Interaction between mitochondrial CYP27B1 and adrenodoxin: role of arginine 458 of mouse CYP27B1

Redox partner adrenodoxin alters cytochrome P450 11B1 ligand binding and inhibition

Human cytochrome P450 11B1 (CYP11B1) generation of the major glucocorticoid cortisol requires two electrons delivered sequentially by the iron‑sulfur protein adrenodoxin. While the expected adrenodoxin binding site is on the opposite side of the heme and 15-20 Å away, evidence is provided that adrenodoxin allosterically impacts CYP11B1 ligand binding and catalysis. The presence of adrenodoxin both decreases the dissociation constant (Kd) for substrate binding and increases the proportion of substrate that is bound at saturation. Adrenodoxin additionally decreases the Michaelis-Menten constant for the native substrate. Similar studies with several inhibitors also demonstrate the ability of adrenodoxin to modulate inhibition (IC50 values). Somewhat similar allosterism has recently been observed for the closely related CYP11B2/aldosterone synthase, but there are several marked differences in adrenodoxin effects on the two CYP11B enzymes. Comparison of the sequences and structures of these two CYP11B enzymes helps identify regions likely responsible for the functional differences. The allosteric effects of adrenodoxin on CYP11B enzymes underscore the importance of considering P450/redox partner interactions when evaluating new inhibitors.

Vitamin D resistant genes - promising therapeutic targets of chronic diseases

Vitamin D is an essential vitamin indispensable for calcium and phosphate metabolism, and its deficiency has been implicated in several extra-skeletal pathologies, including cancer and chronic kidney disease. Synthesized endogenously in the layers of the skin by the action of UV-B radiation, the vitamin maintains the integrity of the bones, teeth, and muscles and is involved in cell proliferation, differentiation, and immunity. The deficiency of Vit-D is increasing at an alarming rate, with nearly 32% of children and adults being either deficient or having insufficient levels. This has been attributed to Vit-D resistant genes that cause a reduction in circulatory Vit-D levels through a set of signaling pathways. CYP24A1, SMRT, and SNAIL are three genes responsible for Vit-D resistance as their activity either lowers the circulatory levels of Vit-D or reduces its availability in target tissues. The hydroxylase CYP24A1 inactivates analogs and prohormonal and/or hormonal forms of calcitriol. Elevation of the expression of CYP24A1 is the major cause of exacerbation of several diseases. CYP24A1 is rate-limiting, and its induction has been correlated with increased prognosis of diseases, while loss of function mutations cause hypersensitivity to Vit-D. The silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and its corepressor are involved in the transcriptional repression of VDR-target genes. SNAIL1 (SNAIL), SNAIL2 (Slug), and SNAIL3 (Smuc) are involved in transcriptional repression and binding to histone deacetylases and methyltransferases in addition to recruiting polycomb repressive complexes to the target gene promoters. An inverse relationship between the levels of calcitriol and the epithelial-to-mesenchymal transition is reported. Studies have demonstrated a strong association between Vit-D deficiency and chronic diseases, including cardiovascular diseases, diabetes, cancers, autoimmune diseases, infectious diseases, etc. Vit-D resistant genes associated with the aforementioned chronic diseases could serve as potential therapeutic targets. This review focuses on the basic structures and mechanisms of the repression of Vit-D regulated genes and highlights the role of Vit-D resistant genes in chronic diseases.

