Complete amino acid sequence of 21-hydroxylase cytochrome P-450 from porcine adrenal microsomes

Adrenal microsomal 21-hydroxylase is essential for biosynthesis or metabolism of various steroid hormones. The complete amino acid sequence of this cytochrome P-450 from pig adrenal was determined by sequence analysis on several sets of proteolytic fragments. The mature protein consists of 492 amino acid residues, corresponding to a molecular weight of 55,484. Seven out of nine total cysteine residues are localized in the amino terminal half of the molecule. The carboxyl terminal half contains only two cysteines, one of which is located at the highly conserved heme-binding region proposed in all cytochromes P-450. A structural comparison between 21-hydroxylase and 17 alpha-hydroxylase reveals that there is a preponderance of sequence homology at the carboxyl terminal region. These studies indicate that a single gene product is expressed for steroid 21-hydroxylase in porcine adrenal glands.     

C21 steroid side chain cleavage enzyme from porcine adrenal microsomes. Purification and characterization of the 17 alpha-hydroxylase/C17,20-lyase cytochrome P-450

The properties and the purity of a cytochrome P-450 (17 alpha-hydroxylase) from porcine adrenal microsomes have been examined following a report that the corresponding enzyme from bovine adrenocortical microsomes is inactive as a 17 alpha-hydroxylase and fails to show a high spin spectrum on addition of substrate, once the enzyme has been purified (Bumpus, J. A., and Dus, K. M. (1982) J. Biol. Chem. 257, 12696-12704). The purity of the porcine enzyme was demonstrated by electrophoresis on polyacrylamide with sodium dodecyl sulfate, immunoelectrophoresis, and NH2-terminal amino acid sequence (16 residues). The pure enzyme shows Mr = 54,000, heme content of greater than 0.8 nmol/nmol of protein, and absorption spectra typical of cytochrome P-450. The enzyme is active with both delta 4 (progesterone) and delta 5 (pregnenolone) substrates as a 17 alpha-hydroxylase and with the corresponding 17 alpha-hydroxysteroids as a C17,20-lyase. All four substrates produce typical type I spectra with the enzyme (so-called high spin form). We conclude that: 1) porcine adrenal microsomes contain a 17 alpha-hydroxylase/C17,20-lyase which is a single protein molecule readily purified to an enzymatically active form; 2) the C17,20-lyase activity is largely suppressed in the microsomes; and 3) the enzyme closely resembles that found in testicular microsomes. We propose that this enzyme be referred to as the adrenal C21 steroid side chain cleavage enzyme.     

The interference of polychlorinated biphenyls (Aroclor 1254) with membrane regulation of the activities of cytochromes P-450C21 and P-450(17) alpha,lyase in guinea-pig adrenal microsomes.

Regulation of cytochromes P-450 21-hydroxylase (P-450C21) and P-450 17 alpha-hydroxylase/C17,20-lyase (P-450(17) alpha,lyase) activities and impairment of this regulation by Aroclor 1254 was studied in guinea-pig adrenal microsomes. In a membrane depleted system, a decrease in the normally predominant, P-450C21 activity and an increase in P-450(17) alpha,lyase activities was observed. The same deviations were observed in intact microsomes with increase in the reaction temperature (0-40 degrees C). Breaks in Arrhenius plots for activities of P-450C21 and P-450(17) alpha,lyase correlate with transition temperatures reported for the microsomal membrane. These results point to: (1) preference of a gel state membrane for catalytic expression of P-450C21 suggesting a clustered organization of this P-450 species with reductase; (2) preference of a fluid membrane for lyase activity suggesting a random collision mechanism for reduction of P-450(17) alpha,lyase. Aroclor 1254 introduced to reaction mixtures containing intact microsomes elicited basically the same changes as caused by depletion of the microsomal membrane or by increase in the incubation temperature. Lack of effect of Aroclor 1254 on P-450C21 and P-450(17) alpha,lyase activities in the membrane depleted system demonstrates that its interference with monooxygenase activities is mediated by the microsomal membrane. The similarities between altered cytochrome P-450 mediated activities in the presence of Aroclor 1254 and the deviations observed in the membrane depleted system or upon increase in the incubation temperature may suggest that this chemical exerts its impacts by influencing membrane fluidity.     

