GH, obtained in castrated males and charac- teristic of females, results in the female pattern of P-450 expres- sion (51, 53, 54). Notably, the administration of sex steroids to hypophysectomized animals is generally without effect on P-450 expression, consistent with these hormones acting through the hypothalamopituitary axis (51, 52, 55, 56). It should be noted that not all gender-determined P-450s are regulated as described above; several appear to be suppressed by GH regardless of the mode of its administration (51, 52).

The P-450 cytochromes responsible for the biosynthesis of steroid hormones are regulated in part by ACTH, which acts intracellularly through CAMP to increase gene transcription (57, 58). However, several of the steroidogenic P-450 genes do not contain the canonical CAMP-responsive element (CRE) found upstream of many other genes regulated by this nucleotide. Re- cent studies on the 17a-hydroxylase gene have identified the CAMP-responsive region and have shown that it binds a 47-kDa protein that may be a member of the proposed CRE-binding protein family (59). Surprisingly, none of the other known ste- roidogenic P-450 genes contain this CAMP-responsive sequence element, suggesting that they may bind one or more unique CRE transcription factors. Indeed, each steroidogenic P-450 appears to have its own CRE and perhaps its own subset of CRE-binding proteins (58).

Structure-Function Relationships Studies on the relationship of structure to function in the P-

450s have been limited by the lack of a three-dimensional struc- ture for a mammalian isoform. Currently, all mammalian models are based on the known structure of P-450cam (60), a cytosolic cytochrome from Pseudomonas putida that shares only 10-20% sequence identity with the mammalian forms (7,61). The validity of using P-450cam as a model has not yet been clearly established, although most structural studies on mammalian P-450s have been supportive, as discussed below. The crystal structure of P- 450cam resembles a triangular prism, with 45% a-helix and 15% antiparallel @-structure. Although the helices are distributed throughout the polypeptide chain, the tertiary structure reveals an asymmetric arrangement, with the helices clustered on one side of the protein and @-structure located predominantly on the opposite side. The heme is positioned between two helices and is held in place by hydrophobic interactions, by hydrogen-bonding interactions between the heme propionates and Arg and His residues, and by a cysteine-thiolate ligand to the iron. In common with other hemoproteins such as catalase and cytochrome c peroxidase, and in contrast to typical cytochromes, the heme of P-450cam is not directly accessible from the surface of the pro- tein; the closest approach i: to what is termed the proximal surface, a distance of about 8 A. Site-directed mutagenesis studies

have demonstrated that several basic amino acids on this surface are involved in the electrostatic interaction of P-450cam with its redox partner, putidaredoxin (62). The lack of surface accessibil- ity to the heme makes it likely that one or more amino acid side chains are involved in the conduction of electrons from putida- redoxin to the heme, but the identity of these residues is not yet known.

Crystallographic and site-directed mutagenesis studies have identified 2 residues critical to both substrate and 0, binding a t the active site of P-450cam. The substrate-binding pocket is lined with hydrophobic residues and is buried within the protein; access is gained via a small, dynamic solvent channel leading to the distal face of the heme. The substrate, camphor, is bound by van der Waals contacts and a single sterically important hydrogen bond with Tyr-96 (60,63). The O,-binding pocket is centered on Thr-252, which forms a hydrogen bond with Gly-248 to produce a local deformation, or kink, in the distal helix. When this Thr is replaced with Ala or Val, the monooxygenase reaction is uncoupled, such that 0, and NADPH consumption is funneled into the production of HzO, (64). These results point to a role for the threonyl hydroxyl group in the cleavage of the dioxygen bond during catalysis. Notably, the substitution of Ser for Thr at this position does not alter monooxygenase activity appreciably.

Cys-357 of P-450cam provides the axial heme ligand that dictates many of the spectral and functional characteristics of cytochrome P-450 (60). The peptide containing this invariant residue is the single most highly conserved P-450 segment and can be readily recognized in P-450s from organisms as diverse as bacteria and man (7, 25, 65). Substitutions of amino acids in this segment by oligonucleotide-directed mutagenesis produce a vari- ety of effects on the spectral and heme-binding properties of the resultant cytochrome. Substitution of His or Tyr for the axial Cys of rat P-450 1A2 apparently prevents heme incorporation into the mutant proteins, as indicated by the minimal ferrous- CO Soret absorbance at either 448 or 420 nm (66). This finding, coupled with the concurrent loss of catalytic activity in the mutant proteins, demonstrates the importance of the cysteine- thiolate ligand to heme binding. Substitutions of more polar for hydrophobic amino acids in this segment also generally results in decreased Soret absorbance and in complete or partial loss of activity toward some, but not all, substrates (66-68).