Binding of cytochrome P450 27C1, a retinoid desaturase, to its accessory protein adrenodoxin

Of the 57 human cytochrome P450 (P450) enzymes, seven are mitochondrial: 11A1, 11B1, 11B2, 24A1, 27A1, 27B1, and 27C1. Mitochondrial P450s utilize an electron transport system with adrenodoxin (Adx) and NADPH-adrenodoxin reductase (AdR). AdR reduces Adx, which then transfers electrons to the P450. The interactions between proteins in the mitochondrial P450 system are largely driven by electrostatic interactions, though the specifics vary depending on the P450. Unlike other mitochondrial P450s, the interaction between P450 27C1, a retinoid 3,4-desaturase expressed in the skin, and Adx remains largely uncharacterized. In this work, we utilized an Alexa Fluor 488 C₅ maleimide-labeled Adx to measure binding affinities between Adx and P450 27C1 or AdR. Both proteins bound Adx tightly, with K_d values < 100 nM, and binding affinities decreased with increasing ionic strength, supporting the role of electrostatic interactions in mediating these interactions. Cross-linking mass spectrometry and computational modeling were performed to identify interactions between P450 27C1 and Adx. While the residues of Adx identified in interactions were consistent with studies of other mitochondrial P450s, the binding interface of P450 27C1 was quite large and supported multiple Adx binding positions, including ones outside of the canonical Adx binding site. Additionally, Adx did not appear to be an allosteric effector of P450 27C1 substrate binding, in contrast to some other mitochondrial P450s. Overall, we conclude that P450-Adx interactions are P450-specific.

Evidence of Allosteric Coupling between Substrate Binding and Adx Recognition in the Vitamin D Carbon-24 Hydroxylase CYP24A1

Metabolic inactivation of 1,25(OH)2D3 requires molecular recognition between the mitochondrial enzyme cytochrome P450 24A1 (CYP24A1) and its cognate redox partner adrenodoxin (Adx). Recent evidence supports a model of CYP24A1 function in which substrate binding and Adx recognition are structurally linked. However, the details of this allosteric connection are not clear. In this study, we utilize chemical cross-linking coupled to mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy, and CYP24A1 functional assays to inform a working model of a CYP24A1-Adx complex. We report that differential cross-linking internal to CYP24A1 points toward an Adx-induced conformational change that perturbs the F and G helices, which are required for substrate binding. Moreover, the modeled complex suggests that a semiconserved nonpolar interaction at the interface may influence CYP24A1 regioselectivity. Taken together, these findings contribute to our understanding of Adx recognition in a critical vitamin D-inactivating enzyme and provide broader insight regarding the variability inherent in CYP-Adx interactions.

Metabolism of 2α-[2-(tetrazol-2-yl)ethyl]-1α,25-dihydroxyvitamin D3 by CYP24A1 and biological activity of its 24R-hydroxylated metabolite

Our previous study revealed that the 2α-substituted vitamin D analog 2α-[2-(tetrazol-2-yl)ethyl]-1α,25(OH)₂D₃ (AH-1) exhibited a higher osteocalcin promoter transactivation activity in human osteosarcoma cells and a greater effect on bone mineral density in a rat model of osteoporosis than 1α,25(OH)₂D₃　without increasing blood calcium concentration. Thus, we hypothesized that AH-1 could be a promising therapeutic agent for osteoporosis without any serious side effects. In this study, we compared the CYP24A1-dependent metabolism of AH-1 with that of 1α,25(OH)₂D₃. The resistance to CYP24A1-dependent metabolism could be an important property of vitamin D analogs in prolonging their biological effects. A kinetic analysis was performed using a membrane fraction prepared from recombinant E. coli expressing human CYP24A1. The k_cat/K_m (μM^-1 min^-1) value for AH-1 was 31% of that for 1α,25(OH)₂D₃, suggesting that AH-1 is not as resistant to CYP24A1-dependent metabolism as the other C2-substituted vitamin D analogs such as eldecalcitol [2β-hydroxypropoxy-1α,25(OH)₂D₃]. The major metabolite of AH-1 was the 24R-hydroxylated metabolite, which had high vitamin D receptor (VDR) binding affinity and high HL-60 cell differentiation activity similar to AH-1 itself. In contrast, 1α,25(OH)₂D₃ was metabolized by multistep monooxygenation reactions, which led to the loss of affinity for VDR. Thus, the greater therapeutic effects of AH-1 than those of 1α,25(OH)₂D₃ in in vivo studies using osteoporosis rat models may be due to 24R-hydroxy-AH-1 whose VDR affinity was 91% of that of AH-1.

