Bovine adrenocortical microsomal hemeproteins P-45017 alpha and P-450C-21. Isolation, partial characterization, and comparison to P-450SCC

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.

17-Hydroxylase: an evaluation of the present view of its catalytic role in steroidogenesis

This survey analyses the evidence that has led to the belief that the catalytic role of 17-hydroxylase in the biosynthesis of cortisol, estradiol, testosterone and dehydroepiandrosterone is confined to two chemical reactions: pregnenolone-->17-hydroxypregnenolone-->dehydroepiandrosterone. This analysis suggests that the evidence supporting this view is not compelling enough to accept it unquestioningly. Different interpretations of the data can suggest other catalytic roles for 17-hydroxylase that are worthy of consideration. One such alternative is proposed.

Electron paramagnetic resonance investigation of cytochrome P-450c21 from bovine adrenocortical microsomes: a new enzymatic activity

Cytochrome P-450c21 (CYP21A1) purified from bovine adrenocortical microsomes was investigated by electron paramagnetic resonance (EPR) spectroscopy to clarify the interactions among heme active center, protein surroundings, water molecules and bound substrates or analogues. The low-spin EPR signals of the oxidized enzyme (as purified) consisted of two species; one at gz = 2.39, gy = 2.23, and gx = 1.925 (component A) and the other at gz = 2.42, gy = 2.23, and gx = 1.916 (component B). The component A is probably representing a product-bound form, whereas the component B indicates either occupation of the substrate-binding site with a substrate analogue or absence of steroid at the site. Upon addition of progesterone, the component A signal completely disappeared and the intensity of high-spin signal (g = 8.06, 3.54) increased. Addition of 17 alpha-hydroxyprogesterone caused a development of a new low-spin signal at gz = 2.42, gy = 2.21, and gx = 1.966 (component C) and a further increase in intensity of the high-spin signal (g = 8.06, 3.54). Addition of 20 beta-hydroxyprogesterone caused an increase in intensity of the component C signal (and the g = 8 high-spin signal) even stronger than did 17 alpha-hydroxyprogesterone. These observations suggest that 20 beta-hydroxyprogesterone binds to the cytochrome P-450c21 active center in a very similar manner as 17 alpha-hydroxyprogesterone does and, therefore, may be a metabolizable substrate. A new enzymatic pathway catalyzed by cytochrome P-450c21 was confirmed with a reconstituted enzymatic system consisting of cytochrome P-450c21, NADPH-cytochrome P-450 reductase and an NADPH-generating system. 20 beta-Hydroxyprogesterone was converted to progesterone via a putative 20 beta-oxidase reaction in a comparable turnover number to that of the 21-hydroxylation of 17 alpha-hydroxyprogesterone.

Steroid 21-hydroxylase is a major autoantigen involved in adult onset autoimmune Addison's disease

An adrenal-specific protein reacting with autoantibodies in the sera of patients with adult onset Addison's disease has been purified from human adrenal glands. The protein, mol.wt. 55K, has the biochemical characteristics of steroid 21-hydroxylase and reacts on Western blots with rabbit antibodies to recombinant 21-hydroxylase. Absorption of the native human 55K adrenal protein with human adrenal autoantibodies prevented the subsequent reaction of the 55K protein with rabbit antibodies to 21-hydroxylase in Western blot analysis. In addition, human adrenal autoantibodies reacted with recombinant 21-hydroxylase expressed in yeast. These data indicate that the adrenal specific enzyme steroid 21-hydroxylase is a major autoantigen involved in adult onset autoimmune Addison's disease.

