Studies on structure-function relationships of human choriogonadotropins with C-terminally shortened α-subunits. II. Stimulation of adenylate cyclase activity and depletion of cytochromeP-450 in the rat testis

Human choriogonadotropin (hCG) analogues, containing the native β-subunit and α-subunits enzymatically shortened by 2–3 amino acid residues, were used for studying influence of hCG on the content of microsomal progesterone-binding cytochromeP-450 in rat tests. When 2–3 residues have been renuwed from the α-subunit, the ability of the hormone analogue to stimulate adenylate cyclase of isolated rat Leydig cells was diminished by 55%. When the hCG analogue containing a des-(88–92)-α chain was applied, the residual activity of the adenylate cyclase was negligible. 18 h after administration to rats in vivo, the hormone species containing des-(Lys-91-Ser-92)-α or des-(90–92)-α, respectively, were found to have induced a decrease in microsomal cytochromeP-450 content with an effectiveness corresponding to their ability of stimulating the adenylate cyclase in vitro. However, when assayed 48 h after application, the desensitization of the microsomal cytochromeP-450 system had persisted in case of the hCG species containing a des-(90–92)-α chain but not in case of hCG consisting of des-(Lys-91-Ser-92)-α and a native β-subunit. From these results, it is concluded that short-term effects of hCG on the microsomal content of progesterone-binding cytochromeP-450 are mediated by the stimulation of adenylate cyclase. In contrast, the long-lasting action of hCG on this system seems not to be exclusively mediated by the increase in intracellular cAMP.     

Studies on structure-function relationships of human choriogonadotropins with C-terminally shortened alpha-subunits. I. Receptor binding and immunologic properties

Recombination products composed of the native beta-subunit and an alpha-subunit with an enzymatically shortened C-terminal region showed a diminished (less than 5 amino acids removed) or - in the case of des-(88-92)-alpha/native beta - a completely abolished ability to bind to testicular LH/hCG receptors of the rat. An antigenic determinant which is present in native hCG but not in the isolated subunits was not or incompletely expressed in the modified hormone species. Antigenic determinants which are characteristic for the isolated alpha-subunit, however, were not affected by removal of the C-terminal residues 88-92. The immunologic experiments indicate that hCG containing an alpha-subunit with a shortened C-terminal region differs from native hCG in its conformation. These conformational changes are probably responsible for the loss in receptor-binding ability.     

Studies of the specific role of the subunits of choriogonadotropin for biological, immunological and physical properties of the hormone. Digestion of choriogonadotropin and its isolated subunits with serine carboxypeptidase