Interestingly, substitutions in several mammalian P-450s at the highly conserved Thr that is thought to help form the 0,- binding pocket (corresponding to Thr-252 of P-450cam) also affected substrate selectivity and binding (69, 70). Most notably, and in contrast to the studies with P-450cam noted above, the substitution of Ser at this position decreased the rate of some reactions but left others unaffected or even slightly enhanced. Moreover, substrate-binding affinities were significantly reduced with this mutant. As might be expected, His and IIe substitutions at this position yielded inactive enzymes; however, enzymes with Val or Asn at this position retained limited activity toward some substrates. Evidently, the hydroxyl moiety is not essential to function in the two mammalian P-450s examined; perhaps the substrate-binding pocket is sufficiently large or flexible that with some substrates it can accommodate 0, binding and catalysis in the absence of the bend in the distal helix otherwise provided by the hydroxyl group hydrogen bond.

Most of the studies on substrate binding to the mammalian P- 450 cytochromes have used chimeric constructs between related isoforms to identify functional segments and residues. Based on these studies, the middle third of the P-450 sequence was assigned to substrate binding. More recent studies have identified specific residues that are responsible for substrate recognition. Kronbach et al. (71) utilized two structurally similar cytochromes that exhibit a 10-fold difference in the K, of progesterone to identify 3 residues that are critical to the binding of this steroid. The segment containing these closely spaced residues corresponds to the region of P-450cam that contains Tyr-96, a residue involved in substrate binding in this cytochrome, as noted above. Similarly, Aoyama et al. (72) identified two amino acids in the NH,-terminal third of the P-450 2B cytochromes that are necessary for testos- terone 16/3-hydroxyIation; a variant with substitutions at these

13472 Minireview: Cytochrome P-450

positions lacked this activity. A noteworthy study by Lindberg and Negishi (73) on two highly similar P-450s that catalyze coumarin and testosterone hydroxylation similarly narrowed sub- strate recognition to 3 residues, one of which conveys greater than 80% of the specificity for testosterone. In all of these studies, a most surprising aspect is the conservative nature of the amino acid substitutions that confer changes in substrate specificity.

REFERENCES4

1. Klingenberg, M. (1958) Arch. Biochem. Biophys. 75,376-386 2. Garfinkel, D. (1958) Arch. Bioehem. Biophys. 77,493-509 3. Omura, T., and Sato, R. (1964) J. Biol. Chem. 239,2370-2378 4. Omura, T., Sato, R., Cooper, D. Y., Rosenthal, O., and Estabrook, R. W.

5. Lu, A. Y. H., and Coon, M. J. (1968) J. Biol. Chem. 2 4 3 , 1331-1332 6. Strobel, H. W., Lu, A. Y. H., Heidema, J., and Coon, M. J. (1970) J. Biol.

7. Black, S. D., and Coon, M. J. (1987) Adu. Enzymol. Relat. Areas Mol. Biol.

8. Ryan, D. E., and Levin, W. (1990) Phrmacol. & Ther. 4 5 , 153-239 9. Guengerich, F. P. (1991) J. Biol. Chem. 266,10019-10022

(1965) Fed. Proc. 2 4 , 1181-1189

Chem. 245,4851-4854

60,35-87

10. Nebert, D. W., Nelson, D. R., Coon, M. J., Estabrook, R. W., Feyereisen, R., Fujii-Kuriyama, Y., Gonzalez, F. J., Guengericb, F. P., Gunsalus, I. C., Johnson,E. F., Lo er J C Sato R , Waterman, M. R., and Waxman, D. J. (1991) DNA CetBlol: 1 0 , l - l k '

11. Dawson, J. H. (1988) Science 240,433-439 12. Pandey, R. N., Kuemmerle, S. C., and Hollenberg, P. F. (1987) Drug Metab.

