Expression and functional study of wild-type and mutant human cytochrome P450c21 in Saccharomyces cerevisiae

Biosynthetic approach to combine the first steps of cardenolide formation in Saccharomyces cerevisiae

A yeast expression plasmid was constructed containing a cardenolide biosynthetic module, referred to as CARD II, using the AssemblX toolkit, which enables the assembly of large DNA constructs. The genes cloned into the vector were (a) a Δ⁵‐3β‐hydroxysteroid dehydrogenase gene from Digitalis lanata, (b) a steroid Δ⁵‐isomerase gene from Comamonas testosteronii, (c) a mutated steroid‐5β‐reductase gene from Arabidopsis thaliana, and (d) a steroid 21‐hydroxylase gene from Mus musculus. A second plasmid bearing an ADR/ADX fusion gene from Bos taurus was also constructed. A Saccharomyces cerevisiae strain bearing these two plasmids was generated. This strain, termed “CARD II yeast”, was capable of producing 5β‐pregnane‐3β,21‐diol‐20‐one, a central intermediate in 5β‐cardenolide biosynthesis, starting from pregnenolone which was added to the culture medium. Using this approach, five consecutive steps in cardenolide biosynthesis were realized in baker's yeast.

CYP21-catalyzed production of the long-term urinary metandienone metabolite 17β-hydroxymethyl-17α-methyl-18-norandrosta-1,4,13-trien-3-one: a contribution to the fight against doping

Anabolic-androgenic steroids are some of the most frequently misused drugs in human sports. Recently, a previously unknown urinary metabolite of metandienone, 17β-hydroxymethyl-17α-methyl-18-norandrosta-1,4,13-trien-3-one (20OH-NorMD), was discovered via LC-MS/MS and GC-MS. This metabolite was reported to be detected in urine samples up to 19 days after administration of metandienone. However, so far it was not possible to obtain purified reference material of this metabolite and to confirm its structure via NMR. Eleven recombinant strains of the fission yeastSchizosaccharomyces pombethat express different human hepatic or steroidogenic cytochrome P450 enzymes were screened for production of this metabolite in a whole-cell biotransformation reaction. 17,17-Dimethyl-18-norandrosta-1,4,13-trien-3-one, chemically derived from metandienone, was used as substrate for the bioconversion, because it could be converted to the final product in a single hydroxylation step. The obtained results demonstrate that CYP21 and to a lesser extent also CYP3A4 expressing strains can catalyze this steroid hydroxylation. Subsequent 5 l-scale fermentation resulted in the production and purification of 10 mg of metabolite and its unequivocal structure determination via NMR. The synthesis of this urinary metandienone metabolite viaS. pombe-based whole-cell biotransformation now allows its use as a reference substance in doping control assays.

Increased TCA cycle activity and reduced oxygen consumption during cytochrome P450-dependent biotransformation in fission yeast

Cytochrome P450s are haem-containing monooxygenases that catalyse a variety of oxidations utilizing a large substrate spectrum and are therefore of interest for biotechnological applications. We expressed human CYP21 in fission yeast Schizosaccharomyces pombe as a eukaryotic model for P450-dependent whole-cell biotransformation. The resulting strain displayed strong steroid hydroxylase activity that was accompanied by contrary effects on respiration and non-respiratory oxygen consumption, which combined to a significant decline in total oxygen consumption of the cells. While production of ROS (reactive oxygen species) decreased, the TCA cycle activity increased, as was shown by metabolic flux (METAFoR) analysis. Pentose phosphate pathway (PPP) activity was found to be negligible, regardless of growth phase, CYP21 expression or biocatalytic activity, indicating that NADPH levels in Sz. pombe are sufficiently high to support an exogenous P450 without adaptations of central carbon metabolism. We conclude from these data that neither oxygen supply nor NADPH availability are limiting factors in P450-dependent biocatalysis in Sz. pombe.