The residues 90-92 can be split off from the C-terminal region of the isolated alpha-subunit of choriogonadotropin (residues 88--92: -Tyr-Tyr-His-Lys-Ser-OH) by means of serine carboxypeptidase (des-Lys91,Ser92-alpha-subunit; des-(90-92)-alpha-subunit). However, when choriogonadotropin is digested by serine carboxypeptidase, only the residues 143-145 (-Leu-Pro-Gln-OH) form the C-terminus of the beta-subunit are released (des-(143-145)-choriogonadotropin). Depending on the pH conditions, glutamine 145 and the residues 143-145, respectively, are liberated by digestion of the isolated beta-subunit (des-Gln145-beta-subunit and des-(143-145)-beta-subunit, respectively). The present study provides evidence that the C-termini of both the isolated subunits and those in choriogonadotropin are probably arranged on the surface of the molecules. The biological activity of des-(143-145)-choriogonadotropin is not significantly decreased. The immunological activity, however, is reduced when measured by complement fixation. In comparison to the native hormone, a four-fold amount of des-(143-145)-choriogonadotropin has to be applied to obtain highest complement fixation. The conformation of des-(143-145)-choriogonadotropin does not seem to differ from that of the native hormone, when estimated both by CD measurements and by Ans-choriogonadotropin fluorescence. The respective determinant therefore seems to depend, at least to some extent, on the sequence of the C-terminal region of the beta-subunit of the hormone; complement fixation, however, does not seem to be affected significantly, when the des-(143-145)-beta-subunit is compared with the native beta-subunit using an antiserum against the native beta-subunit. This provides evidence that this C-terminal determinant is possibly more immunogenic at the hormone than at the isolated beta-subunit. The biological activity of recombined choriogonadotropin in vivo as well as in vitro is markedly reduced when serine 92 is removed from the C-terminus of the alpha-subunit (des-Ser92,Lys91-alpha-native beta-subunit: 36% residual activity in vivo). Biological activity is lost when the residues 88-90 are removed by digestion of the des-Ser92,Lys91-alpha-subunit with carboxypeptidase A. Recombination products between a modified alpha-and the native beta-subunit show a reduced Anschoriogonadotropin fluorescence (des-Lys91,-Ser92-alpha + native beta-subunit: 52%; des-(88-92)-alpha- + native beta-subunit: 23%). The Ans-induced aggregation of choriogonadotropin, however, also takes place in those recombination products which display a low Ans-choriogonadotropin fluorescence, indicating that the reduction is probably not caused by a portion of the molecules losing their binding sites for Ans. Therefore the diminished Ans-choriogonadotropin fluorescence seems to signal small conformational changes. The CD spectra of the native and the des-(90-92)-alpha-subunit, however, seem not to differ significantly. It is shown that the release of amino acids from the C-terminus of the alpha-subunit causes a disturbance of the interaction between the subunits. This seems to prevent an effective conformational change of the beta-subunit which probably is a prerequisite for the binding of the hormone to the receptors of Leydig cells.     

Effect of human chorionic gonadotropin and estradiol in vivo on estradiol binder and cytochrome P-450 concentrations in rat testis

The effects of single administration to adult male rats in vivo of various amounts of human chorionic gonadotropin (HCG) and of single or repeated injections of estradiol on testicular cytoplasmic estradiol binder concentrations and on microsomal progesterone-binding cytochrome P-450 were compared. Half-life periods of HCG-induced loss of estradiol binder and cytochrome P-450 concentrations are identical (6 h) whereas a strong dissociation of these half-life periods are evident after chronic estradiol treatment (less than 2 h for the estradiol binder, about 35 h for cytochrome P-450). Depletion of cytoplasmic estradiol binder is not a sufficient condition for mediation of effects on cytochrome P-450 content. Rate of replenishment of microsomal cytochrome P-450 is similar after HCG or estradiol treatment. Both HCG- and estradiol-induced loss of cytochrome P-450 occur not only in Leydig cells but also in microsomes prepared from seminiferous tubules. Additional information is presented contradicting the hypothesis that loss of cytochrome P-450-dependent steroidogenic enzymes caused by HCG could be mediated by estrogens.     

Phosphorylation of hepatic phenobarbital-inducible cytochrome P-450.

The major phenobarbital-inducible cytochrome P-450 purified from rat liver, a member of family II of the cytochrome P-450 gene superfamily, is rapidly phosphorylated by cAMP-dependent protein kinase. The phosphorylation reaches greater than 0.5 mol phosphate/mol P-450 after 5 min and is accompanied by a decrease in enzyme activity. The serine residue in position 128 was shown to be the sole phosphorylation site and a conformational change of the protein was indicated by a shift of the carbon monoxide difference spectrum of the reduced cytochrome from 450 to 420 nm. Comparison of amino acid sequences of various cytochrome P-450 families revealed a highly conserved arginine residue in the immediate vicinity of the phosphorylated serine residue which constitutes the kinase recognition sequence. It also revealed that only the members of the cytochrome P-450 family II carry this kinase recognition sequence. To find out whether this phosphorylation also occurs in vivo, the exchangeable phosphate pool of intact hepatocytes derived from phenobarbital-pretreated rats was labeled with 32Pi followed by an incubation of the cells with the membrane-permeating dibutyryl-cAMP or with the adenylate cyclase stimulator glucagon to activate endogenous kinase. As a result, a microsomal polypeptide with the same electrophoretic mobility as cytochrome P-450 became strongly labeled. Peptide mapping and immunoprecipitation with monospecific antibodies identified this protein as the major phenobarbital-inducible cytochrome P-450. It becomes phosphorylated at the same serine residues as in the cell-free phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relation between the 7-ethoxyresorufin-O-deethylase activity of the liver microsomal monooxygenase system and the level of methylcholanthrene-induced form of cytochrome P-450