13. Coon, M. J., and Koop, D. R. (1983) in The Enzymes (Boyer, P. D., ed)

14. Jakoby, W. B., and Ziegler, D. M. (1990) J. Biol. Chem. 265,20715-20718 15. White, R. E., and Coon, M. J. (1980) Annu. Reu. Biochem. 49,315-356 16. Schenkman, J. B., Jansson, I., and Robie-Suh, K. M. (1976) Life Sci. 19 ,

17. Pompon, D., and Coon, M. J. (1984) J. Biol. Chem. 259,15377-15385 18. Groves. J. T.. McCluskv. G. A.. White. R. E.. and Coon. M. J. (1978)

Dispos. 15,518-523

Vol. XVI, pp. 645-677, Academic Press, New York

611-624

Biochem. Bzophys. R e i 'Commun. 81,154-160 , ,

20. Larroaue. C.. Lanee. R.. Maurin. L.. Bienvenue. A,. and van Lier. J. E. 19. Blake, R. C., 11, and Coon, M. J. (1989) J. Biol. Chem. 264,3694-3701

(1990) Arch. BioE~em.'Biophys.'282,198-201 . ,

21. Hecker, M., and Ullrich, V. (1989) J. Biol. Chem. 264,141-150 22. Ortiz de Montellano, P. R. (1989) Trends Pharmacol. Sci. 10,354-359 23. Bondon, A., Macdonald, T. L., Harris, T. M., and Guengerich, F. P. (1989)

24. Ortiz de Montellano, P. R., and Reich, N. 0. (1986) in Cytochrome P-450 J . Biol. Chem. 264,1988-1997

pp. 273-314, Plenum Publishing Corp., New York Structure, Mechanism, and Biochemistry (Ortiz de Montellano, P. R., ed)

25. Gonzalez, F. J. (1988) Pharmacol. Reu. 40,243-288

27. Nebert, D. W., and Jones, J. E. (1989) Int. J . Biochem. 2 1 , 243-252 26. Okey, A. S. (1990) Pharmacol. & Ther. 4 5 , 241-298

28. Poland, A,, Glover, E., and Kende, A. S. (1976) J. Biol. Chem. 2 5 1 , 4936-

29. FuJlsawa-Sehara, A., Sogawa, K., Yamane, M., and Fujii-Kuriyama, Y.

30. Wilhelmsson, A., Wikstrom, A.-C., and Poellinger, L. (1986) J. Biol. Chem.

31. Hapgood, J., Cuthill, S., Den+, M., Poellinger, L., and Gustafsson, J. A. 32. Denison, M. S., Fisher, J. M., and Whitlock, J. P., Jr. (1988) J. Biol. Chem.

33. Saatcioglu, F., Perry, D. J., Pasco, D. S., and Fagan, J. B. (1990) J. Biol.

34. Jones, K. W., and Whitlock, J. P., Jr. (1990) Mol. Cell. Biol. 10,5098-5105 35. Gonzalez, F. J., and Nebert, D. W. (1985) Nucleic Acids Res. 13,7269-7288

We regret that, due to space limitations, many important contributions to

4946

(1987) Nucleic Acids Res. 15,4179-4191

261,13456-13463

(1989) Proc. Natl. Acad. Set. U. S. A. 86, 60-64

263 , 17221-17224

Chem. 265,9251-9258

this field could not be noted here.

36.

37. 38.

39.

41. 40.

42.

43.

44.

45.

46.

47.

49. 48.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62. 63. 64.

65.

66.

67.

68.

69. 70.

71.

72.

73.

Mathis, J. M., Houser, W. H., Bresnick, E., Cidlowski, J. A,, Hines, R. N., Prough, R. A,, and Simpson, E. R. (1989) Arch. Biochem. Biophys. 2 6 9 , 93-105

Quattrocbi, L. C., and Tukey, R. H. (1989) Mol. Pharmacol. 36,66-71 Kimura, S., Gonzalez, F. J., and Nebert, D. W. (1986) Mol. Cell. Bzol. 6 ,

Pasco, D. S., Boyum, K. W., Merchant, S. N., Chalberg, S. C., and Fagan,

Silver, G., and Krauter, K. S. (1988) J. Biol. Chem. 2 6 3 , 11802-11807 Silver, G., and Krauter, K. S. (1990) Mol. Cell. Biol. 10,6765-6768 Song, B. J., Matsunaga, T., Hardwick, J. P., Park, S. S., Veech, R. L., Yang,

C. S., Gelboln, H. V., and Gonzalez, F. J. (1987) Mol. Endocrinol. 1,542-

Hong, J., Pan, J., Gonzalez, F. J., Gelboin, H. V., and Yang, C. S. (1987) 547

Porter, T. D., Jfhani, S.'C., and Coon, M. J. (1989) Mol. Pharmacol. 3 6 , Biochem. Bio hys Res Commun. 142,1077-1083

Johansson, I., Lindros, K. O., Eriksson, H., and Ingelman-Sundberg, M.

Song, B.-J., Veech, R. L., Park, S. S., Gelboin, H. V., and Gonzalez, F. J.