Purification and properties of 17 alpha-hydroxylase from microsomes of pig adrenal: a second C21 side-chain cleavage system

We have purified a cytochrome P-450 from microsomes of pig adrenal glands to homogeneity (11.1 n moles heme/mg protein) as demonstrated by electrophoresis on polyacrylamide gels with sodium dodecyl SO4 and by line of identity with an antibody using double diffusion. The enzyme shows both 17 alpha-hydroxylase and C17,20-lyase activities and therefore constitutes a C21 side-chain cleavage system like that previously purified in this laboratory from neonatal pig testis. Antibody to the testicular enzyme cross-reacts (line of identity) with both enzymes. It is concluded that the adrenal enzyme is the same or very similar to the testicular enzyme, that each enzyme possesses two enzymatic activities, and that microsomes provide some regulatory device to limit the lyase activity of the enzyme in vivo. No evidence was found for the usually accepted existence of an adrenal steroid 17 alpha-hydroxylase without lyase activity.     

Microsomal cytochrome P-450 from neonatal pig testis. Purification and properties of A C21 steroid side-chain cleavage system (17 alpha-hydroxylase-C17,20 lyase)

A cytochrome P-450 from neonatal pig testicular microsomes was purified to homogeneity as judged by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels and by double diffusion on agar against antiserum raised in rabbits against the protein. The enzyme shows both 17 alpha-hydroxylase (Vmax = 4.6 nmol of product/min/nmol of P-450, Km = 1.5 microM) and C17,20 lyase (Vmax = 2.6 nmol of product/min/nmol of P-450, Km = 2.4 microM) activities. Both activities require NADPH and a flavoprotein P-450 reductase; microsomal P-450 reductase from pig and rat livers was used in these studies. The enzyme possesses a single subunit of molecular weight 59,000 +/- 1,000 as determined by electrophoresis on polyacrylamide with sodium dodecyl sulfate and by chromatography on sodium dodecyl sulfate-Sephadex. The enzyme is a glycoprotein and contains 8 nmol of heme/mg of protein and 40 nmol of phospholipid/mg of protein. All heme detected by pyridine hemochromogen is accounted for as P-450 by difference spectroscopy of the reduced P-450.carbon monoxide complex. This complex shows an absorbance maximum at 448 nm with no evidence of P-420. These studies raise the possibility that one microsomal protein (cytochrome P-450) may possess two enzymatic activities (hydroxylase and lyase).     

Side-chain cleavage of C21 steroids by testicular microsomal cytochrome P-450 (17 alpha-hydroxylase/lyase): involvement of heme

The two steps in the side-chain cleavage of C21 steroids to give C19 steroids (i.e. 17 alpha-hydroxylation and C17,20 lyase activity) were examined using a highly purified cytochrome P-450 from microsomes of neonatal pig testis to determine the photochemical action spectra for the two reactions. Photochemical action spectra, using either 4-ene (progesterone) or 5-ene (pregnenolone) substrates, showed maximal reversal of inhibition by CO with light of 451 nm. Evidently the heme of cytochrome P-450 is involved in both 17 alpha-hydroxylation and in C17,20-lyase activity as in the case of the side-chain cleavage of cholesterol. Mechanisms proposed to account for enzymatic cleavage of the alpha-ketol side-chain of C21 steroids (C17,20 lyase activity) must be consistent with these findings.     

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.     

Interaction between cytochrome P-450 (P-450C21) and NADPH-cytochrome P-450 reductase from adrenocortical microsomes in a reconstituted system

The interaction between P-450C21 and NADPH-cytochrome P-450 reductase, both purified from bovine adrenocortical microsomes, has been investigated in a reconstituted system with a nonionic detergent, Emulgen 913, by kinetic analysis and gel filtrations. Steady state kinetic data in progesterone 21-hydroxylation showed formation of an equimolar complex between the two enzyme proteins at low Emulgen concentration. Steady state kinetic studies on the electron transfer from NADPH to P-450C21 via the reductase showed that a stable complex formation between the two enzyme proteins was not involved in the steady state electron transfer at high Emulgen concentration. In stopped flow experiments, a time course of the P-450C21 reduction showed biphasic kinetics composed of fast and slow phases. The dependence of kinetic parameters on Emulgen concentration indicates that the fast phase corresponds to the electron transfer within the complex and the slow phase to the electron transfer through a random collision between P-450C21 and the reductase. The stable complex formation between P-450C21 and the reductase has been clearly demonstrated by gel filtration. The stable complex was composed of several molecules of the two enzyme proteins at an equimolar ratio, which was active for progesterone 21-hydroxylation and had a tendency to dissociate at high Emulgen concentration.     

