Stereochemical specificity at carbon-20 and -22 of hydroxylated cholesterols for side-chain cleavage by adrenocortical cytochrome P -450scc

The multistep oxidation of cholesterol to pregnenolone by human cytochrome P450 11A1 is highly processive

Cytochrome P450 (P450, CYP) 11A1 is the classical cholesterol side chain cleavage enzyme (P450scc) that removes six carbons of the side chain, the first and rate-limiting step in the synthesis of all mammalian steroids. The reaction is a 3-step, 6-electron oxidation that proceeds via formation of 22R-hydroxy (OH) and 20R,22R-(OH)2 cholesterol, yielding pregnenolone. We expressed human P450 11A1 in bacteria, purified the enzyme in the absence of nonionic detergents, and assayed pregnenolone formation by HPLC-mass spectrometry of the dansyl hydrazone. The reaction was inhibited by the nonionic detergent Tween 20, and several lipids did not enhance enzymatic activity. The 22R-OH and 20R,22R-(OH)2 cholesterol intermediates were bound to P450 11A1 relatively tightly, as judged by steady-state optical titrations and koff rates. The electron donor adrenodoxin had little effect on binding; the substrate cholesterol showed a ∼5-fold stimulatory effect on the binding of adrenodoxin to P450 11A1. Presteady-state single-turnover kinetic analysis was consistent with a highly processive reaction with rates of intermediate oxidation steps far exceeding dissociation rates for products and substrates. The presteady-state kinetic analysis revealed a second di-OH cholesterol product, separable by HPLC, in addition to 20R,22R-(OH)2 cholesterol, which we characterized as a rotamer that was also converted to pregnenolone at a similar rate. The first oxidation step (at C-22) is the slowest, limiting the overall rate of cleavage. d3-Cholesterol showed no kinetic deuterium isotope effect on C-22, indicating that C-H bond cleavage is not rate-limiting in the first hydroxylation step.

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.

Purification and comparative characterization of cytochrome P-450scc (CYP XIA1) from sheep adrenocortical mitochondria

Cytochrome P-450scc (CYP XI A1) was purified from sheep adrenocortical mitochondria. The purified cytochrome was found to be homogeneous on SDS-polyacrylamide gel electrophoresis and to have a heme content of 20.8 nmol/mg of protein. Its amino acid composition and NH2-terminal amino acid sequence were determined, and compared with those of other known mammalian and fish cytochromes P-450scc. EPR spectra of the cytochrome P-450scc were measured for oxidized and NO-reduced forms in the presence or absence of cholesterol and/or adreno-ferredoxin. Spectral properties of these various forms were very similar to those of the bovine enzyme. Circular dichroism spectra of the purified sheep cytochrome P-450scc in the oxidized and dithionite-reduced forms, and of their complexed forms with cholesterol or adreno-ferredoxin were analyzed in the region from 200 to 700 nm. The difference CD spectrum of the oxidized cytochrome P-450scc complexed with adreno-ferredoxin minus the oxidized form suggests an increase in the high-spin form upon the addition of adreno-ferredoxin. This may suggest a direct influence of the adreno-ferredoxin binding to the heme moiety of the oxidized cytochrome P-450scc.

Cytochrome P-450scc-catalyzed production of progesterone from 22R-hydroxycholest-4-en-3-one by way of 20,22-dihydroxycholest-4-en-3-one

