Evidence for concerted kinetic oxidation of progesterone by purified rat hepatic cytochrome P-450g

Generation of specific antibodies and their use to characterize sex differences in four rat P450 3A enzymes following vehicle and pregnenolone 16alpha-carbonitrile treatment

The purpose of this study was to identify isozyme-specific antibodies and use them to determine the expression levels of four P450 3A enzymes in the livers of vehicle- and pregnenolone 16alpha-carbonitrile (PCN)-treated rats of both sexes, since previous work on mRNA levels has shown considerable sexual dimorphism. Using Western blot analysis with four isozyme-specific antibodies, we show that P450 3A1, 3A2, and 3A9 were expressed in vehicle-treated adult female rats at very low levels whereas P450 3A18 was not detected. PCN treatment of females strongly induced the expression of P450 3A1 in the livers with protein product increases of 214-, 3-, and 5-fold for P450 3A1, 3A2, and 3A9, respectively, and P450 3A18 was induced to 3.7 pmol/mg protein. In contrast, all four P450 3As were detected in livers of vehicle-treated males, in the order of 3A2 >> 3A18 > 3A9 approximately = 3A1. The protein product increases induced by PCN treatment of male rats were 92-, 3-, 6-, and 16-fold for P450 3A1, 3A2, 3A9, and 3A18, respectively.

Structure-function relationships of rat liver CYP3A9 to its human liver orthologs: site-directed active site mutagenesis to a progesterone dihydroxylase

CYP3A9 is an estrogen-inducible ortholog of human liver CYP3A4 with 76.5% sequence identity to CYP3A4. Unlike CYP3A4, it is a very poor testosterone 6beta- and 2beta-hydroxylase, but a relatively better catalyst of progesterone monohydroxylation largely at 6beta, 16alpha, and 21 positions with negligible 6beta, 21-dihydroxylation. We reasoned that such differences in substrate catalyses must be due to differences in the active site architecture of each CYP3A enzyme. Indeed, alignment of CYP3A4 substrate recognition sites (SRSs) with the corresponding regions of CYP3A9 sequence revealed that of the 22 fully divergent residues, 4 reside in SRS regions [P107N (SRS-1), M371G (SRS-5), and L479K and G480Q (SRS-6)]. Accordingly, we substituted these and other divergent CYP3A9 SRS residues with the corresponding residues of CYP3A4 and/or CYP3A5. Our findings of the influence of these site-directed mutations of the CYP3A9 active site on its catalysis of testosterone and three other established but structurally different CYP3A substrates (progesterone, imipramine, and carbamazepine) are described. These findings revealed that some mutations (N107P, N107S, V207T, G371M, and Q480G) not only improved the ability of CYP3A9 to hydroxylate testosterone at the 6beta and 2beta positions, but also converted it into a robust progesterone 6beta, 21-dihydroxylase. The latter in the case of CYP3A9N107P was accompanied by a shift from sigmoidal to hyperbolic enzyme-substrate kinetics. In contrast, the catalytic potential of CYP3A9 mutants K206N, K206S, M240V, and K479L/Q480G was either relatively unchanged or negligible to nonexistent. Together these findings attest to the unique substrate-active site fit of each CYP3A enzyme.

Cytochrome P450 3A9 catalyzes the metabolism of progesterone and other steroid hormones