Follicle-stimulating hormone regulation of cytochrome P-450 side-chain cleavage messenger ribonucleic acid accumulation by porcine granulosa cells isolated from small and medium follicles

The experiments described here were conducted to examine regulation of cytochrome P-450 side-chain cleavage (SCC) mRNA accumulation in porcine granulosa cells isolated from small (1-4-mm) and medium (5-6-mm) follicles. Granulosa cells were cultured under the following conditions: 1) for 48 h or 96 h with 0, 50, or 200 ng/ml porcine FSH; 2) for 96 h with 200 ng/ml FSH and aminoglutethimide (100 microM); and 3) for 96 h with forskolin (100 microM). Total RNA was extracted and examined by Northern and dot-blot hybridization analysis, and culture media were assayed for progesterone concentration. Northern blot analysis revealed a single band approximately 2.1 kb in size. Accumulation of SCC mRNA by granulosa cells was both FSH dose- and culture time-dependent (p less than 0.05) with maximal increases approximately 4.5 times control levels. Aminoglutethimide reduced progesterone production by about 80% while having no effect on granulosa cell accumulation of SCC mRNA compared to cells stimulated with 200 ng/ml of FSH. Forskolin-treated cells produced significantly more progesterone than did cells treated with FSH, but accumulation of SCC mRNA was similar. In response to FSH, concentration of SCC mRNA did not vary with follicle size, but granulosa cells from small follicles produced significantly more progesterone than did those from medium follicles. These results demonstrate that concentration of SCC mRNA in cultured porcine granulosa cells is FSH dose-dependent, does not vary significantly in cells from small- and medium-sized follicles, and is correlated with progesterone production, but may not parallel progesterone secretion. This last observation indicates that control at sites other than SCC mRNA can affect progesterone production.     

Noncoordinate regulation of de novo synthesis of cytochrome P-450 cholesterol side-chain cleavage and cytochrome P-450 17 alpha-hydroxylase/C17-20 lyase in mouse Leydig cell cultures: relation to steroid production

The role of cAMP in the regulation of the amount and synthesis of cytochrome P-450 cholesterol side-chain cleavage (P-450scc) and cytochrome P-450 17 alpha-hydroxylase/C17-20 lyase (P-450(17 alpha) was investigated in mouse Leydig cell cultures. In the absence of cAMP, the amount of immunoreactive P-450(17 alpha) decreased to less than 5% by day 4 and was undetectable between days 7 and 11. In contrast, the amount of immunoreactive P-450scc remained relatively constant throughout the same period. Treatment of Leydig cell cultures for 4 days with 0.05 mM 8-bromo-cAMP initiated on day 7 increased the amount of P-450(17 alpha) with relatively little effect on the amount of P-450scc. The rate of de novo synthesis of each of the P-450 enzymes was studied by determining [35S]methionine incorporation into newly synthesized protein. In the absence of cAMP, de novo synthesis of P-450(17 alpha) ceased while the rate of de novo synthesis of P-450scc increased with time in culture between days 2 and 11. Treatment with cAMP initiated on day 7 of culture caused a time-dependent increase in the rate of de novo synthesis of P-450(17 alpha) on days 9 and 11 equivalent to 40% and 60%, respectively, of that observed in freshly isolated Leydig cells. The rate of de novo synthesis of P-450scc was increased 2-fold relative to untreated cultures on days 9 and 11. De novo synthesis of P-450(17 alpha) ceased when cAMP was removed on day 11 and restored when cAMP was added again on day 13 of culture.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of spin state on reduction of cytochrome P-450 (P-450C21) from bovine adrenocortical microsomes

The effect of spin state on cytochrome P-450 reduction was studied with a reconstituted system consisting of P-450C21 and NADPH-cytochrome P-450 reductase (NADPH:ferricytochrome oxidoreductase, EC 1.6.2.4) purified from bovine adrenocortical microsomes. The absolute high spin contents of substrate-free, progesterone-bound and 17 alpha-hydroxyprogesterone-bound P-450C21 were estimated from the analysis of thermally induced difference spectra to be 25, 78 and 94% at 25 degrees C, respectively, in 50 mM potassium phosphate buffer (pH 7.2) containing 20% glycerol, 0.1 mM EDTA and 0.5% Emulgen 913. The effect of the high spin content on P-450C21 reduction by NADPH in the reconstituted system was analyzed by a steady-state method and by a stopped-flow method at 25 degrees C. The steady-state results showed that the rate of P-450C21 reduction was not affected by the high spin content of substrate-bound P-450C21 but was very slow without a steroid substrate. Biphasic reduction of P450C21 containing two first-order processes was observed in the stopped-flow experiment in the presence of either of the steroid substrates, but the reduction was very slow without the substrate. There were no significant differences in the rate and the amount of the fast phase of reduction between 17 alpha-hydroxyprogesterone-bound and progesterone-bound P-450C21. Both kinetic studies indicate that the spin state does not control the electron transfer from NADPH to P-450C21 via NADPH-cytochrome P-450 reductase but the presence of substrate is essential for the reduction of P-450C21.     