Vitamin D-Dependent Rickets Type 1 Caused by Mutations in CYP27B1 Affecting Protein Interactions With Adrenodoxin

CONTEXT

CYP27B1 converts 25-hydroxyvitamin D3 to active 1,25-dihydroxyvitamin D3, playing a vital role in calcium homeostasis and bone growth. Vitamin D-dependent rickets type 1 (VDDR-1) is a rare autosomal recessive disorder caused by mutations in CYP27B1.

OBJECTIVE

The objective of the study was an enzymatic and structural analysis of mutations in a patient with calcipenic rickets. Design, Setting, Patient, and Intervention: Two siblings presented with calcipenic rickets and normal 1,25-dihydroxyvitamin D3 levels. CYP27B1 gene analysis showed compound heterozygous mutations confirming VDDR-1. We studied wild-type CYP27B1 and mutations H441Y and R459L by computational homology modeling, molecular dynamics simulations, and functional studies using a luciferase assay. The patients were successfully treated with calcitriol.

MAIN OUTCOME

The main outcomes of the study were novel mutations leading to a severe loss of CYP27B1 activities for metabolism of 25-hydroxyvitamin D3.

RESULTS

Mitochondrial cytochrome P450s require adrenodoxin (FDX1) and adrenodoxin reductase. We created models of CYP27B1-FDX1 complex, which revealed negative effects of mutations H441Y and R459L. Upon structural analysis, near-identical folds, protein contact areas, and orientations of heme/iron-sulfur cluster suggested that both mutations may destabilize the CYP27B1-FDX1 complex by negating directional interactions with adrenodoxin. This system is highly sensitive to small local changes modulating the binding/dissociation of adrenodoxin, and electron-transporting efficiency might change with mutations at the surface. Functional assays confirmed this hypothesis and showed severe loss of activity of CYP27B1 by both mutations.

CONCLUSIONS

This is the first report of mutations in CYP27B1 causing VDDR-1 by affecting protein-protein interactions with FDX1 that results in reduced CYP27B1 activities. Detailed characterization of mutations in CYP27B1 is required for understanding the novel molecular mechanisms causing VDDR-1.

Potent antiproliferative effects of 25-hydroxy-16-ene-23-yne-vitamin D₃ that resists the catalytic activity of both CYP27B1 and CYP24A1

The potency of 25-hydroxyvitamin D3 (25(OH)D3) is increased by several fold through its metabolism into 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3) by cytochrome P450 27B1 (CYP27B1). Thus, the pivotal role of 1α-hydroxylation in the activation of vitamin D compounds is well known. Here, we examined the metabolism of 25-hydroxy-16-ene-23-yne-vitamin D3 (25(OH)-16-ene-23-yne-D3), a synthetic analog of 25(OH)D3 in a cell-free system and demonstrated that 25(OH)-16-ene-23-yne-D3 is neither activated by CYP27B1 nor inactivated by cytochrome P450 24A1 (CYP24A1). These findings were also confirmed in immortalized normal human prostate epithelial cells (PZ-HPV-7) which are known to express both CYP27B1 and CYP24A1, indicating that the structural modifications featured in 25(OH)-16-ene-23-yne-D3 enable the analog to resist the actions of both CYP27B1 and CYP24A1. To provide intelligible structure-function information, we also performed molecular docking analysis between the analog and CYP27B1. Furthermore, 25(OH)-16-ene-23-yne-D3 was found to suppress the growth of PZ-HPV-7 cells with a potency equivalent to 1α,25(OH)2D3. The antiproliferative activity of 25(OH)-16-ene-23-yne-D3 was found to be vitamin D receptor (VDR)-dependent as it failed to inhibit the growth of mammary tumor cells derived from VDR-knockout mice. Furthermore, stable introduction of VDR into VDR-knockout cells restored the growth inhibition by 25(OH)-16-ene-23-yne-D3. Thus, we identified 25-hydroxy-16-ene-23-yne-vitamin D3 as a novel non-1α-hydroxylated vitamin D analog which is equipotent to 1α,25(OH)2D3 in its antiproliferative activity. We now propose that the low potency of the intrinsic VDR-mediated activities of 25(OH)D3 can be augmented to the level of 1α,25(OH)2D3 without its activation through 1α-hydroxylation by CYP27B1, but by simply preventing its inactivation by CYP24A1.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