Changes in the metabolic activity of 7-ethoxyresorufin in rat liver microsomes containing different amounts of cytochrome P-450 induced by 3-methylcholanthrene and other polycyclic hydrocarbons (P-450c) were studied. Using antibodies to cytochrome P-450c for the determination of the cytochrome P-450c content and its metabolic role, it was demonstrated that 7-ethoxyresorufin O-deethylation by the liver microsomal monooxygenase system is catalyzed exclusively by cytochrome P-450c. The rate of the substrate metabolism is correlated with the cytochrome P-450c content in microsomal membranes; the cytochrome P-450c activity does not depend on the cytochrome P-450c/NADPH-cytochrome P-450 reductase ratio. The experimental results suggest that the level of 7-ethoxyresorufin metabolism in liver microsomes can be regarded as a measure of the cytochrome P-450c content, whose function is associated with the stimulation of potential carcinogenic and toxic substances.     

Actions of cyclic adenosine monophosphate on the cytodifferentiation of ovarian cells: studies in cultured swine granulosa cells using a novel exogenous adenylate cyclase from Bordetella pertussis

The regulation by cAMP of cholesterol side-chain cleavage activity and the synthesis of immunoisolated cytochrome P-450scc and adrenodoxin proteins was investigated in primary cultures of swine ovarian (granulosa) cells. Administration of a novel adenylate cyclase toxin isolated from Bordetella pertussis increased granulosa-cell cAMP accumulation up to 200-fold over basal. These effects were additive with those of FSH, forskolin, and cholera toxin. In contrast, bacterial extracts BP 347 and BP 348 from mutant strains of B. pertussis that lack either all virulent factors or the adenylate cyclase toxin and hemolysin were devoid of effect. Granulosa-cell cAMP accumulation supported by active bacterial adenylate cyclase was accompanied by 2- to 11-fold, time-dependent increases in [35S]methionine incorporation into immunospecific cytochrome P-450scc and adrenodoxin. These increases in the synthesis of cholesterol side-chain cleavage proteins were associated with enhanced pregnenolone production in response to exogenous sterol substrate, 25-hydroxycholesterol, and augmented progesterone secretion both in the absence and presence of exogenous lipoprotein. Moreover, the effects of Bordetella adenylate cyclase toxin on granulosa cell steroidogenesis were functionally integrated with other regulatory responses, since the non-cAMP dependent effector, estradiol 17-beta, interacted synergistically with bacterial adenylate cyclase in stimulating progesterone production. We conclude that exogenous adenylate cyclase isolated from B. pertussis can be functionally integrated into the cAMP-dependent effector pathway of granulosa cells with a resulting increase in intracellular cAMP concentrations, augmented biosynthesis of progesterone and pregnenolone, enhanced synthesis of immunospecific cytochrome P-450scc and adrenodoxin, and synergistic interactions with a non-cAMP-dependent ovarian effector hormone (estradiol).(ABSTRACT TRUNCATED AT 250 WORDS)     

Expression, purification, and properties of the flavoprotein domain of cytochrome P-450BM-3. Evidence for the importance of the amino-terminal region for FMN binding

