Synthesis and properties of two new steroid hydroperoxides: an alternative pathway for the formation of 6beta-hydroxyprogesterone

Engineering of CYP106A2 for steroid 9α- and 6β-hydroxylation

CYP 106A2 from Bacillus megaterium ATCC 13368 has been described as a 15β-hydroxylase showing also minor 11α-, 9α- and 6β-hydroxylase activity for progesterone conversion. Previously, mutant proteins with a changed selectivity towards 11α-OH-progesterone have already been produced. The challenge of this work was to create mutant proteins with a higher regioselectivity towards hydroxylation at positions 9 and 6 of the steroid molecule. 9α-hydroxyprogesterone exhibits pharmaceutical importance, because it is a useful intermediate in the production of physiologically active substances which possess progestational activity. Sixteen mutant proteins were selected from a library containing mutated proteins created by a combination of site-directed and saturation mutagenesis of active site residues. Four mutant proteins out of these catalyzed the conversion of progesterone to 9α-OH-progesterone as a main product. For further optimization site-directed mutagenesis was performed. The introduction of seven mutations (D217V, A243V, A106T, F165L, T89N, T247V or T247W) into these four mutant proteins led to 28 new variants, which were also used for an in vivo conversion of progesterone. The best mutant protein, F165L/A395E/G397V, showed a ten-fold increase in the selectivity towards progesterone 9α-hydroxylation compared with the wild type CYP106A2. Also 6β-OH-progesterone is a pharmaceutically important compound, especially as intermediate for the production of drugs against breast cancer. For the rational design of mutant proteins with 6β-selectivity, docking of the 3D-structure of CYP106A2 with progesterone was performed. The introduction of three mutations (T247A, A243S, F173A) led to seven new mutant proteins. Clone A243S showed the greatest improvement in 6β-selectivity being more than ten-fold. Finally, an in vivo conversion of 11-deoxycorticosterone (DOC), testosterone and cortisol with the best five mutant proteins displaying 9α- or 6β-hydroxylation, respectively, of progesterone was performed to investigate whether the introduced mutations also effected the conversion of other substrates.

Peroxidation in position C-6 of progesterone-3-ethanolimine is increased by the presence of enzymes generating oxygen radicals

The generation of 6-oxygenated (6 beta-hydroxy, 6 beta-hydroperoxy, and 6-oxo) progesterone derivatives during the hydrolysis of progesterone-3-ethanolimine has been shown to be increased in the presence of xanthine/xanthine oxidase. The combination of xanthine/xanthine oxidase with other enzymes and/or reagents that catalyze transformation (or formation) of oxygen radicals suggested that the most likely oxygen species participating in the 6-oxygenation was the protonated acid of the superoxide anion, i.e., the hydroperoxy radical. The suggestion was further supported by experiments with oxygen scavengers. However, the data presented do not rule out a radical propagation reaction since the steroid compound used may be more reactive than the scavengers tested. A stimulation of 6-oxygenation of progesterone-3-ethanolimine by NADPH-supplemented rat liver microsomes was found. This reaction was inhibited by the only oxygen scavenger (reduced glutathione) found to be effective in the xanthine/xanthine oxidase experiments. The similarities between the two oxygenation systems may implicate a mechanism for 6 beta-hydroperoxidation of 3-oxo-4-ene steroids in rat liver microsomes.

Sterol hydroperoxide metabolism by Salmonella typhimurium

In order to rationalize multiphasic dose-response data evincing mutagenicity towards Salmonella typhimurium TA1537 for sterol hydroperoxides 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide and 3 beta-hydroxycholest-5-ene-7 alpha-hydroperoxide their metabolism by the bacterial test strain was investigated. The 5 alpha-hydroperoxide was isomerized to the 7 alpha-hydroperoxide and reduced to 5 alpha-cholest-6-ene-3 beta,5-diol; the 7 alpha-hydroperoxide was reduced to cholest-5-ene-3 beta,7 alpha-diol and transformed to 3 beta-hydroxycholest-5-en-7-one. The 3 beta,5 alpha-diol and 3 beta,7 alpha-diol were not interconverted nor was either transformed to the 7-ketone.