The catalytic requirements and the role of P450 3A9, a female-specific isoform of CYP3A from rat brain, in the metabolism of several steroid hormones were studied using recombinant P450 3A9 protein. The optimal steroid hormone hydroxylase activities of P450 3A9 required cholate but not cytochrome b5. P450 3A9 was active in the hydroxylation reactions of testosterone, androstenedione, progesterone and dehydroepiandrosterone (DHEA). No activity of P450 3A9 toward cortisol was detectable under our reconstitution conditions. Among all the steroid hormones examined, female-specific P450 3A9 seemed to catalyze most efficiently the metabolism of progesterone, one of the major female hormones, to form three mono-hydroxylated products, 6beta-, 16alpha-, and 21-hydroxyprogesterone. Our data also showed that P450 3A9 can catalyze the formation of a dihydroxy product, 4-pregnen-6beta, 21-diol-3, 20-dione, from progesterone with a turnover number, 1.3 nmol/min/nmol P450. Based on the Vmax/Km values for P450 3A9 using either 21-hydroxprogesterone or 6beta-hydroxyprogesterone as a substrate, 4-pregnen-6beta, 21-diol-3, 20-dione may be formed either by 6beta-hydroxylation of 21-hydroxprogesterone or 21-hydroxylation of 6beta-hydroxyprogesterone. As a major isoform of CYP3A expressed in rat brain, the activities of P450 3A9 toward two major neurosteroids, progesterone and DHEA suggested a possible role for P450 3A9 in the metabolism of neurosteroids.

High-performance liquid chromatographic analysis of corticosteroids

This review presents recent developments in high-performance liquid chromatographic (HPLC) analysis of corticosteroids for the determination of clinically important steroids in biological specimens. Various sample preparation techniques are described.

Inhibition of bovine cytochrome P-450(11 beta) by 18-unsaturated progesterone derivatives

The last step of aldosterone biosynthesis, an 11 beta-hydroxylation followed by two 18-hydroxylations, are catalyzed, in the bovine system, by the same enzyme, the cytochrome P-450(11 beta) (deoxycorticosterone (DOC)-->corticosterone-->18-hydroxycorticosterone-->aldosterone). The 11 beta- and 18-hydroxylase activities were studied separately with a reconstituted enzymic system, using 11-deoxy[14C]corticosterone and [3H]corticosterone, respectively, as substrates. The inhibition of 11 beta-hydroxylase activity by corticosterone was competitive (Ki = 60 microM) showing that transformation of both substrates occurs at the same site. Double-label/double-substrate experiments, using an equimolar mixture of 11-deoxy[14C]corticosterone and [3H]corticosterone, suggested that 18-hydroxycorticosterone is directly formed from 11-deoxycorticosterone without the intermediate corticosterone leaving the enzyme. Inhibitions by 18-vinylprogesterone and 18-ethynylprogesterone, potent inhibitors of aldosterone biosynthesis [Viger, A., Coustal, S., Pérard, S., Piffeteau, A. & Marquet, A. (1989) J. Steroid Biochem. 33, 119-124], were characterized for both activities (11 beta- and 18-hydroxylase). The value of reversible Ki for the 18-hydroxylation (Ki = 5 microM for 18-vinylprogesterone and 30 microM for 18-ethynylprogesterone) is lower than that for the 11 beta-hydroxylation (30 microM and 100-150 microM, respectively); the former inhibitor is stronger than the latter for both steps. The binding of substrates and inhibitors to the active site was also examined by difference absorption spectroscopy. 18-Vinylprogesterone gave rise to a type I spectrum with a Ks value of 35 microM close to that of progesterone, while 18-ethynylprogesterone showed a reverse type I spectrum with a much higher Ks value (140 microM). Based on these results, a hypothetical model, involving a conformational change of the enzyme for the second step, is proposed.

Consequences of challenge infections with Fasciola hepatica on rat liver P450-dependent metabolism of sex hormones

The effect of single or repetitive fluke-infections on rat liver steroid hormone metabolism was studied. Fascioliasis was induced by oral administration of 20 metacercariae of Fasciola hepatica to rats, by week-6 (mono-infected) or 12 and 6 (bi-infected), or 12, 9 and 6 (tri-infected) before killing. Total microsomal cytochrome P450 and P450 isoenzymes were measured spectrophotometrically and by Western-blot analysis, respectively. Progesterone and testosterone metabolism were quantified by normal phase high performance liquid chromatography. In control rats, progesterone and testosterone were mainly converted to 2 alpha- and 16 alpha-hydroxymetabolites. In the liver of mono-infected rats, hepatic cytochrome P450 was significantly decreased by 36-64% whereas the expression of all investigated isoenzymes was decreased by 36-82% with the exception of the unchanged P4502E1. 16 alpha- and 2 alpha-hydroxylations of progesterone and testosterone were significantly decreased by 50-90%, these decreases were correlated with those of P4502B1/2 and P4502C11 isoenzymes, respectively. In bi- and tri-infected rats, steroid hormones were metabolized similarly to control rats. The return of steroid drug metabolizing enzyme activities to control level could be related to the immune response associated to the development of the animal resistance to the parasitic infection.