Immunogold cytochemistry of cytochromes P-450 in porcine adrenal cortex. Two enzymes (side-chain cleavage and 11 beta-hydroxylase) are co-localized in the same mitochondria

In order to study the distribution of mitochondrial cytochromes P-450 in porcine adrenal glands, the glands of anesthetized pigs were fixed in situ. Polyclonal antibodies against two cytochromes P-450, i.e., C27 side-chain cleavage enzyme and 11 beta-hydroxylase, were used to study the distribution of these enzymes in cryosections of the adrenal cortex. Ultrathin cryosections were evaluated by both protein-A/gold/silver immunocytochemistry and immunoelectron microscopy using double labeling with protein-A/colloidal-gold. At light microscopy, the two cytochrome P-450 enzymes were found to be broadly distributed in both the fasciculata and glomerulosa zones of the adrenal cortex. Quantitative immunoelectron microscopy revealed that both enzymes were localized only in mitochondria, in which they were present on the inner aspects of the inner mitochondrial membrane. Both cytochromes P-450 were demonstrable in all of the mitochondria examined, and statistical evaluation of the ratios of the two enzymes present in individual mitochondria yielded a normal distribution curve. Since no evidence was found for the preferential localization of either enzyme in a special population of mitochondria, we conclude that all mitochondria of the adrenal cortex contain both enzymes. We discuss implications of these findings with respect to the regulation of steroidogenesis.     

Cytochrome b5 promotes the synthesis of delta 16-C19 steroids by homogeneous cytochrome P-450 C21 side-chain cleavage from pig testis

Conversion of progesterone to 17 alpha-hydroxyprogesterone plus androstenedione (17 alpha-hydroxylation) and to androstadienone (delta 16 synthetase activity) by microsomes from neonatal pig testis, were both inhibited by antibodies raised against homogeneous cytochrome P-450 C21 side-chain cleavage. Inhibition of the two activities showed the same relationship to the concentration of antibody added. Analogous results were obtained with pregnenolone as substrate. In a reconstituted enzyme system consisting of the homogeneous cytochrome P-450 C21 side-chain cleavage enzyme, P-450 reductase and NADPH, addition of cytochrome b5 resulted in the synthesis of the corresponding delta 16-C19-steroid from progesterone (androstadienone) and pregnenolone (androstadienol). The effect of cytochrome b5 was concentration-dependent and prevented by anti-cytochrome b5. It is concluded that the cytochrome P-450 C21 side-chain cleavage enzyme from pig testicular microsomes is also capable of synthesizing delta 16-C19-steroids and is, therefore, likely to be responsible for the large amounts of the pherormone androstadienone produced by male pigs.     

The domain structure of the cholesterol side-chain cleavage cytochrome P-450 from bovine adrenocortical mitochondria

A homogeneous cytochrome P-450scc preparation with a specific enzyme content of 18 nmol/1 mg protein has been obtained using affinity chromatography on adrenodoxin-Sepharose under optimal conditions of the protein adsorption onto and desorption from the affinity sorbent. The data on the N-terminal amino acid sequence of the enzyme, along with the results of electrophoretic and spectrophotometric analyses favoured the multistage cholesterol transformation to pregnenolone to be catalyzed by single species of cytochrome P-450scc consisting of one polypeptide chain. Limited proteolysis of cytochrome P-450scc with trypsin resulted, at the initial stages, in the formation (in an equimolar ratio) of two large polypeptide fragments, I and II, with Mr 27000 and 22000, respectively. Prolonged action of trypsin led to the digestion of fragment II and the formation of a stoichiometric amount of fragment III, Mr of about 14000. Cytochrome P-450scc converted by trypsin into equimolar mixtures of fragments I and II or I and III retained the major spectral and functional properties of the native protein. The aspartyl-prolyl linkages, sulphhydryl groups, and surface tyrosine residues are distributed nonuniformly among fragments I and II. These data, as well as a different resistance of the fragments to the action of trypsin, suggest that cytochrome P-450scc consists of two independently folded domains linked with a short loop of the polypeptide chain, the domains being rigidly associated under neutral conditions.     