Transient accumulation of a dihydroxylated steroid was found when 22R-hydroxycholest-4-en-3-one was used as the substrate for a reconstituted cholesterol side-chain cleavage system derived from bovine adrenocortical mitochondria. The indications were that the accumulated steroid was an intermediate in the cytochrome P-450scc-catalyzed reaction. The retention time of the accumulated intermediate was identical with that of authentic 20,22-dihydroxycholest-4-en-3-one on HPLC. When 22R-hydroxycholesterol and 22R-hydroxycholest-4-en-3-one were incubated simultaneously, the total amount of reaction products was essentially the same as that observed with 22R-hydroxycholest-4-en-3-one alone. Under the conditions employed, the apparent turnover number of cytochrome P-450scc for 22R-hydroxycholesterol was calculated to be 77 nmol/min/nmol P-450 from the amount of pregnenolone formed, whereas the apparent turnover number for 22R-hydroxycholest-4-en-3-one was 64 nmol/min/nmol P-450 with respect to the intermediate formation and 77 nmol/min/nmol P-450 with respect to the progesterone formation. The apparent turnover number for 20,22-dihydroxycholest-4-en-3-one was about 125 nmol/min/nmol P-450, which was not significantly different from that of 20,22-dihydroxycholesterol. The apparent Km for 22R-hydroxycholesterol was about 20 microM and those for 22R-hydroxycholest-4-en-3-one and 20,22-dihydroxycholest-4-en-3-one were 50 and 40 microM, respectively. Thus, 22R-hydroxycholest-4-en-3-one was efficiently metabolized to progesterone by way of 20,22-dihydroxycholest-4-en-3-one by cytochrome P-450scc.

Sensitive assay of cytochrome P450scc activity by high-performance liquid chromatography

We have developed a simple procedure for analyzing the reaction intermediates and product of the cholesterol side-chain cleavage system by high-performance liquid chromatography with uv absorption monitoring. After the cholesterol side-chain cleavage system had been incubated and the reaction then halted by heat treatment, the product was converted into 3-one-4-en steroid showing intense absorption at 240 nm upon reaction with cholesterol oxidase. The converted steroids were then analyzed by normal-phase HPLC. In consequence, the catalytic activity of the reconstituted adrenocortical cytochrome P450scc system was readily assayed with a sensitivity more than 10-fold higher by this conversion. Also, it was shown that 22R-hydroxy-cholest-4-en-3-one could serve as a good substrate for cytochrome P450scc and that the 20R,22R-dihydroxy derivative could be clearly detected as a reaction intermediate in the reconstituted system.

Electron paramagnetic resonance study of ferrous cytochrome P-450scc-nitric oxide complexes: effects of 20(R),22(R)-dihydroxycholesterol and reduced adrenodoxin

Electron paramagnetic resonance (EPR) spectra of ferrous-nitric oxide (14NO and 15NO) cytochrome P-450scc complexed with 20(R),22(R)-dihydroxycholesterol were measured at 77 K with X-band (9.35 GHz) microwave frequency. The EPR spectra clearly showed the spin system to have rhombic symmetry (gx = 2.068, gz = 2.001, gy = 1.961, and Az = 1.89 mT for 14NO) and were distinct from those of 20(S)-hydroxycholesterol complexes. The unique nature of the 20(S)-hydroxycholesterol complexes indicates that 20(S)-hydroxycholesterol is not a proper intermediate in the cholesterol side-chain cleavage reaction. In addition, among various steroid complexes of ferrous-NO species having rhombic symmetry, the EPR spectra of 20(R),22(R)-dihydroxycholesterol complexes were significantly different from those of 22(R)-hydroxycholesterol complexes, suggesting that upon 20S-hydroxylation of 22(R)-hydroxycholesterol the conformation of the active site changes so as to facilitate subsequent cleavage of the C20-C22 bond of the cholesterol side chain. Addition of reduced adrenodoxin to the ferrous-NO cytochrome P-450scc complex in the presence of cholesterol caused a complete shift of the gx = 2.070 signal to gx = 2.075, indicating a reorientation of cholesterol in the substrate-binding site of the enzyme upon adrenodoxin binding. Without reduced adrenodoxin, the process of reorientation of cholesterol in the substrate-binding site was very slow, requiring more than 50 h of incubation at 0 degrees C. The present observations suggest that adrenodoxin may have another positive role in the cholesterol side-chain cleavage reaction, in addition to transferring an electron to the heme of cytochrome P-450scc.