Fatty acid monooxygenation by P450BM-3: product identification and proposed mechanisms for the sequential hydroxylation reactions

The soluble P450 isolated from Bacillus megaterium (the product of the CYP 102 gene) (P450BM-3) is a catalytically self-sufficient fatty acid hydroxylase which converts lauric, myristic, and palmitic acids to omega-1, omega-2, and omega-3 hydroxy analogs. The percentage distribution of the regioisomers depends on the substrate chain length. Lauric and myristic acids were preferentially metabolized to their omega-1 hydroxy counterparts while no hydroxylation occurred when capric acid was used as the substrate. Palmitic acid, when present at concentrations greater than the concentration of oxygen in the reaction medium (greater than 250 microM), was hydroxylated to its omega-1, omega-2, and omega-3 hydroxy analogs, with the percentage distribution of the regioisomers being 21:44:35, respectively. No omega hydroxylation of any of the fatty acids was detected. When the concentration of palmitic acid was less than the concentration of oxygen in the reaction mixture, it was noted that a number of additional products were formed. Under these conditions, unlike lauric and myristic acids, it was observed that palmitic acid was first converted to its monohydroxy isomers which were subsequently metabolized to a mixture of 14-ketohexadecanoic, 15-ketohexadecanoic, 13-hydroxy-14-ketohexadecanoic, 14-hydroxy-15-ketohexadecanoic, and 13,14-dihydroxyhexadecanoic acids with a relative distribution of 8:2:40:30:20, respectively. Thus, P450BM-3 is able not only to monohydroxylate a variety of fatty acids but also to further metabolize some of these primary metabolites to secondary and tertiary products. The present paper characterizes the products formed during the sequential hydroxylation of palmitic acid and proposes reaction pathways to explain these results.

A hypervariable region of P450IIC5 confers progesterone 21-hydroxylase activity to P450IIC1

Cytochrome P450IIC5 is a hepatic progesterone 21-hydroxylase while the 95% identical P450IIC4 has a greater than 10-fold higher Km for progesterone 21-hydroxylation and the 74% identical P450IIC1 does not hydroxylate progesterone at detectable rates. Previous work demonstrated that the apparent Km of P450IIC4 for progesterone 21-hydroxylation can be markedly improved by replacing a valine at position 113 with an alanine which is present at this position in P450IIC5. In the present studies, a single point mutation in cytochrome P450IIC1 that changed valine at position 113 to alanine conferred progesterone 21-hydroxylase activity to this enzyme. Although the catalytic activity was less than that of P450IIC5, these results indicate the residue 113 plays a critical role in the determination of the substrate/product selectivity in subfamily IIC P450s. By alignment with the sequence of P450cam, the segment of the polypeptide, residues 95-123, containing residue 113 corresponds to a substrate-contacting loop in the bacterial enzyme. The region containing residue 113, which is highly variable among family II P450s, may also be a substrate-contacting loop in the mammalian cytochromes P450. The exchange of this hypervariable region of cytochrome P450IIC1, residues 95-123, with that of P450IIC5 enhanced the 21-hydroxylase activity of the cells transfected with this chimera to levels similar to those of cells transfected with the plasmid encoding P450IIC5. Kinetic analysis of microsomes isolated from the transfected cells showed that the apparent Km for progesterone 21-hydroxylation of the chimera was indistinguishable from that of P450IIC5.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of cytochrome P-450 (SCC) from pig testis mitochondria

Cytochrome P-450 was purified from pig testis mitochondria to a specific content of 13.1 n mol/mg of protein. The purified preparation was found to contain a single species of P-450, on sodium dodecyl sulfate polyacrylamide gel electrophoresis, with an apparent molecular weight of about 53000 +/- 2000. The cholesterol side chain-cleavage system could be reconstituted by mixing the purified cytochrome P-450, adrenodoxin reductase, adrenodoxin, cholesterol and NADPH. The rate of conversion of cholesterol to pregnenolone was 6.2 n mol/min/n mol of P-450 under the conditions employed. The absorption spectrum of the oxidized cytochrome P-450 had maxima at 416, 530 and 568 nm. The reduced CO-complex of the cytochrome P-450 exhibited an absorption maximum at 448 nm. The purified P-450 was subjected to microsequence analysis and its NH2-terminal amino acid sequence was found to show considerable homology with that of bovine adrenal P-450 (SCC).