Ontogenic development of liver progesterone metabolism in female sheep. Contribution of cytochrome P4502B and P4503A subfamilies

Age-related changes in progesterone hepatic metabolism were measured in Lacaune ewes in the foetal, neonatal (1 and 4 weeks), growing (7 months), pregnant (11 months) and adult (6 years) stages. 6 beta-Hydroxylation and 20 alpha-reduction were found to be the most efficient metabolic process in ovine microsomes. These activities were detected in 3-month-old foetuses and they increased rapidly during the first month of life, in a similar manner to the developmental expression of the cytochrome P4503A subfamily. 16 alpha- and 21-hydroxylation of progesterone were characterized by low, constant turn over in sheep liver microsomes during development. The hepatic ovine P4502B isozyme was purified to electrophoretic homogeneity by means of successive DEAE cellulose, hydroxylapatite and CM cellulose chromatographic separations. This hemoprotein had an apparent molecular weight of 51 kDa and was characterized by spectral data, NH2-terminal amino-acid sequence, immunological and catalytic properties. The relative contribution of this form and of the previously purified ovine P4503A subfamily was investigated in liver progesterone metabolism by immunoinhibition studies using polyclonal antibodies raised in rabbits and from the existence of induction and of significant correlations between microsomal activity and specific P450 content. In sheep liver microsomes, it would appear that cytochrome P4502B is involved in progesterone 21-hydroxylation whereas P4503A participates in the 6 beta- and 16 alpha-hydroxylation and possibly in the reductive conversion of progesterone in its 20 alpha-hydroxy derivative.

Cytochrome P-450(17 alpha,lyase)-mediating pathway of androgen synthesis in bovine adrenocortical cultured cells

Cytochrome P-450(17 alpha,lyase) mediating pathway of dehydroepiandrosterone (DHA) formation from pregnenolone was investigated in primary cultures of bovine adrenocortical fasciculata-reticularis cells. To determine whether DHA formation proceeds predominantly by successive monooxygenase reactions without 17 alpha-hydroxypregnenolone leaving P-450(17 alpha,lyase) the cells were incubated with [14C]pregnenolone and 17 alpha-[3H]hydroxypregnenolone in the presence of Trilostane. Results of the double-substrate double-label experiments indicate that in the presence of high concentration of pregnenolone most of DHA was formed, directly from pregnenolone by the successive reactions. Since the concentration of pregnenolone usually exceeds that of 17 alpha-hydroxypregnenolone in the adrenal glands, DHA is concluded to be formed predominantly by successive reactions from pregnenolone without 17 alpha-hydroxypregnenolone leaving P-450(17 alpha,lyase) in vivo. By chronic ACTH treatment, the activities of 17 alpha-hydroxylation and DHA formation in adrenocortical cultured cells became higher concomitantly with the increase of P-450(17 alpha,lyase) content. Most of DHA was found to be formed by successive reactions from pregnenolone even under such conditions.