Comparison of the amino acid sequences of several microsomal cytochrome P-450 reductases to the flavoprotein domain (BMR) of cytochrome P-450BM-3 has revealed that this class of flavoproteins contains evolutionarily conserved regions that are important for their interaction with nucleotide substrates and cofactors. In order to understand the properties of BMR, the region encoding this protein, beginning at residue Lys-472 of cytochrome P-450BM-3, was subcloned and expressed in Escherichia coli. The recombinant protein (more than 50% of host-soluble proteins) was purified to homogeneity using conventional purification procedures. BMR (Mr 66,000) showed typical flavoenzyme absorbance spectra, contained FAD and FMN in a stoichiometry of 1:1, and catalyzed reduction of several artificial electron acceptors with rates comparable to those of the microsomal NADPH-cytochrome P-450 oxidoreductase. Limited trypsinolysis of BMR, under non-denaturing conditions, revealed that the protease removed the NH2-terminal 122 residues. This region was postulated to contain amino acids that are important for FMN binding (Porter, T. D. (1991) Trends Biochem. Sci. 16, 154-158). Consistent with this hypothesis, the major tryptic product of BMR (BMR-52, Mr 52,000) contained only FAD, in an equimolar ratio to the protein. Also, like the FMN-depleted microsomal NADPH-cytochrome P-450 oxidoreductase (Kurzban, G. P., Howarth, J., Palmer, G., and Strobel, H. W. (1990) J. Biol. Chem. 265, 12272-12279), BMR-52 was active for only catalyzing ferricyanide reduction. These data provide strong experimental evidence for a discrete multidomain structure of BMR, as proposed for the membrane-bound reductases, with an amino-terminal FMN binding region and carboxyl-terminal FAD- and NADPH binding regions. Thus, BMR strongly resembles the microsomal cytochrome P-450 reductase and offers an opportunity to better understand the structure-function relationships of this class of flavoproteins.     

Cytochrome P-450 arachidonic acid epoxygenase. Regulatory control of the renal epoxygenase by dietary salt loading.

The rat kidney microsomal epoxygenase catalyzed the asymmetric epoxidation of arachidonic acid to generate as major products: 8(R),9(S)-, 11(R),12(S)- and 14(S),15(R)-epoxyeicosatrienoic acids with optical purities of 97, 88, and 70%, respectively. Inhibition studies utilizing a panel of polyclonal antibodies to several rat liver cytochrome P-450 isoforms, indicated that the renal epoxygenase(s) belongs to the cytochrome P-450 2C gene family. Dietary salt, administered either as a 2-2.5% (w/v) solution in the drinking water or as a modified solid diet containing 8% NaCl (w/w), resulted in marked and selective increases in the renal microsomal epoxygenase activity (416 and 260% of controls, for the liquid and solid forms of NaCl, respectively) with no significant changes in the microsomal omega/omega-1 oxygenase or in the hepatic arachidonic acid monooxygenase reaction. Immunoblotting studies demonstrated that dietary salt induced marked increases in the concentration of a cytochrome P-450 isoform(s) recognized by polyclonal antibodies raised against human liver cytochrome P-450 2C10 or rat liver cytochrome P-450 2C11. Comparisons of the stereochemical selectivity of the induced and non-induced microsomal epoxygenase(s) with that of purified rat liver cytochrome P-450 2C11 suggest that the salt-induced protein(s) is catalytically and structurally different from liver cytochrome P-450 2C11. The in vivo significance of dietary salt in regulating the activities of the kidney endogenous arachidonic acid epoxygenase was established by the demonstration of a salt-induced 10-20-fold increase in the urinary output of epoxygenase metabolites. These results, in conjunction with published evidence demonstrating the potent biological activities of its metabolites, suggest a role for the epoxygenase in the renal response to dietary salt.     