Purification and properties of steroid 17 alpha-hydroxylase from calf testis

Steroid 17 alpha-hydroxylase has emerged as a key enzyme in steroidogenic cells: (i) it represents the branch point between the 17-deoxy (mineralo) and the 17-hydroxy (gluco) corticosteroid pathways in the adrenal cortex; (ii) the corresponding specific cytochrome (P-450(17 alpha] is highly dependent upon hormonal regulation; and (iii) the enzyme also catalyzes the steroid 17-20 lyase reaction, leading to the major androgens in the testis. As a prerequisite to the study of its regulation in intact cell, 17 alpha-hydroxylase was purified from calf testis microsomal preparations. Following five chromatographic steps, the enzyme was obtained as an apparently homogeneous protein of Mr = 57 kDa upon gel electrophoresis. The procedure yielded a recovery of about 10% as judged by cytochrome P-450 assay. Whereas 17 alpha-hydroxylase specific activity was about 30-fold enriched during the purification, that of the C17-20 lyase was increased by about 6-fold, strongly suggesting that its organelle environment may modulate the enzymatic activity. The purified enzyme yielded a 20 N-terminal amino-acid sequence showing a complete homology with that of its adrenal counterpart and a polyclonal antibody raised against our preparation revealed a 57 kDa protein band in bovine adrenocortical microsomal extracts, upon immunoblotting experiments. It was thus concluded that bovine 17 alpha-hydroxylase activity is supported by highly similar if not identical enzymatic proteins in both testis and adrenal cortex tissues. The purified P-450(17 alpha) preparation is now being used in reconstitution experiments which suggest that microsomal components may contribute to a different expression of the enzyme specificity in its native testis or adrenocortical intracellular environment, respectively.

Immunohistochemical localization of cytochrome P-450C21 in human adrenal cortex and its relation to endocrine function

Cytochrome P-450C21 was successfully demonstrated in the human adrenal glands by a peroxidase-antiperoxidase method. All three cortical layers were stained in the normal adrenal glands, particularly the zonae glomerulosa and reticularis. Well-stained and faintly stained cells were intermingled in the zona fasciculata, suggestive of intrazonal variations. The immunoreactivity was particularly intense at the site of ACTH action, i.e., cells in micronodules and cells around myelolipomatous lesions in adrenocortical hyperplasia of Cushing's disease and sites of regeneration in the normal adrenal glands. In adrenocortical adenomas with Cushing's syndrome and primary aldosteronism, cells with large nuclei were generally stained well. In the adrenocortical tissue adjacent to a functioning adenoma, the immunoreactivity was observed only in the zona glomerulosa, especially in cases of primary aldosteronism. This finding is consistent with morphologic observations.

The function of NADH-semidehydroascorbate reductase and ascorbic acid in corticosteroid hydroxylation

We have demonstrated in rat adrenal (Natarajan, R.D. and Harding, B.W. (1985) J. Biol. Chem. 260, 3902-3905) that NADH-semidehydroascorbate reductase and ascorbate participate in an electron transport pathway (ETP) supplying reducing equivalents from NADH to cytochrome P-450scc. Here, we demonstrate that this ascorbate dependent ETP also supplies reducing equivalents to cytochrome P-450(11 beta/18) in both rat adrenal and bovine adrenal cortex. The activity is dependent upon addition of catalase or upon 'cold shock' treatment of isolated mitochondria. Comparison of the rates of 11 beta- and 18-hydroxylation supported by this ETP and by the classical pathway supported by various TCA cycle intermediates suggests that in vivo the ascorbate dependent pathway may be essential for maximal flow of reducing equivalents to the mitochondrial hydroxylases. Partial reconstitution of the ascorbate dependent 11 beta/18-hydroxylase activity was achieved with purified bovine outer mitochondrial and inner mitochondrial membranes fortified with supernatant from sonified mitochondria all preincubated with phosphatidyl choline. These preparations no longer require catalase or 'cold shock' treatment. Ascorbate and NADH-semidehydroascorbate reductase are unable to support 17 alpha- or 21-hydroxylase activity in isolated bovine adrenal cortical microsomes whether incubated with purified outer mitochondrial membranes or not.