From cholesterogenesis to steroidogenesis: role of riboflavin and flavoenzymes in the biosynthesis of vitamin D

Flavin-dependent monooxygenases and oxidoreductases are located at critical branch points in the biosynthesis and metabolism of cholesterol and vitamin D. These flavoproteins function as obligatory intermediates that accept 2 electrons from NAD(P)H with subsequent 1-electron transfers to a variety of cytochrome P450 (CYP) heme proteins within the mitochondria matrix (type I) and the (microsomal) endoplasmic reticulum (type II). The mode of electron transfer in these systems differs slightly in the number and form of the flavin prosthetic moiety. In the type I mitochondrial system, FAD-adrenodoxin reductase interfaces with adrenodoxin before electron transfer to CYP heme proteins. In the microsomal type II system, a diflavin (FAD/FMN)-dependent cytochrome P450 oxidoreductase [NAD(P)H-cytochrome P450 reductase (CPR)] donates electrons to a multitude of heme oxygenases. Both flavoenzyme complexes exhibit a commonality of function with all CYP enzymes and are crucial for maintaining a balance of cholesterol and vitamin D metabolites. Deficits in riboflavin availability, imbalances in the intracellular ratio of FAD to FMN, and mutations that affect flavin binding domains and/or interactions with client proteins result in marked structural alterations within the skeletal and central nervous systems similar to those of disorders (inborn errors) in the biosynthetic pathways that lead to cholesterol, steroid hormones, and vitamin D and their metabolites. Studies of riboflavin deficiency during embryonic development demonstrate congenital malformations similar to those associated with genetic alterations of the flavoenzymes in these pathways. Overall, a deeper understanding of the role of riboflavin in these pathways may prove essential to targeted therapeutic designs aimed at cholesterol and vitamin D metabolism.

Vitamin D binding protein and monocyte response to 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D: analysis by mathematical modeling

Vitamin D binding protein (DBP) plays a key role in the bioavailability of active 1,25-dihydroxyvitamin D (1,25(OH)₂D) and its precursor 25-hydroxyvitamin D (25OHD), but accurate analysis of DBP-bound and free 25OHD and 1,25(OH)₂D is difficult. To address this, two new mathematical models were developed to estimate: 1) serum levels of free 25OHD/1,25(OH)₂D based on DBP concentration and genotype; 2) the impact of DBP on the biological activity of 25OHD/1,25(OH)₂D in vivo. The initial extracellular steady state (eSS) model predicted that 50 nM 25OHD and 100 pM 1,25(OH)₂D), <0.1% 25OHD and <1.5% 1,25(OH)₂D are ‘free’ in vivo. However, for any given concentration of total 25OHD, levels of free 25OHD are higher for low affinity versus high affinity forms of DBP. The eSS model was then combined with an intracellular (iSS) model that incorporated conversion of 25OHD to 1,25(OH)₂D via the enzyme CYP27B1, as well as binding of 1,25(OH)₂D to the vitamin D receptor (VDR). The iSS model was optimized to 25OHD/1,25(OH)₂D-mediated in vitro dose-responsive induction of the vitamin D target gene cathelicidin (CAMP) in human monocytes. The iSS model was then used to predict vitamin D activity in vivo (100% serum). The predicted induction of CAMP in vivo was minimal at basal settings but increased with enhanced expression of VDR (5-fold) and CYP27B1 (10-fold). Consistent with the eSS model, the iSS model predicted stronger responses to 25OHD for low affinity forms of DBP. Finally, the iSS model was used to compare the efficiency of endogenously synthesized versus exogenously added 1,25(OH)₂D. Data strongly support the endogenous model as the most viable mode for CAMP induction by vitamin D in vivo. These novel mathematical models underline the importance of DBP as a determinant of vitamin D ‘status’ in vivo, with future implications for clinical studies of vitamin D status and supplementation.