Cytochrome P-450scc a review of the specificity and properties of the cholesterol binding site

Cytochrome P-450scc is unusual among members of this class of enzymes in showing a high degree of substrate specificity. Features of the cholesterol structure which are particularly important for binding include the 3 beta-hydroxyl, the delta 5-ring configuration, and the side-chain organization in the 20-22 region. Regarding the ring system, binding appears to require planarity and limited size at the 4-5-6 carbons (the A-B ring juncture). In the region of the 3 beta-hydroxyl, a "cleft" in the binding site extends about 4 A beyond the hydroxyl and can accommodate two additional ether-linked carbons. Evidence indicates that an enzyme residue hydrogen-bonds to the oxygen of the 3 beta hydroxyl, providing much of the energy for the initial enzyme-substrate interaction. The cytochrome shows less specificity for the side-chain structure, except in the region of carbons 20-22 where hydroxylation/side-chain cleavage takes place. The binding cleft for the side-chain is limited to approximately the length of the isocaproic group but can accommodate structural variations beyond the 22-position. Evidence indicates that the region near the 20-22 bond is more limited in size, and that an amino acid residue near the heme iron binds strongly and stereospecifically to the 22R-hydroxyl of the cleavage intermediates, 22R-hydroxycholesterol and 20 alpha, 22R-dihydroxycholesterol. The 22R-hydrogen of cholesterol is very close to the heme iron (approximately 3 A), while the 22S-hydrogen is slightly further (about 4 A). The size and bonding properties of the steroid binding/active site suggest a mechanism which accounts for the stereospecificity and sequence of reactions catalyzed by cytochrome P-450scc.

Lysis of platelets and erythrocytes by the incorporation of a unique oxygenated sterol: 22R-hydroxycholesterol

We have found that an oxygenated sterol, 22R-hydroxycholesterol [(22R)-5-cholestene-3 beta, 22-diol], lyses not only platelets but also erythrocytes in a dose-dependent manner. The lysis of platelets and erythrocytes were evidenced by the release of intracellular proteins, lactate dehydrogenase and hemoglobin, respectively. Their morphological change was shown by scanning electron microscopy. Elevated temperature was required for the lysis, probably to redistribute the sterol in the lipid bilayers in the plasma membranes. When the sterol was incorporated at low temperature, the temperature had to be raised to readily lyse cells. This lytic effect was, surprisingly enough, restricted to the R-isomer; the S-isomer was only marginally effective. Furthermore, sitosterol and other oxygenated sterols, with a hydroxyl group at different positions in the side chain of cholesterol, were much less lytic, regardless of the configuration of the hydroxyl group introduced. A possible mechanism for this interesting phenomenon will be discussed in relation with a structural alteration in lipid bilayers in plasma membranes brought about by the incorporation of this unique oxygenated sterol.

Microbial side-chain degradation of sterols

Apart from the broadly used diosgenin and some further natural compounds sterols gained an increasing importance as raw material for the synthesis of steroid drugs. Parallel to the elucidation of the pathways of the enzymatic degradation of sterols technical processes were developed for a specific degradation of the side-chain to useful primary products. A review is given on the present state of this field and the trends to further improvements.

Cytochrome P-450 from mitochondria of bovine adrenal cortex: comparison of cholesterol side-chain cleavage P-450 with steroid 11beta-hydroxylation P-450 and immunochemical cross-reactivity between adrenal mitochondrial and liver microsomal cytochromes P-450

Adrenocortical mitochondrial cytochrome P-450 specific to the cholesterol side-chain cleavage (desmolase) reaction differs from that for the 11beta-hydroxylation reaction of deoxycorticosterone. The former cytochrome appears to be more loosely bound to the inner membrane than the latter. Upon ageing at 0 degrees C or by aerobic treatment with ferrous ions, the desmolase P-450 was more stable than the 11beta-hydroxylase P-450. By utilizing artificial hydroxylating agents such as cumene hydroperoxide, H2O2, and sodium periodate, the hydroxylation reaction of deoxycorticosterone to corticosterone in the absence of NADPH was observed to a comparable extent with the reaction in the presence of adrenodoxin reductase, adrenodoxin and NADPH. However, the hydroxylation reaction of cholesterol to pregnenolone was not supported by these artificial agents. Immunochemical cross-reactivity of bovine adrenal desmolase P-450 with rabbit liver microsomal P-450LM4 was also investigated. We found a weak but significant cross-reactivity between the adrenal mitochondrial P-450 and liver microsomal P-450LM4, indicating to some extent a homology between adrenal and liver cytochromes P-450.

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.