Mutagenic sterol hydroperoxides

Sterol hydroperoxides 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide and 3 beta-hydroxycholest-5-ene-7 alpha-hydroperoxide show weak dose-response direct mutagenicity towards Salmonella typhimurium strain TA 1537 in a liquid medium incubation bioassay. Responses were compromised by metabolism of the sterol hydroperoxides and by phase separation during the incubation period. Mutagenicity responses were increased by added superoxide dismutase but diminished by added rat liver S9 enzymes and abolished by added catalase. Catalase also abolished the stimulatory effect of superoxide dismutase. These results indicate that superoxide and peroxide be implicated in the mutagenicity responses.

Investigation of rat liver microsomal 6 beta-hydroxylation of 4-androstene-3,17-dione and 4-pregnene-3,20-dione using methodology which excludes steroid-3-imine induced introduction of the 6 beta-hydroxyl group

To avoid artefactual 6 beta-hydroxylation of 3-oxo-4-ene steroids due to steroid-3-imine formation and rearrangement a combined extraction and liquid chromatography purification procedure for incubated rat liver microsomes has been worked out. With this procedure no nonenzymatic 6 beta-hydroxylation could be observed. Conventional termination of incubations with male rat liver microsomes (105,000 g sediments) and 4-14 C-labelled 4-androstene-3,17-dione (or progesterone) by solvent extraction and evaporation might lead to a 30% overestimation of the formation of 6 beta-hydroxy-derivatives at substrate saturation. Furthermore this work-up procedure produces 6-oxo-derivatives which were not observed when the new procedure was used. By elimination of the artefactual 6-oxygenation some properties of the male rat liver microsomal 4-androstene-3,17-dione 6 beta-hydroxylase have been studied, and the activity has been compared to the artefact produced by solvent extraction and evaporation. Using the combined extraction-liquid chromatography purification it was demonstrated that the microsomal 6 beta-hydroxylase active on 4-androstene-3,17-dione and progesterone was inhibited to 95% by carbon monoxide. This makes previous suggestions regarding participation of a non cytochrome P450-dependent 4-androstene-3,17-dione 6 beta-hydroxylase less likely.

6-Oxygenation of 3-oxo-4-ene-steroids in high yields after 3-imine-formation

3-Imine formation between primary amines and 3-oxo-4-ene-steroids, followed by hydrolysis of the imines (either spontaneously during work up or induced by acetic acid) has been shown to cause 6-oxygenation of the steroids tested (17 beta-hydroxy-4-androsten-3-one, 4-androstene-3,17-dione, 4-pregnene-3,20-dione and 4-cholesten-3-one). The main products are the 6 beta-hydroxy- and the 6-oxo-derivatives of the respective steroid. These derivatives were identified by chromatographic mobilities and by gas chromatography-mass spectrometry. The formation of 6 beta-hydroperoxy-derivatives is suggested and these derivatives were tentatively identified. The highest yields of 6-oxygenated products (30-50%) were found when cadaverine and spermine were reacted with the steroids. The addition of reduced glutathione during hydrolysis of the steroid 3-imines of cadaverine, hexylamine and ethanolamine as well as addition of ascorbic acid during the hydrolysis of the steroid 3-imines of cadaverine substantially reduced the 6-oxygenation. Steroid 3-imine formation and hydrolysis which yields 6-oxygenated derivatives has also been shown to occur during work up (evaporation) of organic solvent extracts of rat liver microsomes (105,000 g sediments) to which 17 beta-hydroxy-4-androsten-3-one, 4-androstene-3,17-dione, 4-pregnene-3,20-dione or 4-cholesten-3-one respectively had been added. It is concluded that there is a risk that these organic reactions are mistaken for enzymatic conversions during in vitro investigations of 3-oxo-4-ene-steroids.