Biochemical properties of cytochrome P-450 in relation to steroid oxygenation

The P-450 cytochromes have been characterized biochemically in recent years as a family of monooxygenases that reductively activate molecular oxygen for insertion into steroids and other physiologically occurring lipids. Many of these enzymes are also known to bind and oxygenate a host of foreign compounds, including alcohol, drugs, pesticides, anesthetics, and mutagens. Some of the poorly understood variations in congenital adrenal hyperplasia may represent nutritional effects on the P-450 oxygenase systems or the ability of xenobiotics to interfere with normal steroid metabolism by these versatile cytochromes.

Regulation of synthesis and activity of bovine adrenocortical NADPH-cytochrome P-450 reductase by ACTH

Primary, monolayer cultures of bovine adrenocortical cells have been utilized to demonstrate that ACTH regulates the synthesis and thus the activity of NADPH-cytochrome P-450 reductase. The temporal pattern of induction correlates well with that previously reported for cytochromes P-45017 alpha and P-450 C21, microsomal steroid hydroxylases serviced by P-450 reductase. Results of in vitro translation studies suggest that ACTH regulates P-450 reductase synthesis at the transcriptional level and show that the adrenocortical enzyme is synthesized as the mature form. The action of ACTH to induce P-450 reductase synthesis in the adrenal cortex can be mimicked by dibutyryl cAMP, but not by phenobarbital which induces P-450 reductase synthesis in liver.

Regulation of the biosynthesis of cytochromes P-450 involved in steroid hormone synthesis

The actions of ACTH to regulate the synthesis of the various enzymes involved in steroid hormone biosynthesis have been studied using bovine adrenocortical cells in monolayer culture. ACTH causes an increase in the synthesis of both the mitochondrial and the microsomal forms of cytochrome P-450 involved in steroid hormone biosynthesis, as well as of the iron-sulfur protein involved in transferring electrons to the mitochondrial forms of cytochrome P-450, namely, adrenodoxin. This increased synthesis is reflective of an increase in translatability of mRNA species specific for these various proteins, and appears in each case to be mediated by cyclic AMP. Whereas the mitochondrial proteins are synthesized as precursors of higher molecular weight which are processed upon insertion into the mitochondria, the microsomal proteins are synthesized as species identical in molecular weight to the mature forms. In order to determine whether the action of ACTH to increase the rate of synthesis of these proteins is the result of an increase in the levels of specific mRNA species, cDNA clones complementary to these mRNA species are being isolated. These probes will also make it possible to characterize the genes encoding the steroidogenic enzymes, as well as to identify regulatory elements which control their transcription.

Peptide alignment of the porcine 21-hydroxylase cytochrome P-450 using a cDNA sequence of the corresponding bovine enzyme

The amino acid sequence deduced from a cDNA clone of the bovine adrenal steroid 21-hydroxylase cytochrome P-450 has been utilized to align peptide sequences derived from the corresponding porcine enzyme. Homology analysis revealed that only fifty percent of the amino acid sequence predicted by the cDNA clone overlapped with peptide sequences from the porcine enzyme. The homology in the remaining portions of the bovine sequence was restored by considering amino acid sequences predicted by the two additional DNA reading frames of the cDNA sequence. Forty eight percent of the bovine sequences predicted by the two alternate reading frames showed strong homology with the porcine peptide sequences. A minimum of 4 nucleotide sequencing errors account for the observed reading frame alterations and the approximate position of each error in the bovine cDNA sequence has been established.

Regiospecific hydroxylation of lauric acid at the (omega-1) position by hepatic and kidney microsomal cytochromes P-450 from rainbow trout

Microsomes from liver or kidney of untreated rainbow trout hydroxylated lauric acid specifically at the (omega-1) position. Turnover numbers for liver (2.72 min-1) and kidney (14.1 min-1) were decreased seven- and twofold, respectively, following treatment with beta-naphthoflavone. Laurate hydroxylation activity from untreated trout hepatic microsomes was sensitive to inhibition by SKF-525A, but was not sensitive to metyrapone and only partially inhibited by alpha-naphthoflavone. The temperature optimum of laurate (omega-1) hydroxylation in trout liver microsomes was 25-30 degrees C. The Km and Vmax for (omega-1)- hydroxylaurate formation was 50 microM and 1.63 nmol min-1 mg-1, respectively, in liver and 20 microM and 3.95 nmol min-1 mg-1, respectively, in kidney from untreated trout microsomes. (omega-1) Hydroxylation of laurate, in both liver and kidney microsomes, was sensitive to an antibody raised against a previously purified cytochrome P-450 isozyme (LM2) of trout liver microsomes, which has been shown to be active towards aflatoxin B1. Antibody to the major isozyme of cytochrome P-450 ( LM4b , active towards benzo(a)pyrene) induced by beta-naphthoflavone did not inhibit (omega-1) hydroxylation of laurate in microsomes from untreated or beta-naphthoflavone-treated trout.