Structural basis for three-step sequential catalysis by the cholesterol side chain cleavage enzyme CYP11A1

Mitochondrial cytochrome P450 11A1 (CYP11A1 or P450 11A1) is the only known enzyme that cleaves the side chain of cholesterol, yielding pregnenolone, the precursor of all steroid hormones. Pregnenolone is formed via three sequential monooxygenation reactions that involve the progressive production of 22R-hydroxycholesterol (22HC) and 20α,22R-dihydroxycholesterol, followed by the cleavage of the C20-C22 bond. Herein, we present the 2.5-Å crystal structure of CYP11A1 in complex with the first reaction intermediate, 22HC. The active site cavity in CYP11A1 represents a long curved tube that extends from the protein surface to the heme group, the site of catalysis. 22HC occupies two-thirds of the cavity with the 22R-hydroxyl group nearest the heme, 2.56 Å from the iron. The space at the entrance to the active site is not taken up by 22HC but filled with ordered water molecules. The network formed by these water molecules allows the "soft" recognition of the 22HC 3β-hydroxyl. Such a mode of 22HC binding suggests shuttling of the sterol intermediates between the active site entrance and the heme group during the three-step reaction. Translational freedom of 22HC and torsional motion of its aliphatic tail are supported by solution studies. The CYP11A1-22HC co-complex also provides insight into the structural basis of the strict substrate specificity and high catalytic efficiency of the enzyme and highlights conserved structural motifs involved in redox partner interactions by mitochondrial P450s.

Mechanism of steroidogenic electron transport: role of conserved Glu429 in destabilization of CYP11A1-adrenodoxin complex

In the present work the role of conserved residue E429 of cytochrome P45011A1 has been studied. The charge neutralization of E429Q results in 3-fold decrease of K(d) as well as V(max) compared to the wild type hemoprotein indicating tighter binding and, as the result, the impaired dissociation of oxidized adrenodoxin from the complex. As cytochrome P45011A1-adrenodoxin complex formation is driven primarily by electrostatic interactions, the low activity of E429Q mutant is completely restored to that of wild type hemoprotein by increasing of ionic strength. The charge neutralization of the corresponding residue of rat cytochrome P45011B2 has the same effect: the activity is 10-fold decreased but it is restored by increasing of ionic strength without effect on the ratio of products formed. Thus, this is the first report on identification of residues involved in modulation of dissociation of redox partner from the complex with cytochrome P450s.

Metabolism of substrates incorporated into phospholipid vesicles by mouse 25-hydroxyvitamin D3 1alpha-hydroxylase (CYP27B1)