Localization of cytochrome c-binding domain on NADPH-cytochrome P-450 reductase

A covalent complex between purified rat liver microsomal NADPH-cytochrome P-450 reductase and horse cytochrome c was formed through cross-linking studies with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide at low ionic strength. The purified cross-linked derivative shows that this product is a 1:1 complex containing one molecule each of the flavoprotein and cytochrome. The covalent complex had almost completely blocked the electron transfer from NADPH to exogenous cytochrome c or the rabbit liver microsomal cytochrome P-450 induced by phenobarbital, indicating that the cross-linked cytochrome c covers the electron-accepting site of the reductase. These results suggest that the covalently cross-linked derivative is a valid model of the noncovalent electron transfer complex. Although the exact number and site of the cross-linked location were not determinable, in cytochrome c the amide bond originates from Lys-13 and in reductase it might be at any one of six different side chain carboxyl groups in the two neighboring cluster acidic residues, Asp-207, -208, and -209, and Glu-213, Glu-214, and Asp-215. It is therefore proposed that the six clustered carboxyl groups on reductase are in an exposed location near the area where one heme edge comes close to the molecular surface.     

Conversion of lysine 91 to methionine or glutamic acid in human choriogonadotropin alpha results in the loss of cAMP inducibility.

Human choriogonadotropin (hCG) is a heterodimeric glycoprotein hormone. The alpha subunit comprises 92 amino acids, of which 6 are Lys residues (Morgan, F.G., Birken, S., and Canfield, R.E. (1975) J. Biol. Chem. 250, 5247-5258). Our photoaffinity-labeling studies indicate that several of these Lys residues make contact with the lutropin receptor and are covalently cross-linked to the receptor. Lys-91 of the alpha subunit is of interest because deletion of the two alpha C-terminal residues, Lys-91 and Ser-92, results in a significant reduction in the bioactivity of lutropin and thyrotropin (Cheng, K.-W., Glazer, A.N., and Pierce, J.G. (1973) J. Biol. Chem. 248, 7930-7937). To determine the importance of Lys-alpha 91, we substituted it with Arg, Met, or Glu. The resulting mutant alpha cDNA constructs were co-transfected into CHO cells with the wild type hCG beta cDNA construct. Secreted hCG dimers were assayed for binding to receptors on porcine granulosa cells and stimulation of cAMP synthesis in a murine Leydig tumor cell line. The natural hCG, wild type hCG, and all mutant hCGs recognized the receptor, although with somewhat divergent affinities. However, there was a striking difference in the ability of cAMP induction. The natural hCG, wild type hCG, and Lys-91----Arg mutant hCG induced cAMP synthesis, whereas the Lys-91----Met and Lys-91----Glu mutants did not. These results demonstrate that Lys-91 is important for receptor modulation in the stimulation of cAMP synthesis.     

The mechanism of hydroperoxide-dependent reactions with participation of cytochrome P-450

Cytochrome P-450 destruction kinetics by cumene hydroperoxide (CHP) has been studied at 25 degrees C in phosphate buffer, pH 7.25-7.50, in various systems: intact and induced rat or rabbit microsomes, highly purified LM2- and LM2- and LM4-forms of cytochrome P-450 from rabbit liver microsomes. The destruction kinetics is characterized by three phases in all systems. The CHP-influenced cytochrome P-450 destruction is a radical chain process with linear termination of the chains. The acidic phospholipids, phosphatidylserine and phosphatidylinositol and total microsomal phospholipids containing the acidic lipid components activate cytochrome P-450 in the hydroxylation of aniline and naphthalene by CHP. Phosphatidylcholine and sphingomyelin have no effect upon the cytochrome P-450 activity in the type I and II substrates oxidation by CHP. The phase transitions of the microsomal phospholipids influence the interaction of cytochrome P-450 with its reductase, altering the activation energy of type I substrates oxidation. The type II substrate oxidation is not affected by phase transitions in the full microsomal hydroxylating system.     

Spin state transitions of liver microsomal cytochrome P-450.