The role of cytochrome b5 in adrenal microsomal steroidogenesis

The role of cytochrome b5 in adrenal microsomal steroidogenesis was studied in guinea pig adrenal microsomes and also in the liposomal system containing purified cytochrome P-450s and NADPH-cytochrome P-450 reductase. Preincubation of the microsomes with anti-cytochrome b5 immunoglobulin decreased both 17 alpha- and 21-hydroxylase activity in the microsomes. In liposomes containing NADPH-cytochrome P-450 reductase and P-450C21 or P-450(17) alpha,lyase, addition of a small amount of cytochrome b5 stimulated the hydroxylase activity while a large amount of cytochrome b5 suppressed the hydroxylase activity. The effect of cytochrome b5 on the rates of the first electron transfer to P-450C21 in liposome membranes was determined from stopped flow measurements and that of the second electron transfer was estimated from the oxygenated difference spectra in the steady state. It was indicated that a small amount of cytochrome b5 activated the hydroxylase activity by supplying additional second electrons to oxygenated P-450C21 in the liposomes while a large amount of cytochrome b5 might suppress the activity through the interferences in the interaction between the reductase and P-450C21.

The sexually differentiated metabolism of [6,7-3H]17 beta-oestradiol in rats: male-specific 15 alpha- and male-selective 16 alpha-hydroxylation and female-selective catechol formation

The oxygenated-metabolite profiles of exogenous 17 beta-oestradiol (E2) in adult male and female Wistar rats have been characterized and major sex-dependent biotransformations observed which correlate with the regioselectivities of known sexually differentiated hepatic P450. [6,7-3H]E2 (27 micrograms/kg) was given i.v. The metabolites of E2 were rapidly and extensively excreted in bile (46 and 78% of the dose over 1 and 6 h, respectively). Female rats metabolized E2 by one major pathway: oxidation to oestrone (E1) followed by C-2 hydroxylation and O-methylation; the principal aglycones (0-1 h bile collections) were E1 (14%), 2-hydroxyE1 (2-OHE1) (42%) and 2-methoxyE1 (24%). Male rats metabolized E2 principally by two parallel composite pathways of E1 hydroxylation which yielded a complex mixture of mono- and di-oxygenated compounds: 15 alpha-OHE1 (33%), 2,15 alpha-diOHE1 (7%), and 2-methoxy-15 alpha OHE1 (14%); 16 alpha-OHE1 (13%), 2,16 alpha-diOHE1 (4%) and 2-methoxy-16 alpha-OHE1 (2%). 15 alpha-Hydroxylation was unique to males. The balance of aromatic and alkyl hydroxylation in males was dose-dependent: at 3 mg/kg, 15 alpha-hydroxylation was decreased approx. 50% in favour of 2-hydroxylation whilst 16 alpha-hydroxylation was largely unaffected. The male-specific 15 alpha-hydroxylation and male-predominant 16 alpha-hydroxylation of E1 derived from E2 in vivo may be ascribable to the male-specific isoforms P450IIC13 and P450IIC11, respectively.

Altered ovarian progesterone secretion induced by cadmium fails to interfere with embryo transport in the oviduct of the rat

The effect of Cadmium (Cd) on embryo transport through the oviduct and on ovarian progesterone (P) secretion were studied in the rat. Animals were given 2.5, 5, 10 mg/kg CdCl2 or 1.0 mL/kg NaCl sc on day 1 of pregnancy. On days 1, 2, 3, 4, and 5, they were anesthetized with pentobarbital, cannulae were inserted in one of the utero-ovarian veins, and 5-minute blood samples were taken from the ovary. Ovarian venous outflow was recorded, P was determined from the blood fractions, and secretion rates were calculated. P levels were determined in peripheral blood. Body weights and the wet weight of adrenals, ovaries, and oviducts were checked; oviducts and uterine horns were flushed; and number, location, and developmental stage of embryos were observed. Cd content of the oviducts was measured. Cd accumulated dose and time dependently in oviducts and induced a dose-dependent depression and delay in the rise of ovarian P secretion during days 1 through 5 of pregnancy. In the peripheral blood, P levels also failed to rise until day 4 of pregnancy in Cd-treated rats. In embryo transfer, however, no alteration could be observed. It is hypothesized that lack of vascular contact in the oviduct makes it possible for the preimplantation embryos to escape toxic effects of Cd.

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.