CYP27B1 catalyzes the 1alpha-hydroxylation of 25-hydroxyvitamin D3 to 1alpha,25-dihydroxyvitamin D3, the hormonally active form of vitamin D3. To further characterize mouse CYP27B1, it was expressed in Escherichia coli, purified and its activity measured on substrates incorporated into phospholipid vesicles, which served as a model of the inner mitochondrial membrane. 25-Hydroxyvitamin D3 and 25-hydroxyvitamin D2 in vesicles underwent 1alpha-hydroxylation with similar kinetics, the catalytic rate constants (k(cat)) were 41 and 48mol/min/mol P450, respectively, while K(m) values were 5.9 and 4.6mmol/mol phospholipid, respectively. CYP27B1 showed inhibition when substrate concentrations in the membrane were greater than 4 times K(m), more pronounced with 25-hydroxyvitamin D3 than 25-hydroxyvitamin D2. Higher catalytic efficiency was seen in vesicles prepared from dioleoyl phosphatidylcholine and cardiolipin than for dimyristoyl phosphatidylcholine vesicles. CYP27B1 also catalyzed 1alpha-hydroxylation of vesicle-associated 24R,25-dihydroxyvitamin D3 and 20-hydroxyvitamin D3, and 25-hydroxylation of 1alpha-hydroxyvitamin D3 and 1alpha-hydroxyvitamin D2, but with much lower efficiency than for 25(OH)D3. This study shows that CYP27B1 can hydroxylate 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 associated with phospholipid membranes with the highest activity yet reported for the enzyme. The expressed enzyme has low activity at higher concentrations of 25-hydroxyvitamin D in membranes, revealing that substrate inhibition may contribute to the regulation of the activity of this enzyme.

Crystal structure of CYP24A1, a mitochondrial cytochrome P450 involved in vitamin D metabolism

Cytochrome P450 (CYP) 24A1 catalyzes the side-chain oxidation of the hormonal form of vitamin D. Expression of CYP24A1 is up-regulated to attenuate vitamin D signaling associated with calcium homeostasis and cellular growth processes. The development of therapeutics for disorders linked to vitamin D insufficiency would be greatly facilitated by structural knowledge of CYP24A1. Here, we report the crystal structure of rat CYP24A1 at 2.5 A resolution. The structure exhibits an open cleft leading to the active-site heme prosthetic group on the distal surface that is likely to define the path of substrate access into the active site. The entrance to the cleft is flanked by conserved hydrophobic residues on helices A' and G', suggesting a mode of insertion into the inner mitochondrial membrane. A docking model for 1alpha,25-dihydroxyvitamin D(3) binding in the open form of CYP24A1 that clarifies the structural determinants of secosteroid recognition and validates the predictive power of existing homology models of CYP24A1 is proposed. Analysis of CYP24A1's proximal surface identifies the determinants of adrenodoxin recognition as a constellation of conserved residues from helices K, K'', and L that converge with an adjacent lysine-rich loop for binding the redox protein. Overall, the CYP24A1 structure provides the first template for understanding membrane insertion, substrate binding, and redox partner interaction in mitochondrial P450s.

Structural and dynamic implications of an effector-induced backbone amide cis-trans isomerization in cytochrome P450cam

Experimental evidence has been provided for a functionally relevant cis-trans isomerization of the Ile88-Pro89 peptide bond in cytochrome P450(cam) (CYP101). The isomerization is proposed to be a key element of the structural reorganization leading to the catalytically competent form of CYP101 upon binding of the effector protein putidaredoxin (Pdx). A detailed comparison of the results of molecular dynamics simulations on the cis and trans conformations of substrate- and carbonmonoxy-bound ferrous CYP101 with sequence-specific Pdx-induced structural perturbations identified by nuclear magnetic resonance is presented, providing insight into the structural and dynamic consequences of the isomerization. The mechanical coupling between the Pdx binding site on the proximal face of CYP101 and the site of isomerization is described.

A functional proline switch in cytochrome P450cam

The two-protein complex between putidaredoxin (Pdx) and cytochrome P450(cam) (CYP101) is the catalytically competent species for camphor hydroxylation by CYP101. We detected a conformational change in CYP101 upon binding of Pdx that reorients bound camphor appropriately for hydroxylation. Experimental evidence shows that binding of Pdx converts a single X-proline amide bond in CYP101 from trans or distorted trans to cis. Mutation of proline 89 to isoleucine yields a mixture of both bound camphor orientations, that seen in Pdx-free and that seen in Pdx-bound CYP101. A mutation in CYP101 that destabilizes the cis conformer of the Ile 88-Pro 89 amide bond results in weaker binding of Pdx. This work provides direct experimental evidence for involvement of X-proline isomerization in enzyme function.