Spin state transitions of membrane-bound cytochrome P-450 were investigated by difference spectrophotometry using the 'D'-charge transfer absorbance band at 645 nm as a measure of the amount of hemin iron present in the 5-coordinated state. The magnitude of the 'D'-absorbance band in the absence of exogenous substrates, e.g., the concentration of native high spin cytochrome P-450, was evaluated from the difference in absorbance at 645 nm between ferric cytochrome P-450 and the carbon monoxide derivative of the pigment in its ferrous state. The contribution of the native high spin species to the total cytochrome P-450 content of microsomes was calculated to be between 40% and 65% after induction with phenobarbital and polycyclic hydrocarbons, respectively. Up to 80% of the cytochrome P-450 was found to be present in the high spin state after the addition of exogenous substrates. Further, the steady state concentrations of high spin cytochrome P-450, observed in the presence of reduced pyridine nucleotides, suggest that the rate limiting step for microsomal mixed function oxidation reactions is variable and dependent on the substrate under investigation.     

Selective chemical modification of cytochrome P-450SCC lysine residues. Identification of lysines involved in the interaction with adrenodoxin

Selective chemical modification of cytochrome P-450SCC has been carried out with lysine-modifying reagents. Modification of cytochrome P-450SCC with succinic anhydride was shown to result in loss of its ability to interact with intermediate electron transfer protein - adrenodoxin. To identify amino acid residues involved in charge-ion pairing with complementary carboxyl groups of adrenodoxin, cytochrome P-450SCC complex with adrenodoxin was modified with succinic anhydride. Adrenodoxin was then removed and cytochrome P-450 was additionally modified with isotopically labelled reagent. Subsequent chymotryptic hydrolysis of [14C]succinylated cytochrome P-450SCC and separation of digest obtained by combining various types of HPLC resulted in seven major radioactive peptides. The amino acid sequence of the peptides was determined by microsequencing. The major amino groups modified with radioactive succinic anhydride were found to be at Lys-73, -109, -110, -126, -145, -148 and -154 in the N-terminal sequence of cytochrome P-450SCC molecule and at Lys-267, -270, -338 and -342 in the C-terminal sequence. The role of electrostatic interactions in fixation of cytochrome P-450SCC complex with adrenodoxin is discussed.     

Evidence that covalent binding of metabolically activated phenol to microsomal proteins is caused by oxidised products of hydroquinone and catechol

The metabolic activation of [14C]phenol resulting in covalent binding to proteins has been studied in rat liver microsomes. The covalent binding was dependent on microsomal enzymes and NADPH and showed saturation kinetics for phenol with a Km-value of 0.04 mM. The metabolites hydroquinone and catechol were formed at rates which were 10 or 0.7 times that of the binding rate of metabolically activated phenol. The effects of cytochrome P-450 inhibitors and cytochrome P-450 inducers on the metabolism and binding of phenol to microsomal proteins, suggest that cytochrome P-450 isoenzyme(s) other than P-450 PB-B or P-450 beta NF-B catalyses the metabolic activation of phenol. Furthermore, reconstituted mixed-function oxidase systems containing cytochrome P-450 PB-B and P-450 beta NF-B were (on basis of cytochrome P-450 content) 6 and 11 times less active in catalysing the formation of hydroquinone than microsomes. The isolated metabolites hydroquinone and catechol bound more extensively to microsomal proteins than phenol and the binding of these was not stimulated by NADPH. The binding occurring during the metabolism of phenol could be predicted by the rates of formation of hydroquinone and catechol and the rates by which the isolated metabolites were bound to proteins.     