Cytochrome P450 2D catalyze steroid 21-hydroxylation in the brain

mRNA of cytochrome P450 21-hydroxylase (P450c21) is expressed in the brain, but little is known about the enzymatic properties of P450c21 in the brain. In the present study, we showed, by using various recombinant cytochrome P450 (CYP)2D enzymes and anti-CYP2D4- or P450c21-specific antibodies, that rat brain microsomal steroid 21-hydroxylation is catalyzed not by P450c21, but by CYP2D isoforms. Rat CYP2D4 and human CYP2D6, which are the predominant CYP2D isoforms in the brain, possess 21-hydroxylation activity for both progesterone and 17alpha-hydroxyprogesterone. In rat brain microsomes, these activities were not inhibited by anti-P450c21 antibodies, but they were effectively inhibited by the CYP2D-specific chemical inhibitor quinidine and by anti-CYP2D4 antibodies. mRNA and protein of CYP2D4 were expressed throughout the brain, especially in cerebellum, striatum, pons, and medulla oblongata, whereas the mRNA and protein levels of P450c21 were extremely low or undetectable. These results support the idea that CYP2D4, not P450c21, works as steroid 21-hydroxylase in the brain. Allopregnanolone, a representative gamma-aminobutyric acid receptor modulator, was also hydroxylated at the C-21 position by recombinant CYP2D4 and CYP2D6. Rat brain microsomal allopregnanolone 21-hydroxylation was inhibited by fluoxetine with an IC(50) value of 2 microm, suggesting the possibility that the brain CYP2D isoforms regulate levels of neurosteroids such as allopregnanolone, and that this regulation is modified by central nervous system-active drugs such as fluoxetine.

Carriers for type II 3beta-hydroxysteroid dehydrogenase (HSD3B2) deficiency can only be identified by HSD3B2 genotype study and not by hormone test

OBJECTIVE

We investigated adrenal steroidogenic function relevant to 3beta-hydroxysteroid dehydrogenase (HSD3B2) activity in vivo and HSD3B2 genotype in clinically normal family members of patients with HSD3B2 genotype-proven HSD3B2 deficiency congenital adrenal hyperplasia (CAH) to determine whether genotype-proven carriers for HSD3B2 deficiency exhibit decreased enzyme activity analogous to the mildly decreased adrenal 21-hydroxylase activity in the carriers of CYP21 gene mutation.

DESIGN/PATIENTS

Nineteen adult family members (ages median/range: 37/19-56 years) including 13 females and six males of six unrelated patients with HSD3B2 genotype-proven HSD3B2 deficiency were studied.

MEASUREMENTS

All family members had HSD3B2 DNA analysis and an ACTH stimulation test (Cortrosyn 0.25 mg IV bolus) for determination of adrenal HSD3B activity.

RESULTS

Ten of 13 females and five of six males were carriers of a proven or predictably deleterious mutation in one allele of the HSD3B2 gene, which was identified in the probands. ACTH-stimulated levels of 17-hydroxypregnenolone (delta5-17P), 17-hydroxyprogesterone (17-OHP), cortisol (F), dehydroepiandrosterone (DHEA) and androstenedione (delta4-A) and ratios of delta5-17P to 17-OHP, delta5-17P to F and DHEA to delta4-A, as well as increments of delta5-17P and DHEA values (ACTH-stimulated - baseline) in the genotype-proven female carriers (age, mean +/- SD: 36 +/- 6.7 years) and male carriers (age, mean +/- SD: 37 +/- 6.7 years) did not differ significantly from age-matched normal females (35 +/- 5.4 years, n = 20) and normal males (35 +/- 6 years, n = 10), respectively. There were no significant differences in any of the ACTH-stimulated hormonal levels or ratios between the female carriers with a seriously deleterious genotype (n = 5) and the female carriers with mildly deleterious genotypes (n = 5). These hormonal levels and ratios in three genotype-normal females and one genotype-normal male overlapped with those of the carriers.

CONCLUSION

These data suggest that normal adrenal HSD3B2 activity is maintained in the genotype-proven carriers because heterodimers of mutant and wild-type HSD3B2 enzymes may be stable and exhibit similar activity compared to homodimers of wild-type enzymes, possibly by a relatively rate-unlimited effect of haplo-wild-type enzyme activity. However, we cannot preclude entirely the possibility of a limited expression of another HSD3B activity under ACTH stimulation contributing to the normal adrenal HSD3B activity in vivo in the HSD3B2 genotype-proven heterozygotes. Which mechanism plays a role in maintaining normal enzyme activity in the heterozygotes remains to be elucidated. The hormone findings in the genotypic-proven carriers for HSD3B2 deficiency also indicate that carriers for this disorder cannot be detected by a hormone test and can only be detected by HSD3B2 genotype study.

