The immunocytochemical detection of cytochrome P-450 monooxygenase in the lungs of fetal, neonatal, and adult hamsters

Antibodies against rabbit cytochrome P-450 reductase (reductase), cytochrome P-450 isozyme 2 (P-450 IIB), and cytochrome P-450 isozyme 5 (P-450 IVB) were used to detect homologous enzymes in the developing lung of the Syrian golden hamster. No immunocytochemical labeling was observed on gestational days 11, 12, and 13. On gestational day 14, light immunoperoxidase labeling for reductase and P-450 IIB was observed over cells lining the trachea and cranial portions of lobar bronchi. On gestational day 15, these enzymes were detected in conducting airways at all anatomic levels, and in the media of the pulmonary vein and its branches. Light labeling for P-450 IVB was first observed over cells lining the trachea and lobar bronchi on gestational day 15, but the smallest bronchioles and the media and endothelium of the pulmonary vein did not label for this enzyme until gestational day 16 (neonatal day 1). Type II pneumocytes and the pleural mesothelium first labeled for each of the three enzymes on neonatal day 1. Although the mesothelium no longer labeled for reductase or P-450 IIB in hamsters 3.5 wk old, the other labeling sites persisted in adult hamsters. Because cytochrome P-450 enzymes are associated with the endoplasmic reticulum, an ultrastructural examination of differentiating secretory cells was made to detect its appearance. At each conducting airway level, smooth endoplasmic reticulum was present in the cells 2 d before cytochrome P-450 enzymes could be detected immunocytochemically. The appearance of these enzymes paralleled the development of the hamster lung; they were first present in the trachea and lobar bronchi, then in the bronchioles, and finally in the alveoli.

Immunochemical characterization of multiple forms of cytochrome P-450 in rabbit nasal microsomes and evidence for tissue-specific expression of P-450s NMa and NMb

Two unique forms of cytochrome P-450 (P-450), designated NMa and NMb, were recently isolated in this laboratory from nasal microsomes of rabbits. In the present study, polyclonal antibodies to the purified nasal cytochromes were prepared. Immunochemical analysis with specific rabbit anti-NMa and sheep anti-NMb antibodies indicated that P-450 isozymes identical to or having a high structural homology with NMa are present in both olfactory and respiratory mucosa, as well as in liver, but NMb was detected only in the olfactory mucosa. Neither form was detected in other tissues examined, including brain, esophageal mucosa, heart, intestinal mucosa, kidney, and lung. The specific occurrence of NMb in the olfactory mucosa was further substantiated by the detection and specific inhibition by anti-NMb of the formation of unique NMb-dependent metabolites of testosterone in olfactory microsomes but not in microsomes from liver or respiratory mucosa. Similar experiments with antibodies to previously purified rabbit hepatic P-450 isozymes indicated that not all of the hepatic cytochromes are expressed in the nasal tissues. Thus, P-450 isozymes structurally homologous to hepatic forms 2, 3a, and 4, but not 3b and 6, were found in the olfactory mucosa. On the other hand, only form 2 was detected in the respiratory mucosa. Immunoquantitation experiments revealed that NMa and NMb are the major P-450 forms in olfactory microsomes, whereas NMa and P-450 form 2 (or its homolog) constitute the major portion of the respiratory nasal microsomal P-450. The level of NMa in the liver is relatively low, accounting for less than 3% of total microsomal P-450 in this tissue. In addition, evidence is provided that NMa is the major catalyst in the dealkylation of two nasal carcinogens, hexamethylphosphoramide and phenacetin, in both olfactory and respiratory nasal microsomes.

Distribution of cytochromes P-450, cytochrome b5, and NADPH-cytochrome P-450 reductase in an entire human liver

In rat liver there appear to be significant differences between lobes in the concentration of individual cytochrome P-450 isozymes (Sumner and Lodola, Biochem Pharmacol 36: 391-393, 1987). Because studies in patients often rely on small pieces of liver obtained from diverse anatomical locations, it seemed important to determine if the cytochromes P-450 were also heterogeneously distributed in human liver. Accordingly, tissue was obtained from ten different locations in a single human liver including those most commonly biopsied by percutaneous needles, and by surgeons during laparotomy. The differences observed between locations in the microsomal concentrations of carbon monoxide-binding protein (total cytochrome P-450), cytochrome b5, and NADPH-cytochrome P-450 reductase appeared to be small and were not statistically significant. Likewise, no significant differences were observed between locations in the specific content of HLp, HLp3, HLj, HLx or P450MP. However, the specific concentrations of HLd varied almost 2-fold between the microsomes and this was statistically significant in some cases (P less than 0.05). Our results suggest that, in human livers, regional differences in the content of cytochromes P-450 are generally small but may be significant for some isozymes. With the exception of HLd, tissue obtained by percutaneous or surgical liver biopsies is probably representative of the entire organ with regard to the enzymes assayed.

