P450 enzymes are able to incorporate one of the two oxygen atoms of an O 2 molecule into a broad variety of substrates with concomitant reduction of the other oxy- gen atom by two electrons to H 2 O. The most common re- action catalyzed is hydroxylation, but P450 enzymes can perform a wide spectrum of reactions including N-oxida- tion, sulfoxidation, epoxidation, N-, S-, and O-dealkyla- tion, peroxidation, deamination, desulfuration and de- halogenation (Ortiz de Montellano, 1995; Bernhardt, 1996). Apart from the monooxygenase activity of the ma- jority of P450s, a few family members like CYP5A1 (Hau- rand and Ullrich, 1985) or CYP8A1 (DeWitt and Smith, 1983) catalyze intramolecular transfer of an oxygen atom (see Guengerich, 1996). Currently the sequences of more than 2000 members of the P450 superfamily are known (http://drnelson.utmem.edu/CytochromeP450.html), and many more are to be expected as genome sequencing projects advance. To aid in communication, a standard- ized curated nomenclature has been established several years ago (Nelson et al., 1996) that uses the abbreviation CYP for cy tochrome P 450 followed by the number of the respective P450 family, a letter designating the subfamily and another number that identifies the individual mem- bers in a subfamily, e.g. CYP1A1 or CYP11B2. The func- tions of P450s are very broad: besides being involved in steroidogenesis in mammals (see below), they are essen- tial for drug metabolism, blood hemostasis, cholesterol biosynthesis and other reactions. Insect P450s are also actively studied because of their importance in agricul- tural problems. P450s of higher plants have attracted much attention in recent years (Harvey et al., 2002), and many P450s with novel metabolic activities have been found and characterized (Durst and Nelson, 1995). Vari- ous P450s have been shown to be involved in the syn- thesis of numerous secondary metabolites of higher plants like lignins, flavonoids, terpenoids, alkaloids, phy- toalexins, and of plant hormones including gibberellins, abscisic acid, and brassinosteroids. In fungi, P450s are key players in ergosterol biosynthesis and, consequently, fungal CYP51 (lanosterol 14α-demethylase) is the pri- mary target of antifungal triazole drugs. Bacterial P450s participate in antibiotic synthesis and catabolic reac- tions. Eukaryotic P450s can be subdivided into two ma- jor groups according to their subcellular localization: mi- crosomal and mitochondrial P450s, and both utilize NADPH as the electron donor of the monooxygenation reactions, whereas most bacterial P450s receive elec- trons from NADH. Two soluble components, the flavopro- Biol. Chem., Vol. 383, pp. 1537 – 1551, October 2002 · Copyright © by Walter de Gruyter · Berlin · New York Matthias Bureik, Michael Lisurek and Rita Bernhardt* Universität des Saarlandes, FR 8.8 Biochemie, Postfach 151150, D-66041 Saarbrücken, Germany * Corresponding author Major advances have been made during the last decade in our understanding of adrenal steroid hor- mone biosynthesis. Two key players in these path- ways are the human mitochondrial cytochrome P450 enzymes CYP11B1 and CYP11B2, which catalyze the final steps in the biosynthesis of cortisol and aldos- terone. Using data from mutations found in patients suffering from steroid hormone-related diseases, from mutagenesis studies and from the construction of three-dimensional models of these enzymes, structural information could be deduced that provide a clue to the stereo- and regiospecific steroid hydrox- ylation reactions carried out by these enzymes. In this review, we summarize the current knowledge on the physiological function and the biochemistry of these enzymes. Furthermore, the pharmacological and tox- icological importance of these steroid hydroxylases, the means for the identification of their potential in- hibitors and possible biotechnological applications are discussed. Key words: Aldosterone / Aldosterone synthase / Blood pressure / Cortisol / Cytochrome P450 / Steroid-11β hydroxylase. Introduction In 1958, Garfinkel (Garfinkel, 1958) and Klingenberg (Klin- genberg, 1958) detected a carbon monoxide binding pig- ment in liver microsomes of pigs and rats, which was re- ducible by either NADPH or dithionite and displayed an absorption maximum of the reduced CO-bound complex at 450 nm. However, it was not until 1964 that the hemo- protein nature of this CO-binding pigment could be veri- fied by Omura and Sato (Omura and Sato, 1964). Today we know that the name-giving spectrum is caused by a thiolate anion acting as the 5 th ligand to the heme moiety, and the cytochrome P450 superfamily has been found to comprise a highly diversified set of proteins. Cytochrome Review The Human Steroid Hydroxylases CYP11B1 and CYP11B2 Brought to you by | Karolinska Institute Authenticated Download Date | 6/1/15 4:32 PM