The enantiomer of progesterone (ent-progesterone) is a competitive inhibitor of human cytochromes P450c17 and P450c21

Human cytochrome P450c17 (17alpha-hydroxylase, 17,20-lyase) (CYP17) and cytochrome P450c21 (21-hydroxylase) (CYP21) differ by only 14 amino acids in length and share 29% amino acid identity. Both enzymes hydroxylate progesterone at carbon atoms that lie only 2.6A apart, but CYP17 also metabolizes other steroids and demonstrates additional catalytic activities. To probe the active site topologies of these related enzymes, we synthesized the enantiomer of progesterone and determined if ent-progesterone is a substrate or inhibitor of CYP17 and CYP21. Neither enzyme metabolizes ent-progesterone; however, ent-progesterone is a potent competitive inhibitor of CYP17 (K(I)=0.2 microM). The ent-progesterone forms a type I difference spectrum with CYP17, but molecular dynamics simulations suggest different binding orientations for progesterone and its enantiomer. The ent-progesterone also inhibits CYP21, with weaker affinity than for CYP17. We conclude that CYP17 accommodates the stereochemically unnatural ent-progesterone better than CYP21. Enantiomeric steroids can be used to probe steroid binding sites, and these compounds may be effective inhibitors of steroid biosynthesis.

CYP21 mutations and congenital adrenal hyperplasia

Congenital adrenal hyperplasia (CAH) is a common autosomal recessive disorder caused mainly by defects in the steroid 21-hydroxylase (CYP21) gene. More than 90% of CAH cases are caused by mutations of the CYP21 gene on chromosome 6p21.3. The wide range of CAH phenotypes is associated with multiple mutations known to affect 21-hydroxylase enzyme activity. To date, 56 different CYP21 mutations have been reported, mostly point mutations, but small deletions or insertions have been described too, as well as complete gene deletions. Fifteen mutations, constituting 90-95% of alleles, are derived from intergenic recombination of DNA sequences between the CYP21 gene and the highly homologous CYP21P pseudogene, while the remaining are spontaneous mutations. A reliable and accurate detection of CYP21 mutations is not only important for clinical diagnosis, but also for carrier detection as there is a high variability in the basal level of 17-hydroxyprogesterone between normal and heterozygous individuals. Several strategies based on polymerase chain reaction (PCR)-driven amplification with allele-specific oligonucleotides to the CYP21 gene have been developed. It has been demonstrated that one reaction for PCR amplification of the CYP21 gene and the chimeric CYP21P/CYP21 gene using mixed primers in combination with nested PCR and single-strand conformation polymorphism is considered highly efficient and accurate for molecular diagnosis of CAH due to 21-hydroxylase deficiency.

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency

More than 90% of cases of congenital adrenal hyperplasia (CAH, the inherited inability to synthesize cortisol) are caused by 21-hydroxylase deficiency. Females with severe, classic 21-hydroxylase deficiency are exposed to excess androgens prenatally and are born with virilized external genitalia. Most patients cannot synthesize sufficient aldosterone to maintain sodium balance and may develop potentially fatal "salt wasting" crises if not treated. The disease is caused by mutations in the CYP21 gene encoding the steroid 21-hydroxylase enzyme. More than 90% of these mutations result from intergenic recombinations between CYP21 and the closely linked CYP21P pseudogene. Approximately 20% are gene deletions due to unequal crossing over during meiosis, whereas the remainder are gene conversions--transfers to CYP21 of deleterious mutations normally present in CYP21P. The degree to which each mutation compromises enzymatic activity is strongly correlated with the clinical severity of the disease in patients carrying it. Prenatal diagnosis by direct mutation detection permits prenatal treatment of affected females to minimize genital virilization. Neonatal screening by hormonal methods identifies affected children before salt wasting crises develop, reducing mortality from this condition. Glucocorticoid and mineralocorticoid replacement are the mainstays of treatment, but more rational dosing and additional therapies are being developed.