Formation of imine derivatives between biologically occurring amines and oxo-steroids

Various biologically occurring amines have been shown to react with 3-oxo-17-oxo and 20-oxo-steroids during solvent evaporation at room temperature to form a complex pattern of products in each case. Using aliphatic monoamines it was shown that imine derivatives are first formed which rapidly rearrange into yet unidentified compounds. The extent of the overall transformation of the oxo-steroid varied with the amine investigated. Thus histamine gave rise to a 95% conversion of 17 beta-hydroxy-4-androsten-3-one (testosterone), whereas epinephrine caused 5% transformation under the same conditions. Imine formation as an experimental error in studies on the biochemistry of steroids is suggested.

Biosynthesis of ascaridole: iodide peroxidase-catalyzed synthesis of a monoterpene endoperoxide in soluble extracts of Chenopodium ambrosioides fruit

Ascaridole, an asymmetric monoterpene endoperoxide with anthelmintic properties, occurs as a major constituent (60-80%) in the volatile oil of American wormseed fruit (Chenopodium ambrosioides: Chenopodiaceae), and as a lesser component in the leaf pocket oil of the boldo tree (Peumus boldus: Monimiaceae). Determination of optical activity and chromatographic resolution of naturally occurring ascaridole, and several synthetic derivatives, showed that both wormseed and boldo produce ascaridole in racemic form. The biosynthesis of ascaridole from the conjugated, symmetrical diene alpha-terpinene (a major component of the oil from wormseed) was shown to be catalyzed by a soluble iodide peroxidase isolated from homogenates of C. ambrosioides fruit and leaves. The enzymatic synthesis of ascaridole was confirmed by capillary gas-liquid chromatography and mass spectrometry of the product, which was also shown to be racemic. Optimal enzymatic activity occurred at pH 4.0 in the presence of 2.5 mM H2O2 and 1 mM NaI. Soluble enzyme extracts were fractionated by gel filtration on both Sephacryl S-300 and Sephadex G-100, and were shown to consist of a high-molecular-weight peroxidase component (Mr greater than 1,000,000, 30% of total activity) and two other peroxidase species having apparent molecular weights of 62,000 and 45,000 (major component). Peroxidase activity was susceptible to proteolytic destruction only after periodate treatment, suggesting an association of the enzyme(s) with polysaccharide material. Ascaridole biosynthesis from alpha-terpinene was inhibited by cyanide, catalase, and reducing agents, but not by compounds that trap superoxide or quench singlet oxygen. A peroxide transfer reaction initiated by peroxidase-generated I+ is proposed for the conversion of alpha-terpinene to ascaridole.

Formation of 6 beta-hydroperoxyprogesterone in rat liver microsomes

To verify the concept that molecular oxygen can be enzymically introduced as such into the steroid nucleus, radioactive progesterone and also 5-pregnene-3,20-dione were incubated with microsomal preparations of rat liver. In both cases, a significant amount of radioactive 6 beta-hydroperoxyprogesterone was isolated, together with 6 beta-hydroxyprogesterone and 6-oxoprogesterone. Addition of p-hydroxymercuribenzoate and mercurichloride significantly inhibited the formation of the hydroperoxide, whereas potassium cyanide and carbon monoxide were only partially inhibitory. Addition of 6 beta-hydroperoxyprogesterone to cytochrome P-450 containing fractions of hepatic microsomes induced a Type I difference spectrum characterized by an absorption maximum at 392 nm, a minimum at 420 nm, and an isosbestic point at 407 nm. At 4 degrees C, its apparent dissociation constant was found to be in the same order of magnitude as that of 6 beta-hydroxyprogesterone, namely 1.33 microM. A new pathway in rat liver for the formation of both 6 beta-hydroxyprogesterone and 6-oxoprogesterone via the 6 beta-hydroperoxide as a common precursor, is proposed.