In situ localization and distribution of xenobiotic-activating enzymes and aryl hydrocarbon hydroxylase activity in lungs of untreated rats

The present investigation was undertaken to more precisely establish where xenobiotics can be oxidatively metabolized and bioactivated within the lung. To accomplish this, antibodies raised against NADPH-cytochrome P-450 reductase (EC 1.6.2.4) and cytochromes P-450 BNF-B, PB-B, and PCN-E (the major forms of cytochrome P-450 induced by beta-naphthoflavone, phenobarbital, and pregnenolone-16 alpha-carbonitrile, respectively) that had been purified to apparent homogeneity from rat liver microsomes were used to determine the localizations and distributions of these enzymes immunohistochemically at the light microscopic level within lungs of untreated rats. Additionally, the intrapulmonary sites at which benzo(alpha)pyrene undergoes hydroxylation were identified in situ by means of fluorescence histochemistry. Immunohistochemical staining for NADPH-cytochrome P-450 reductase and cytochromes P-450 BNF-B, PB-B, and PCN-E was detected in bronchial epithelial cells, both ciliated and nonciliated (Clara) bronchiolar epithelial cells, and type II pneumocytes as well as other cells in the alveolar wall. Results of microfluorometric analyses of the immunofluorescence staining intensities of bronchial epithelial cells, Clara cells, and type II pneumocytes demonstrated further that Clara cells bound the antibodies raised to NADPH-cytochrome P-450 reductase and cytochrome P-450 PB-B to significantly greater extents than did bronchial epithelial cells and type II pneumocytes. Thus, in lungs of untreated rats, Clara cells contain the greatest amounts of these two enzymes. In marked contrast, the antibodies directed against cytochromes P-450 BNF-B and PCN-E were each bound to similar extents by bronchial epithelial cells, Clara cells, and type II pneumocytes. In agreement with immunohistochemical observations on the intrapulmonary localizations of NADPH-cytochrome P-450 reductase and cytochromes P-450 BNF-B, PB-B, and PCN-E in untreated rats, benzo(alpha)pyrene was hydroxylated in situ by bronchial and bronchiolar epithelial cells and alveolar wall cells, especially type II pneumocytes. These immunohistochemical and histochemical findings, thus, demonstrate that bronchial epithelial cells, Clara and ciliated bronchiolar epithelial cells, and type II pneumocytes as well as other alveolar wall cells represent sites for the in vivo oxidative metabolism and bioactivation of xenobiotics in lungs of untreated rats.

The cosubstrate NADP(H) protects lysine 601 in the porcine NADPH-cytochrome P-450 reductase against pyridoxylation

Lys601 in NADPH-cytochrome P-450 reductase is modified by reductive alkylation with pyridoxal 5'-phosphate (pyridoxylation). Lys601 is protected against modification by the cosubstrate NADP(H).

Effects of phenobarbital, dietary protein intake, and ewe liveweight on ovulation rate and concentrations of plasma FSH and hepatic microsomal enzymes

Coopworth ewes were differentially fed from December 1985 to April 1986 to produce two liveweight classes: fat (55-60 kg) and thin (40-45 kg). The ewes were then fed on either high-protein (22%) or low-protein (12%) diets, with or without phenobarbital treatment for 10 days commencing on Day 7 of the oestrous cycle. Phenobarbital treatment caused an increase in ovulation rate that was most pronounced in thin ewes and those on the low-protein diet. Ewe liveweight produced an increase in ovulation rate, but increased dietary protein in the particular formulations used had no effect. Hepatic enzyme concentrations were increased by phenobarbital treatment and, to a lesser extent, by dietary protein intake and ewe liveweight. However, these metabolic changes and the increase in ovulation rate were not accompanied by interpretable changes in the plasma FSH concentrations.