Eliasson, E., Johansson, I., and Ingelman-Sundberg, M. (1990) Proc. Natl.

Chao, H., and Chung, L. W. K. (1982) Mol. Pharmacol. 21,744-752 Waxman, D. J., Dannan, G. A., and Guengerich, F. P. (1985) Biochemistry

Wong, G., KawaJlri, K., and Negishi, M. (1987) Biochemistry 2 6 , 8683-

Waxman, D. J., LeBlanc G. A,, Morrissey, J. J., Staunton, J., and Lapen-

McClellan-Green, P. D., Linko, P., Yeowell, H. N., and Goldstein, J. A.

Morgan, E. T., MacGeoch, C., and Gustafsson, J.-A. (1985) J. Biol. Chem.

1471-1477

J. B. (1988) J. Biol. Chem. 263,8671-8676

61-65

(1990) Biochem. Biophys. Res. Commun. 173,331-338

(1989) J. Biol. Chem. 264,3568-3572

Acad. Sci. U. S. A. 87,3225-3229

24,4409-4417..

8690

son, D. P. (1988) J. Bibl. Chem. 263,11396-11406

(1989) J. Biol. Chem. 264,18960-18965

260. 11895-1 1898 Zaphiropoulos, P..G., Mode, A., Strom, A., Moller, C., Fernandez, C., and

Colby, H. D., Gaskm, J. H., and Kitay, J. 1. (1973) Endocrrnology 92,769- Gustafsson, J.-A; (1988) Proc. Natl. Acad. Sci. U. S. A. 8 5 , 4214-4217

"., . " ~ ~

774 Kamitaki, T., Shimada, M., Maeda, K., and Kato, R. (1985) Biochem.

Simpson, E. R., and Waterman, M. R. (1988) Annu. Reu. Physiol. 50,427-

Simpson, E. R:, Lund, J., Ahlgren, R., and Waterman, M. R. (1990) Mol.

Lund, J., Ahlgren, R., Wu, D., Kagimoto, M., Simpson, E. R., and Water-

Poulos, T. L., Finzel, B. C., Gunsalus, I. C., Wagner, G. C., and Kraut, J.

Haniu, M., Armes, L. G., Yasunobu, K. T., Shastry, B. A,, and Gunsalus,

Stayton, P. S., and Sligar, S. G. (1990) Biochemistry 29,7381-7386 Atkins, W. M., and Sligar, S. G. (1988) J. Biol. Chem. 2 6 3 , 18842-18849 Imai, M., Shimada, H., Watanabe, Y., Matsushima-Hibiya, Y., Makino, R.,

Koga, H., Horiucbi, T., and Ishimura, Y. (1989) Proc. Natl. Acad. Sci.

Kalb, V. F., and Loper, J. C. (1988) Proc. Natl. A d . Sci. U. S. A. 8 5 , U. S. A. 8 6 , 7823-7827

Shimizu, T., Hirano, K., Takahashi, M., Hatano, M., and Fujii-Kuriyama, 7221-7225

Furuya, H., Shimizu, T., Hatano, M., and Fujii-Kuriyama, Y. (1989) Y. (1988) Biochemistry 27,4138-4141

Furuya, H., Shimizu, T., Hirano, K., Hatano, M., Fujii-Kuriyama, Y., Raag, Biochem. Biophys. Res. Commun. 160,669-676

R., and Poulos, T. L. (1989) BtochemLstry 28,6848-6857 Imai, Y., and Nakamura, M. (1988) FEBS Lett. 234,313-315 Im?jLY,and Nakamura, M. (1989) Biochem. Biophys. Res. Commun. 1 5 8 ,

Biophys. Res. Commun. 130 , 1247-1253

440

Cell. Endocrtnol. 70, C25-C28

man, M. R. (1990) J. Biol. Chem. 265,3304-3312

(1985) J. Biol. Chem. 2 6 0 , 16122-16130

I. C. (1982) J. Biol. Chem. 257,12664-12671

Kronbach, T., Larabee, T. M., and Johnson, E. F. (1989) Proc. Natl. Acad. '/17-'izZ

Aoyama, T., Korzekwa, K., Nagata, K., Adesnik, M., Reiss, A,, Lapenson, Sei. U. S. A. 86,8262-8265

D. P., Gillette, J., Gelboin, H. V., Waxman, D. J., and Gonzalez, F. J. (1989) J. Biol. Chem. 264,21327-21333

Lindberg, R. L. P., and Negishi, M. (1989) Nature 339,632-634