Characterization of the consequence of a novel Glu-380 to Asp mutation by expression of functional P450c21 in Escherichia coli

P450c21 catalyzes an important step in steroid synthesis. Its deficiency leads to symptoms of steroid imbalance. To obtain enough P450c21 for structure and function studies, we developed a method to express P450c21 in Escherichia coli. The 5'-region of the human P450c21 cDNA was modified to ensure efficient translation and the C terminus of the protein was extended with four His residues for easy purification. Mutant proteins with substitutions at residues 172 and 281 exhibited decreased enzymatic activities similar to those found in mammalian cells. One new mutation changing Glu-380 to Asp (D380) caused 3-fold reduction in enzymatic activity. The amount of apoprotein production detected by immunoblotting and the affinity of the mutant protein towards substrate as measured by Km were normal. The defect lies in the decreased ability of the apoprotein to bind heme, which was measured by CO difference and substrate-binding spectra. The D380 mutant protein had 3-fold reduction in peak heights in both spectra. This reduced heme binding resulted in 3-fold lower enzymatic activity.

Molecular modelling of steroidogenic cytochromes P450 from families CYP11, CYP17, CYP19 and CYP21 based on the CYP102 crystal structure

The results of homology modelling of mammalian steroidogenic cytochromes P450 (CYP) from families CYP11, CYP17, CYP19 and CYP21 are reported, based on a novel protein sequence alignment with CYP102, a bacterial P450 of known crystal structure. The molecular models generated from the CYP102 crystal structure template are consistent with experimental information from site-directed mutagenesis studies, steroidal substrate specificity and active site inhibitor studies. Interactive docking studies with both substrates and inhibitors of these enzymes indicate key residue interactions with the putative active site regions of each isoform investigated, which point to potential determinants of substrate specificity within these related enzymes.

Function and membrane topology of wild-type and mutated cytochrome P-450c21

We have studied membrane topology of cytochrome P-450c21 (P450c21) using the approaches of mutagenesis and protease digestion. P450c21 is located at the cytoplasm with an N-terminal hydrophobic domain integrated into microsomal membranes. When this hydrophobic domain was replaced by a secretory signal peptide, P450c21 was translocated into the lumen and lost enzymic activity. No other topogenic sequence was detected in the bulk of the P450c21 peptide. A mutant protein with Pro-30 replaced by Leu (L30) corresponding to the mutation found in the diseased state was created. L30 protein lost 90% of enzymic activity, while a double mutant (L30R32) with an additional Leu-32 to Arg mutation had slightly higher residual enzymic activity. Apart from lower activity, L30 was also present in the cell at a lower level than wild-type P450c21. This lower level is probably due to increased degradation, as L30 is synthesized at a normal rate. Both L30 and L30R32 proteins, however, were integrated into membranes normally. Therefore the Pro-30 --> Leu mutation did not affect membrane integration, but affected the abundance and enzymic activity of P450c21.

The common I172N mutation causes conformational change of cytochrome P450c21 revealed by systematic mutation, kinetic, and structural studies

We have investigated the structure and function of P450c21 with regard to a conserved site around Ile-172 by site-directed mutagenesis making single amino acid substitutions of residues 169 173. Substitutions of Ile-171 and -172 resulted in production of mutant proteins with dramatic reductions in enzymatic activities, indicating the importance of these two residues in maintaining the structure and function of P450c21. The I171N protein was present at a slightly lower level, due to a decreased rate of protein synthesis. The I172N apoprotein was synthesized at the normal rate, but its heme-bound P450 form was present at a much lower level. This I172N protein was tightly integrated into the membrane of endoplasmic reticulum, similar to the wild type P450c21, as shown by immunofluorescence detection, alkaline extraction, and cellular fractionation. Kinetic studies indicated that I172N had a lower Vmax value. In addition, the I172N protein was more sensitive to proteinase K digestion, indicating a possible alteration of conformation. This conformational change may result in the lower yield of the I172N hemoprotein and the reduced catalytic activity.