NADPH-cytochrome P-450 reductase is not a component of the liver microsomal steroid 5-alpha reductase

Liver microsomal steroid 5-alpha-reduction is catalyzed by a NADPH-dependent enzyme system. The requirement of NADPH-cytochrome P-450 reductase to shuttle reduction equivalents from NADPH to steroid 5-alpha-reductase was investigated using an inhibitory antibody against NADPH-cytochrome P-450 reductase. This antibody preparation inhibited cytochrome c reduction in microsomes from female rat liver with an I50 of 0.75 mg antibody/mg of microsomal protein. Benzphetamine N-demethylation and testosterone 6-beta-hydroxylation, two cytochrome P-450-mediated oxidative reactions, were inhibited by the antibody. On the other hand, testosterone 5-alpha-reductase was not affected by the antibody. These results suggest that NADPH-cytochrome P-450 reductase is not an obligatory component of the liver microsomal steroid 5-alpha-reduction.

Identification and induction of cytochromes P450, P450IIE1 and P450IA1 in rabbit bone marrow

A number of drugs and xenobiotics which require metabolic activation to reactive metabolites by cytochrome P450 are toxic to hemopoietic tissue. In the present study we used immunoblot analysis and specific catalytic activities to identify two isozymes of cytochrome P450 (P450IIE1 and P450IA1) in rabbit bone marrow microsomal preparations. P450IIB1, P450IIC1, P450IIIA6 and P450IA2 were below detectable levels using immunoblot analysis. P450IIE1 was present at a concentration of 0.0065 nmol/mg of microsomal protein in marrow microsomes from untreated rabbits, and it was induced 12.9-fold by acetone treatment. The change in the P450IIE1 apoprotein determined immunochemically in bone marrow microsomes after acetone treatment was paralleled by a 9.4-fold increase in benzene hydroxylation, a P450IIE1-dependent activity. Aroclor 1254 pretreatment induced P450IA1 in marrow microsomes 11-fold to a specific content of about 0.025 nmol/mg of microsomal protein. 7-Ethoxyresorufin deethylation, a P450IA1-dependent activity, was induced 15.7-fold by Aroclor 1254 treatment. The increases in the enzymic activities are consistent with an increase in catalytically competent holoenzymes. The presence of these two isozymes in bone marrow could result in the metabolic activation of compounds in the bone marrow producing toxic metabolites at the site of toxicity.

Effect of 2-butanol and 2-butanone on rat hepatic ultrastructure and drug metabolizing enzyme activity

The effect of a single oral dose of 2-butanol (2.2 ml/kg) or 2-butanone (1.87 ml/kg) on hepatic ultrastructure and drug-metabolizing enzyme activity was studied in the rat. A 135-197% increase in acetanilide hydroxylase activity was found in rats sacrificed 12-40 h after dosing with 2-butanol or 2-butanone. A 40-h pretreatment with 2-butanone produced a 155% increase in aminopyrine N-demethylase activity. NADPH-cytochrome c reductase activity and the concentrations of cytochromes P-450 and b5 were largely unaltered 2-40 h after dosing with either agent. Electron microscopic examination of hepatocytes from rats sacrificed 16 h after 2-butanol or 2-butanone revealed a marginal increase in the prevalence of smooth endoplasmic reticulum. However, by 40 h, there was a marked proliferation of the smooth endoplasmic reticulum and reduction in rough endoplasmic reticulum in response to both agents. The most marked potentiation of CCl4 hepatotoxicity occurred when rats were pretreated with 2-butanol or 2-butanone 16 h before CCl4 administration. The coincidental finding of maximal CCl4-induced hepatic injury and elevation of microsomal xenobiotic activity within the same time frame following 2-butanol or 2-butanone supports the hypothesis that aliphatic alcohols and ketones potentiate CCl4 hepatotoxicity by enhancing biotransformation of the halocarbon to cytotoxic metabolites.