Induction of 17 alpha-hydroxylase (cytochrome P-450(17)alpha) activity by adrenocorticotropin in bovine adrenocortical cells maintained in monolayer culture

Using bovine adrenocortical cells in monolayer culture it has been shown that treatment with adrenocorticotropin (ACTH) causes a dramatic increase in 17 alpha-hydroxylase activity. In postmitochondrial supernatant fractions (PMS) prepared from cells maintained in culture, there was a 15-fold increase in 17 alpha-hydroxylase activity 36 h following initiation of ACTH treatment compared with the activity measured in PMS prepared from control cells. In the continued presence of ACTH, 17 alpha-hydroxylase activity declined; however, even after 60 h of exposure to ACTH, 17 alpha-hydroxylase activity was eight times higher than that present in control cells. The dramatic increase in 17 alpha-hydroxylase activity provides an explanation for the previously observed phenomenon that following initiation of ACTH treatment of bovine adrenocortical cells in monolayer culture there is a shift in the pattern of corticosteroid secretion from approximately equal amounts of cortisol and corticosterone to almost exclusively cortisol. Thus, the modulation of 17 alpha-hydroxylase activity by ACTH action appears to serve a key regulatory role in the pattern of corticosteroid production. Soluble cytosolic factors apparently do not participate in the regulation of 17 alpha-hydroxylase activity in the bovine adrenal cortex. Increases in the magnitude of substrate-induced absorbance changes are indicative that the increase in 17 alpha-hydroxylase activity is due, at least in part, to an elevation of cytochrome P-450(17)alpha synthesis.

Insights into the active site of the cytochrome P-450 haemoprotein family--a unifying concept based on structural considerations

1. Cytochromes P-450CAM of Pseudomonas putida and P-450LM-2 of rabbit liver, and to a lesser extent also adrenocortical mitochondrial cytochromes P-450(11) beta and P-450SCC, were investigated by active site-targeted reagents and by immunochemical techniques. The results of these studies and of the alignment of the amino acid sequences of cytochromes P-450CAM and of phenobarbital-induced rat-liver P-450LM support the following conclusions. 2. Cytochrome P-450 haemoproteins follow a common architectural design, most readily apparent in the surroundings of the haem group. Sequence homology and associated folding leads to immunochemical similarities revealed by radioimmune assay. 3. The haem domain is partially exposed on the surface of the protein. This part appears to be tightly structured from several distinct portions of the polypeptide chain which carry the haem catalytic site and the substrate binding site, respectively. 4. After photocovalent labelling with substrate- or inhibitor-derived arylazido reagents, a substantial portion of the haem domain may be excised by BrCN or formic acid and purified without loss of haem ('haemopeptide'). A major antibody-binding site is associated with this fragment. 5. Exploration of the haem pocket of cytochrome P-450CAM, using bromoacetyl and mercapto derivatives of its substrate, camphor, revealed an activated SH group responsible for binding and precision alignment of camphor toward the haem iron, presumably by forming a transient thiohemiketal bond. This cysteine may also function in translocation of nascent product for facilitated release. Specific terpenoid substrates of other cytochrome P-450 haemoproteins may be bound in a similar manner. 6. From radiolabelled peptide studies the substrate-binding cysteine was identified as residue 56 in the amino acid sequence of cytochrome P-450CAM. The haemchelating cysteine occupies position 134 or 146 in cytochrome P-450CAM and position 152 or 173 in cytochrome P-450LM, respectively.