Hormonal regulation of cytochrome P450 enzymes, cholesterol side-chain cleavage and 17 alpha-hydroxylase/C17-20 lyase in Leydig cells

Testosterone biosynthesis in Leydig cells is dependent on two cytochrome P450 enzymes, cholesterol side-chain cleavage (P450scc) and 17 alpha-hydroxylase/C17-20 lyase (P450(17 alpha]. The expression of these two enzymes is differentially regulated by LH acting via its second messenger, cyclic adenosine 3',5'-monophosphate (cAMP), and by specific steroid hormones. P450scc is constitutively expressed in normal mouse Leydig cells and in MA-10 tumor Leydig cells. Chronic cAMP stimulation increases the steady state levels of P450scc mRNA and de novo P450scc protein synthesis. In contrast, cAMP is obligatory for de novo synthesis of P450(17 alpha) in normal mouse Leydig cells; P450(17 alpha) synthesis ceases in the absence of luteinizing hormone or cAMP. MA-10 tumor Leydig cells do not express P450(17 alpha) even after treatment with cAMP. The amount of P450(17 alpha) in Leydig cells is negatively regulated by testosterone acting by two distinct mechanisms. At low concentrations, testosterone acts via the androgen receptor to repress cAMP-induced synthesis of P450(17 alpha), whereas at high concentrations this steroid increases the rate of degradation of the enzyme by an oxygen-mediated mechanism. Both constitutive and cAMP-induced synthesis of P450scc protein and steady state levels of mRNA are modulated by glucocorticoids. In normal mouse Leydig cells, glucocorticoids repress P450scc synthesis and steady state levels of P450scc mRNA, whereas glucocorticoids stimulate P450scc synthesis and levels of P450scc mRNA in the tumor Leydig cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification and some properties of cytochrome P-450 specific for steroid 17 alpha-hydroxylation and C17-C20 bond cleavage from guinea pig adrenal microsomes

The energy requirements for mitochondrial protein synthesis were investigated in isolated rat liver mitochondria. Controlled changes in coupling efficiency were obtained by titration with FCCP in the presence of various substrates. No relationship was observed between the efficiency of oxidative phosphorylation and the inhibition of protein synthesis. With succinate-ADP as the substrate the ADP:O ratio was decreased by 70–80% with no effect on protein synthesis. In contrast, with acetate-ADP as substrate, a 10–20% reduction in the ADP:O ratio gave complete inhibition of protein synthesis. The data suggest that the rate of ATP production is more important for maintenance of protein synthesis than the efficiency of coupling $\text{per se}$. Thus, certain substrates can support maximal rates of protein synthesis even in relatively poorly coupled mitochondria. Analysis of mitochondrial translation products formed in the presence of increasing FCCP concentrations also showed that decreased efficiency of oxidative phosphorylation had no influence on the nature of the products.     

Affinity alkylation of the active site of C21 steroid side-chain cleavage cytochrome P-450 from neonatal porcine testis: a unique cysteine residue alkylated by 17-(bromoacetoxy)progesterone

The affinity alkylating progesterone analogue 17-(bromoacetoxy)progesterone has been used to label the active site of a microsomal cytochrome P-450 enzyme from neonatal pig testis. The enzyme causes removal of the C20 and C21 side chains from the substrates progesterone and pregnenolone by catalyzing both 17-hydroxylase and C17,20-lyase reactions, which produce the corresponding C19 steroidal precursors of testosterone. The progesterone analogue causes simultaneous inactivation of the two catalytic activities of the enzyme by a first-order kinetic process that obeys saturation kinetics. Progesterone and 17-hydroxyprogesterone each protect the enzyme against inactivation. The progesterone and analogue is a competitive inhibitor of the enzyme with Ki values of 8.4 microM and 7.8 microM for progesterone and 17-hydroxyprogesterone, respectively. The enzyme inactivation and kinetic data are consistent with a theory proposing that the analogue and the two substrates compete for the same active site. The radioactive analogue 17-[( 14C]bromoacetoxy)progesterone causes inactivation of the enzyme with incorporation of 1.5-2.2 mol of the analogue per mole of inactivated enzyme. When this experiment is carried out in the presence of a substrate, then 0.9-1.2 mol of radioactive analogue is incorporated per mole of inactivated enzyme. The data suggest that the analogue can bind to two different sites, one of which is related to the catalytic site. Radiolabeled enzyme samples, from reactions of the 14C-labeled analogue with the enzyme alone or with enzyme in the presence of a substrate, were subjected to amino acid analysis and also to tryptic digestion and peptide mapping.(ABSTRACT TRUNCATED AT 250 WORDS)