Hepatic mitochondrial cytochrome P-450 system. Distinctive features of cytochrome P-450 involved in the activation of aflatoxin B1 and benzo(a)pyrene

Rat liver mitoplasts containing less than 1% microsomal contamination contain cytochrome P-450 at 25% of the microsomal level and retain the capacity for monooxygenase activation of structurally different carcinogens such as aflatoxin B1 (AFB1), benzo(a)pyrene (BaP), and dimethylnitrosamine. Both phenobarbital (PB) and 3-methylcholanthrene (3-MC) induce the level of mitochondrial cytochrome P-450 by 2.0- to 2.5-fold above the level of control mitoplasts. The enzyme activities for AFB1 (3-fold) and BaP (16-fold) metabolism were selectively induced by PB and 3-MC, respectively. Furthermore, the metabolism of AFB1 and BaP by intact mitochondria was supported by Krebs cycle substrates but not by NADPH. Both PB and 3-MC administration cause a shift in the CO difference spectrum of mitoplasts (control, 448 nm; PB, 451 nm; and 3-MC, 446 nm) suggesting that they induce two different forms of mitochondrial cytochromes P-450. Mitoplasts solubilized with cholate and fractionated with polyethylene glycol exhibit only marginal monooxygenase activities. The activity, however, was restored to preparations from both PB-induced and 3-MC-induced mitochondrial enzymes (AFB1 activation, ethylmorphine, and benzphetamine deamination and BaP metabolism) by addition of purified rat liver cytochrome P-450 reductase, and beef adrenodoxin and adrenodoxin reductase. The latter proteins failed to reconstitute activity to purified microsomal cytochromes P-450b and P-450c that were fully active with P-450 reductase. Monospecific rabbit antibodies against cytochrome P-450b and P-450c inhibited both P-450 reductase and adrenodoxin-supported activities to similar extents. Anti-P-450b and anti-P-450c provided Ouchterlony precipitin bands against PB- and 3-MC induced mitoplasts, respectively. We conclude that liver mitoplasts contain cytochrome P-450 that is closely similar to the corresponding microsomal cytochrome P-450 but can be distinguished by a capacity to interact with adrenodoxin. These inducible cytochromes P-450 are of mitochondrial origin since their levels in purified mitoplasts are over 10 times greater than can arise from the highest possible microsomal contamination.     

Metabolization of Porphyrinogenic Agents in Brain: Involvement of the Phase I Drug Metabolizing System. A Comparative Study in Liver and Kidney

(1) We evaluated the involvement of brain mitochondrial and microsomal cytochrome P-450 in the metabolization of known porphyrinogenic agents, with the aim of improving the knowledge on the mechanism leading to porphyric neuropathy. We also compared the response in brain, liver and kidney. To this end, we determined mitochondrial and microsomal cytochrome P-450 levels and the activity of NADPH cytochrome P-450 reductase. (2) Animals were treated with known porphyrinogenic drugs such as volatile anaesthetics, allylisopropylacetamide, veronal, griseofulvin and ethanol or were starved during 24 h. Cytochrome P-450 levels and NADPH cytochrome P-450 reductase activity were measured in mitochondrial and microsomal fractions from the different tissues. (3) Some of the porphyrinogenic agents studied altered mitochondrial cytochrome P-450 brain but not microsomal cytochrome P-450. Oral griseofulvin induced an increase in mitochondrial cytochrome P-450 levels, while chronic Isoflurane produced a reduction on its levels, without alterations on microsomal cytochrome P-450. Allylisopropylacetamide diminished both mitochondrial and microsomal cytochrome P-450 brain levels; a similar pattern was detected in liver. Mitochondria cytochorme P-450 liver levels were only diminished after chronic Isoflurane administration. In kidney only mitochondrial cytochrome P-450 levels were modified by veronal; while in microsomes, only acute anaesthesia with Enflurane diminished cytochrome P-450 content. (4) Taking into account that δ-aminolevulinic acid would be responsible for porphyric neuropathy, we investigated the effect of acute and chronic δ-aminolevulinic acid administration. Acute δ-aminolevulinic acid administration reduced brain and liver cytochrome P-450 levels in both fractions; chronic δ-aminolevulinic acid administration diminished only liver mitochondrial cytochrome P-450. (5) Brain NADPH cytochrome P-450 reductase activity in animals receiving allylisopropylacetamide, dietary griseofulvin and δ-aminolevulinic acid showed a similar profile as that for total cytochrome P-450 levels. The same response was observed for the hepatic enzyme. (6) Results here reported revealed differential tissue responses against the xenobiotics assayed and give evidence on the participation of extrahepatic tissues in porphyrinogenic drug metabolization. These studies have demonstrated the presence of the integral Phase I drug metabolizing system in the brain, thus, total cytochrome P-450 and associated monooxygenases in brain microsomes and mitochondria would be taken into account when considering the xenobiotic metabolizing capability of this organ. Dedicated to the memory of Dr. Susana Afonso     