Targeting of heterologous membrane proteins into proliferated internal membranes in Saccharomyces cerevisiae

Overproduction of chimeric proteins containing the HMG2/1 peptide, which comprises the seven transmembrane domains of Saccharomyces cerevisiae 3-hydroxy-3-methylglutaryl-CoA reductase isozymes 1 and 2, has previously been observed to induce the proliferation of internal endoplasmic reticulum-like membranes. In order to exploit this amplified membrane surface area for the accommodation of heterologous microsomal proteins, we fused sequences coding for human cytochrome P4501A1 (CYP1A1) to sequences encoding the HMG2/1 peptide and expressed the hybrid genes in yeast. The heterologous hybrid proteins were targeted into strongly proliferated membranes, as shown by electron microscopic and immunofluorescent analysis. Fusion proteins comprising the whole CYP1A1 polypeptide (HMG2/1-CYP1A1) exhibited 7-ethoxyresorufin-O-deethylase activity, whereas fusion proteins lacking the N-terminal 56 amino acids of CYP1A1 (HMG2/1-delta CYP1A1) were inactive and appeared to be unable to incorporate protoheme. Similar amounts of heterologous protein were detected in cells expressing HMG2/1-CYP1A1, HMG2/1-delta CYP1A1 and CYP1A1, respectively. Replacement of the N-terminal membrane anchor domain of human NADPH-cytochrome P450 oxidoreductase by the HMG2/1 peptide also resulted in a functional fusion enzyme, which was able to interact with HMG2/1-CYP1A1 and the yeast endogenous P450 enzyme lanosterol-14 alpha-demethylase.

Structure and expression of the CYP21 (P450c21, steroid 21-hydroxylase) gene with respect to its deficiency

Steroid 21-hydroxylase (P450c21) deficiency is the major cause of a common genetic disease, congenital adrenal hyperplasia, with the symptoms of virilization due to steroid imbalance. We have devised a fast diagnostic method to detect common mutations in the c21B gene by a two-step gene amplification procedure coupled to restriction digestion. This procedure does not require isotopes and is suitable for routine use in a hospital setting. In addition, we have developed a procedure for the production of active P450c21 in E. coli. We tested many different vector and bacterial strain combinations to find out the best condition for P450c21 expression. The bacteria harboring the P450c21 expression plasmid were grown in a rich media supplemented with trace metals, heme biosynthesis precursor delta-levulinic acid, and induced with IPTG at 20 degrees C for 48 h. We found that low growth temperature and long induction time were important for abundant synthesis of P450c21 in E. coli.

Steroid 21-hydroxylase is a major autoantigen involved in adult onset autoimmune Addison's disease

An adrenal-specific protein reacting with autoantibodies in the sera of patients with adult onset Addison's disease has been purified from human adrenal glands. The protein, mol.wt. 55K, has the biochemical characteristics of steroid 21-hydroxylase and reacts on Western blots with rabbit antibodies to recombinant 21-hydroxylase. Absorption of the native human 55K adrenal protein with human adrenal autoantibodies prevented the subsequent reaction of the 55K protein with rabbit antibodies to 21-hydroxylase in Western blot analysis. In addition, human adrenal autoantibodies reacted with recombinant 21-hydroxylase expressed in yeast. These data indicate that the adrenal specific enzyme steroid 21-hydroxylase is a major autoantigen involved in adult onset autoimmune Addison's disease.

Mutations of P450c21 (steroid 21-hydroxylase) at Cys428, Val281, and Ser268 result in complete, partial, or no loss of enzymatic activity, respectively

Steroid 21-hydroxylase deficiency is the major cause of congenital adrenal hyperplasia (CAH), a common genetic disease. To define the relationship between gene mutations and enzyme deficiency, we generated missense mutations of the 21-hydroxylase cDNA at three different sites and characterized the mutant proteins after expressing them in cultured mammalian and yeast cells. Among them, Ser268 and Val281 have been found to be mutated in CAH patients, whereas Cys428 has been implicated as the heme ligand. Our results show mutations at these sites result in complete, partial, or no loss of the enzymatic activity. All the Cys428 mutants had neither enzymatic activity nor P450 absorption, thus supporting the notion that Cys428 is the heme ligand. All the 268-mutants exhibited the same activity as normal 21-hydroxylase, demonstrating that the clinically observed Ser268----Thr change represents a polymorphism rather than the cause of the enzyme deficiency. The 281-mutants had normal Km but greatly reduced Vmax values that also paralleled the reduction in the heme content, in the order Val281 (normal, 100%) greater than Ile281 (50%) greater than Leu281 (20%) greater than Thr281 (10%). Our findings suggest that the methyl group at the beta-carbon of Val281 is required for heme incorporation and consequently enzymatic activity.