A microcompartmentation analysis of intermediate leakage response to substrate excess in a membrane-bound bifunctional enzyme: local control of hydroxyprogesterone channeling efficiency during cytochrome P450XVII-catalysed androgen biosynthesis

Evidence is presented for the first time that the cytochrome P450XVII-catalysed androgen formation from progesterone (P) in rat testicular microsomal membranes represents a metabolic sequence that exhibits the ability of intrinsic regulation of intermediate transfer and product formation efficiency. Exposure of this system, which catalyses a hydroxylation and oxidative cleavage reaction sequence, to increasing P concentration results in a decreased specific retention of the putative intermediate, 17 alpha-hydroxyprogesterone (HP) in the membrane compartment, and in a decreased HP conversion to androgens in favour of increasing HP transfer into the extramembrane space. This behaviour results in a decreased ratio of product vs. intermediate formation rates, which is interpreted as a partial "uncoupling" of the normal hydroxylation and cleavage reaction sequence catalysed by P450XVII. A similar pattern can likewise be observed in isolated testicular Leydig cells after exposure to increasing P concentrations under more physiological continuous-flow conditions. Further calculations indirectly indicate that the specific retention of HP in the membrane compartment can partially be attributed to its specific association with the P450XVII during catalysis. The results strongly suggest the existence of a local "channel" that becomes more leaky and therefore less effective if loaded with high influx rates. This pattern may be related to significant but incomplete competition of exogenously entering P and endogenously formed and transiently bound HP for oxygen attack at the P450XVII active site.

Steroidogenesis in liposomal system containing adrenal microsomal cytochrome P-450 electron transfer components

Purified adrenal microsomal P-450C21 and/or P-45017 alpha,lyase were incorporated with purified NADPH-cytochrome-P-450 reductase into liposome membranes composed of phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine at a molar ratio of 5:3:1. The rate dependences of reduction of liposomal P-450C21 in the fast phase as well as progesterone hydroxylation activities of P-450C21 and P-45017 alpha,lyase on the reductase concentration in the liposome membranes suggested that electrons were delivered through random collisions between the reductase and cytochrome P-450s in the liposome membranes. A rapid exchange of the steroid metabolic intermediate between vesicles was observed in a reaction system consisting of P-450C21-proteoliposomes and P-45017 alpha,lyase-proteoliposomes. Using the combined liposomal system, it was definitely proved that androstenedione was formed from progesterone mainly by a successive hydroxylation reaction without the intermediate 17 alpha-hydroxyprogesterone leaving from P-45017 alpha,lyase. It was also found that 21-hydroxylation of 17 alpha-hydroxyprogesterone into 11-deoxycortisol was inhibited by a physiological concentration of progesterone.

Specificity of steroid binding to testicular microsomal cytochrome P-450. Relation of steroid structure to type-I spectral responses after correction for hydrophobic association with the membrane

On the basis of the concept that steroids accumulate in the lipid phase of endoplasmic reticulum membranes and approach the active sites of steroidogenic cytochromes P-450 from a hydrophobic environment, we describe a procedure that allows calculation of spectral dissociation constants Ks for steroid interaction with testicular microsomal cytochrome P-450 after correction for hydrophobic association of ligand with the membrane. Maximal type-I spectral responses, apparent Ks, and partition into microsomal lipids were determined for 36 steroids, and corrected Ks values were derived from these primary data. Partition coefficients range from 60 to 62,000, and corrected Ks range from 60 microM to 25 mM steroid concentration in the lipid phase. Full spectral properties depend on a side-chain (1-3 carbon atoms) at the C17-position which may be hydrophobic or may bear a 20-oxo or 20 beta-hydroxy, but not a 20 alpha-hydroxy group. Binding constants are especially sensitive towards modifications of ring A structure (aromatization or 5 beta-, but not 5 alpha-reduction) and of the side-chain length. Androgens, with the exception of those bearing a 17 beta-acetoxy or 17 beta-propionyloxy group, are poorly accommodated by this cytochrome P-450.