Sterol metabolism. 8. Conversion of cholesterol 20 alpha-hydroperoxide to 20 alpha, 21- and 20 alpha, 22R-dihydroxycholesterol by adrenal cortex mitochondria

Isotope-Labeling Studies Support the Electrophilic Compound I Iron Active Species, FeO(3+), for the Carbon-Carbon Bond Cleavage Reaction of the Cholesterol Side-Chain Cleavage Enzyme, Cytochrome P450 11A1

The enzyme cytochrome P450 11A1 cleaves the C20-C22 carbon-carbon bond of cholesterol to form pregnenolone, the first 21-carbon precursor of all steroid hormones. Various reaction mechanisms are possible for the carbon-carbon bond cleavage step of P450 11A1, and most current proposals involve the oxoferryl active species, Compound I (FeO(3+)). Compound I can either (i) abstract an O-H hydrogen atom or (ii) be attacked by a nucleophilic hydroxy group of its substrate, 20R,22R-dihydroxycholesterol. The mechanism of this carbon-carbon bond cleavage step was tested using (18)O-labeled molecular oxygen and purified P450 11A1. P450 11A1 was incubated with 20R,22R-dihydroxycholesterol in the presence of molecular oxygen ((18)O2), and coupled assays were used to trap the labile (18)O atoms in the enzymatic products (i.e., isocaproaldehyde and pregnenolone). The resulting products were derivatized and the (18)O content was analyzed by high-resolution mass spectrometry. P450 11A1 showed no incorporation of an (18)O atom into either of its carbon-carbon bond cleavage products, pregnenolone and isocaproaldehyde . The positive control experiments established retention of the carbonyl oxygens in the enzymatic products during the trapping and derivatization processes. These results reveal a mechanism involving an electrophilic Compound I species that reacts with nucleophilic hydroxy groups in the 20R,22R-dihydroxycholesterol intermediate of the P450 11A1 reaction to produce the key steroid pregnenolone.

Cholesterol hydroperoxides as substrates for cholesterol-metabolizing cytochrome P450 enzymes and alternative sources of 25-hydroxycholesterol and other oxysterols

The interaction of the primary autoxidation products of cholesterol, namely 25- and 20ξ-hydroperoxides, with the four principal cholesterol-metabolizing cytochrome P450 enzymes is reported. Addition of cholesterol 25-hydroperoxide to the enzymes CYP27A1 and CYP11A1 induced well-defined spectral changes while generating 25-hydroxycholesterol as the major product. The 20ξ-hydroperoxides induced spectral shifts in CYP27A1 and CYP11A1 but glycol metabolites were detected only with CYP11A1. CYP7A1 and CYP46A1 failed to give metabolites with any of the hydroperoxides. A P450 hydroperoxide-shunt reaction is proposed, where the hydroperoxides serve as both donor for reduced oxygen and substrate. CYP27A1 was shown to mediate the reduction of cholesterol 25-hydroperoxide to 25-hydroxycholesterol, a role of potential significance for cholesterol-rich tissues with high oxidative stress. CYP27A1 may participate in the removal of harmful autoxidation products in these tissues, while providing a complementary source of 25-hydroxycholesterol, a modulator of immune cell function and mediator of viral cell entry.

Other conceivable renditions of some of the oxidative processes used in the biosynthesis of steroid hormones

The generally accepted version (GAV) of the chemical processes by which the steroid hormones are biosynthesized cannot be considered to be an inerrant description of in vivo processes. Customarily this version is derived by piecing together the results obtained from several independent artificial in vitro incubation experiments. Extrapolation of such results from in vitro to in vivo requires untested assumptions which introduce varying degrees of uncertainty. In vitro incubation experiments reveal only what is possible; not what actually prevails in situ. Presented here are hypothetical alternative renditions of some of the oxidative processes involved in steroidogenesis. These versions suggest that some cytochrome P-450's catalyze the introduction of both oxygen atoms of dioxygen into an appropriate sterol precursor. The products are conceived as oxygen free radicals (peroxy or 1,2-cyclic peroxy) which serve as the "reactive intermediates" (the precursors) for the hormones. The true intermediates are not stable, isolable, hydroxylated compounds as they are customarily portrayed in the GAV. Central to these new renditions is the hypothesis that the appropriate P-450 introduces dioxygen into the precursor yielding either: A, a 20 peroxy sterol species or B, a species oxygenated at both C-17 and C-20 or C, a species oxygenated at both C-20 and C-21. In this hypothesis, A would serve as the precursor for progesterone, B, for the C19-androgens and C18-estrogens and C, for the mineralocorticoids (corticosterone and aldosterone) and the glucocorticoid (cortisol). How this version of steroidogenesis can be used to understand the etiologies of various genetically derived enzyme deficiency diseases of the adrenal and ovaries will be discussed. If as proposed here, the various polyfunctional cytochromes (P-450(scc), P-450(c17,) P-45011B1 (P-450(cortisol)), P-45011B2 (P-450(aldo)), etc.) catalyze conversions that are different from simple hydroxylations, the labels usually given these deficiency diseases may not be appropriate. More importantly, these new conceptions may clarify the etiology of some of the characteristic symptoms of these diseases that are not now adequately explained by the GAV.

New assumptions about oxidative processes involved in steroid hormone biosynthesis: is the role of cytochrome P-450-activated dioxygen limited to hydroxylation reactions or are dioxygen insertion reactions also possible?