Kinetic Studies of 25-Hydroxy-19-nor-vitamin D3 and 1α,25-Dihydroxy-19-nor-vitamin D3 Hydroxylation by CYP27B1 and CYP24A1

Our previous study demonstrated that 25-hydroxy-19-nor-vitamin D(3) [25(OH)-19-nor-D(3)] inhibited the proliferation of immortalized noncancerous PZ-HPV-7 prostate cells similar to 1 alpha,25-dihydroxyvitamin D(3) [1 alpha,25(OH)(2)D(3)], suggesting that 25(OH)-19-nor-D(3) might be converted to 1 alpha,25-dihydroxy-19-nor-vitamin D(3) [1 alpha,25(OH)(2)-19-nor-D(3)] by CYP27B1 before exerting its antiproliferative activity. Using an in vitro cell-free model to study the kinetics of CYP27B1-dependent 1 alpha-hydroxylation of 25(OH)-19-nor-D(3) and 25-hydroxyvitamin D(3) [25(OH)D(3)] and CYP24A1-dependent hydroxylation of 1 alpha,25(OH)-19-nor-D(3) and 1 alpha,25(OH)(2)D(3), we found that k(cat)/K(m) for 1 alpha-hydroxylation of 25(OH)-19-nor-D(3) was less than 0.1% of that for 25(OH)D(3), and the k(cat)/K(m) value for 24-hydroxylation was not significantly different between 1 alpha,25(OH)(2)-19-nor-D(3) and 1 alpha,25(OH)(2)D(3). The data suggest a much slower formation and a similar rate of degradation of 1 alpha,25(OH)(2)-19-nor-D(3) compared with 1 alpha,25(OH)(2)D(3). We then analyzed the metabolites of 25(OH)D(3) and 25(OH)-19-nor-D(3) in PZ-HPV-7 cells by high-performance liquid chromatography. We found that a peak that comigrated with 1 alpha,25(OH)(2)D(3) was detected in cells incubated with 25(OH)D(3), whereas no 1 alpha,25(OH)(2)-19-nor-D(3) was detected in cells incubated with 25(OH)-19-nor-D(3). Thus, the present results do not support our previous hypothesis that 25(OH)-19-nor-D(3) is converted to 1 alpha,25(OH)(2)-19-nor-D(3) by CYP27B1 in prostate cells to inhibit cell proliferation. We hypothesize that 25(OH)-19-nor-D(3) by itself may have a novel mechanism to activate vitamin D receptor or it is metabolized in prostate cells to an unknown metabolite with antiproliferative activity without 1 alpha-hydroxylation. Thus, the results suggest that 25(OH)-19-nor-D(3) has potential as an attractive agent for prostate cancer therapy.

Role of Gln 85 of human CYP27A1 in 25-hydroxyvitamin D(3)-binding and protein folding

CYP27A1 catalyzes vitamin D(3) 25-hydroxylation and further hydroxylation at C-1alpha, C-24 or C-26(27). Molecular modeling of human CYP27A1 and docking with 25-hydroxyvitamin D(3) predicted that Gln 85 might be important for 1alpha-hydroxylation activity of CYP27A1 by forming a hydrogen bond with the 25-OH group of 25-hydroxyvitamin D(3). Expectedly, the mutant Q85H expressed in Escherichia coli showed no detectable 1alpha-hydroxylation activity toward 25-hydroxyvitamin D(3). In addition, Q85H prefers 24-hydroxylation toward 25-hydroxyvitamin D(3) whereas the wild-type prefers 26(27)-hydroxylation. A molecular modeling study also suggests that Gln 85 of CYP27A1 simultaneously interacts with Asn 107 and the hydroxyl group of the substrate. The fact that Q85L did not contain a heme molecule suggests that the hydrogen bond between Gln 85 and Asn 107 is important for protein folding of CYP27A1. Based on these results, it is possible that Gln 85 plays essential roles in both substrate-binding and protein folding.