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 cytochromes in microsomal fractions of germinating mung beans.

Detailed studies of microsomal cytochromes from mung-bean radicles showed the presence of cytochrome P-420, particularly in dark-grown seedlings, accompanied by smaller quantities of cytochrome P-450. Similar proportions of cytochrome P-420 to cytochrome P-450 were found spectrophotometrically in vivo with whole radicles and hypocotyls. Assayed in vitro, maximum concentrations of both cytochromes were attained after 4 days of growth, before undergoing rapid degradation. Illumination of seedlings stabilized cytochrome P-450 and decreased the amount of cytochrome P-420. Three b cytochromes were present in the microsomal fraction, namely cytochromes b-562.5 (Em + 105 +/- 23 mV), b-560.5 (Em + 49 +/- 13 mV) and b5 (Em - 45 +/- 14 mV), all at pH 7.0. Of the b cytochromes, cytochrome b5 alone undergoes a rapid degradation after day 4, Changes in cytochrome b concentrations were confined to the microsomal fraction: mitochondrial b cytochrome concentrations were unaltered with age. Protohaem degradation (of exogenous methaemalbumin) was detected in microsomal fractions of mung beans. The rates of degradation were highest in extracts of young tissue and declined after day 4. The degradation mechanism and products did not resemble those of mammalian haem oxygenase.     

Glucagon: structure-function relationships investigated by sequence deletions

A series of glucagon analogues, des-(1-4)-glucagon, des-(5-9)-glucagon, des-(10-15)-glucagon, des-(16-21)-glucagon, des-(22-26)-glucagon and des-(27-29)-glucagon, were prepared by condensation of synthetic fragments and characterized biologically and immunologically. Fully synthetic glucagon was also characterized. The potencies with regard to glucagon receptor binding in purified rat liver plasma membranes were, in decreasing order: synthetic glucagon 108%, des-(1-4)-glucagon 5.7%, des-(27-29)-glucagon 0.92%, des-(5-9)-glucagon 0.47%, des-(10-15)-glucagon 0.0028%, des-(16-21)-glucagon 0.0017% and des-(22-26)-glucagon 0.00060% relative to that of natural porcine glucagon. Des-(27-29)-glucagon was the only analogue that activated the adenylate cyclase in rat liver plasma membranes or stimulated the lipolysis in isolated free fat cells from rat epididymal fat pad. The potencies were 0.16% and 0.20% of that of glucagon, respectively. Des-(1-4)-glucagon was a glucagon antagonist in the adenylate cyclase assay. The immunoreactivities of the glucagon analogues were determined with two commonly used anti-glucagon sera, K 5563 and K 4023, directed towards the C-terminus and some segment in the sequence 2-23, respectively. In the K 5563 assay, des-(27-29)-glucagon and des-(22-26)-glucagon had potencies of 0.0009% and less than 0.09% of that of glucagon, respectively. The remaining analogues had potencies varying from 45% to 141% of that of glucagon. In the K 4023 assay, the analogues showed a non-linear dilution effect. The combined results indicate a partition within the glucagon molecule with regard to receptor binding and adenylate cyclase activation. The region 10-26 appears to be the most important for receptor binding, whereas 1-4 is essential for adenylate cyclase activation. The C-terminal segment 27-29 is important for the maintenance of full receptor binding but non-essential for adenylate cyclase activation.