The traditional conception of the chemical pathways leading to the formation of the steroid hormones is derived by piecing together the results of several independent in vitro incubation experiments. The results of these experiments have led to the assumption that some relevant cytochrome P-450's (P-450scc, P-450arom, P-450aldo, etc.) are "polyfunctional" and catalyze several successive hydroxylation reactions, which lead to the formation of the hormonal products. This essay offers an alternative view. It advances the suggestion that the oxygenated intermediates in the relevant biosynthetic conversions are reactive species that are formed by addition of both atoms of dioxygen onto two neighboring carbon atoms of steroidal precursors. Space-filled Stuart molecular models, generated by a computer program, suggest that the oxidized intermediates resemble hydroperoxides or cyclic peroxides (1,2-dioxanes). For the aromatization process required for estrogen biosynthesis, the atoms of dioxygen are bonded to C-2 and C-19 of the C19-precursor. For aldosterone formation, dioxygen is bonded to C-11 and C-18 of an appropriate precursor. Moreover, the results obtained from a computer program that provides information about "molecular mechanics" (bond angles and bond distances as well as total potential energies for each conformation of a molecule) suggest that consideration be given to the possibility that cortisol also can be biosynthesized by P-450-activated dioxygen addition to C-11 and C-17 of an appropriate precursor. Neither the traditional view of steroidogenic pathways nor the suggestions advanced here have been established by compelling experimental findings. Both hypotheses are saddled with untested assumptions, which are necessary because the dynamic processes can only be discerned by indirect means. The origins of some naturally occurring steroids hydroxylated at C-17, C-18 and C-19 are examined in the light of the suggestions made in this essay.

Reflections on sterol sidechain cleavage process catalyzed by cytochrome P450(scc)

The essay examines the evidence upon which the presently accepted version of the mechanism of the cytochrome P450(scc)-catalyzed-cleavage of the sidechain of cholesterol is based. This analysis indicates that the generally held view of the process (two consecutive hydroxylations, followed by cleavage of the resulting glycol) most likely does not describe the true mechanism. The available evidence can not be used to support this traditional notion. Two alternative hypotheses are proposed.

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.

Hydroperoxides as inactivators of aromatase: 10 beta-hydroperoxy-4-estrene-3,17-dione, crystal structure and inactivation characteristics

The crystal structure of 10 beta-hydroperoxy-4-estrene-3,17-dione (10 beta-OOH) was determined, and its inhibition of human placental aromatase was investigated. In the absence of added NADPH, 10 beta-OOH caused a time-dependent loss of aromatase activity (e.g., 50% loss after 90 s with 2.16 microM 10 beta-OOH). Protection against this loss of activity was provided when a substrate, androstenedione, was included in the incubation. Centrifugation and resuspension of the 10 beta-OOH-treated microsomes in fresh buffer failed to restore the activity, but partial recovery could be effected by dithiothreitol. Experiments to detect destruction of aromatase protoheme were done but were inconclusive. In the presence of NADPH, 10 beta-OOH did not cause a time-dependent loss of activity but was instead a competitive inhibitor (Ki = 330 nM) of androstenedione (Km = 21 nM) aromatization. The added NADPH was not utilized for the aromatization of 10 beta-OOH to estrogens, and enhanced reduction of 10 beta-OOH to 10 beta-hydroxy-4-estrene-3,17-dione could not be detected. In addition, microsomes alone were incapable of using 10 beta-OOH to support the aromatization of androstenedione. Cumene hydroperoxide and H2O2 were also investigated as inactivators of aromatase. Losses of activity comparable to those found for 10 beta-OOH could only be observed at 500-1000-fold higher concentrations of these agents, and no protection was provided by either androstenedione or NADPH. Extensive destruction of microsomal protoheme was found with these nonsteroidal agents.

Mechanism of cholesterol side-chain cleavage. II. The enzymic hydroperoxide-glycol rearrangement of the epimeric 20-hydroperxycholesterols in 18O-enriched water

Incubation of 20 alpha-hydroperoxycholesterol (I) and its 20 beta-isomer, 20 beta-hydroperoxy-20 isocholesterol (II) with adrenocortical mitochondrial preparations in the absence of molecular oxygen, in normal and 18O-enriched water, gave 20 alpha, 22R-dihydroxycholesterol (III) from I and 20 beta,21-dihydroxy-20-iso-cholesterol (IV) from II. Mass spectral analysis of the persilylated glycol products III and IV showed no uptake of 18O, indicating that the oxygen atoms of the C20-, C22- and C21-hydroxyl groups originated from the 20-hydroperoxy atomic oxygen complex is the intermediate in the enzymic oxidative reactions of cholesterol side-chain cleavage.

Inhibition of the conversion of cholesterol to pregnenolone in bovine adrenocortical preparations

The inhibition of the conversion of [4-14C] cholesterol to [4-14C] pregnenolone by a number of steroids has been studied in bovine adrenocortical mitochondrial acetonedried preparations. At equimolar substrate and inhibitor concentrations (3.3 muM) the most potent inhibitors were cholesterol derivatives containing a nitrogen function at c-22, followed by derivatives containing oxygen functions at c-22 or c-20 or both. The presence of a hydroxyl group at c-17 or the replacement of the 3beta-hydroxyl group by fluorine reduced the inhibitory efficacy. In the presence of inhibitors that were also relatively good substrates of the cholesterol side-chain cleavage system, such as some cholesterol derivatives hydroxylated in the side-chain,the rate of [4-14C] pregnenolone formation increased with time as the inhibitor was consumed. (20S)-20,21-Dihydroxycholesterol exerted such an effect on the kinetics of [4-14C]pregnenolone formation, and yielded 21-hydroxypregnenolone which was identified by gas chromatography-mass spectrometry. The synthesis of (20R)-22-ketocholesterol, of (20R,22R)-22hydroxycholesterol, (20R,22S)-hydroxycholesterol, and of (20S)-desmosterol is described.