Growth inhibition and induction of phenotypic alterations by L-histidinol in B16 mouse melanoma cells

L-Histidinol, a histidine analog was recently shown to be an inducer of differentiation in the promyelocytic cell line HL-60. In the present study we show that L-histidinol inhibits the growth of B16 melanoma cells in vitro. Growth inhibition is accompanied by phenotypic alterations that include a marked increase in the activities of NADPH cytochrome c reductase and gamma glutamyl transpeptidase, lipid accumulation and cell enlargement. These phenotypic alterations are similar to those induced by other chemical inducers of differentiation in melanoma cells.

Heterogeneity of smooth endoplasmic reticulum from rat liver studied by two-phase partitioning

Smooth microsomal membranes, prepared from rat liver by sucrose-density-gradient centrifugation, were subfractionated by counter-current distribution in an aqueous two-phase system consisting of poly(ethylene glycol) and Dextran T500. A comparison of the distribution curves of marker enzymes, together with theoretically calculated curves, indicated the presence of at least five membrane subfractions, differing in the ratios of the marker enzymes. Glucose-6-phosphatase and arylesterase distributed in one manner, and NADPH-cytochrome c reductase and NADH-ferricyanide reductase in another. Evidence for further heterogeneities in the distribution of marker enzymes in smooth microsomes was obtained by analysing the membrane domain structure using a recently described method [Albertsson (1988) Q. Rev. Biophys. 21, 61-98]. Phenobarbital treatment did not influence the behaviour of the marker enzymes.

Human NADPH-P450 oxidoreductase: complementary DNA cloning, sequence and vaccinia virus-mediated expression and localization of the CYPOR gene to chromosome 7

The cDNA containing the full coding sequence of human NADPH-P450 oxidoreductase was isolated and completely sequenced. The cDNA contained 2398 base pairs, including 9 and 358 base pairs of 5' and 3' noncoding sequences, respectively. The human NADPH-P450 oxidoreductase protein deduced from the cDNA has 677 amino acids, with a calculated molecular weight of 76,656. The cDNA nucleotide and deduced amino acid sequences displayed 83 and 92% similarities, respectively, with those of the rat NADPH-P450 oxidoreductase. By use of somatic cell hybrids, the NADPH-P450 oxidoreductase gene was regionally localized to human chromosome 7 (7p15-q35). The levels of NADPH-P450 oxidoreductase protein and mRNA were analyzed in 13 human liver specimens and less than 3-fold variation was found among the different livers. The NADPH-P450 oxidoreductase cDNA was inserted into vaccinia virus and expressed in cell culture. The cDNA-expressed enzyme was active in reducing the electron acceptor cytochrome c. In addition, the NADPH-P450 oxidoreductase stimulated the enzymatic activity of vaccinia virus-expressed human P3(450) when both recombinant viruses were used to coinfect human cells in culture. An approximate equal mole level of NADPH-P450 oxidoreductase and P3(450) was required to achieve maximal activity for both ethoxycoumarin O-deethylase and aryl hydrocarbon hydroxylase.

Distribution of endoplasmic reticulum and calciosome markers in membrane fractions isolated from different regions of the canine brain

Four regions of the canine brain (frontal lobe, parieto-occipital lobe, brainstem, and cerebellum) were each fractionated by differential centrifugation into a crude mitochondrial pellet (P2) and a crude microsomal pellet (P3). Markers of endoplasmic reticulum (glucose-6-phosphate phosphatase and rotenone-insensitive NADPH cytochrome c reductase) and markers of the 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store ([3H]IP3 binding and IP3-induced Ca2+ release) were measured. No correlation was found between the two classes of markers, which suggests that the IP3 receptor does not belong to the endoplasmic reticulum in canine brain. Cerebellum P2 and P3 fractions displayed levels of [3H]IP3 binding 10- to 30-fold higher, and rates of IP3-induced Ca2+ release greater than 15-fold faster than the homologous cerebrum and brainstem fractions. Actively accumulated Ca2+ was only partially released by IP3, both before and after saponin disruption of the plasma membrane compartment. The proportion of the IP3-sensitive Ca2+ store relative to that of the total (IP3-sensitive and IP3-insensitive) Ca2+ store was variable; i.e., it was larger in cerebellum P2 (approximately 90%) than in cerebrum fractions (less than 30%). Cerebellum fractions constitute the best source from which an IP3-sensitive Ca2+ storing organelle can be purified.

Purification and catalytic properties of a cross-linked complex between adrenodoxin reductase and adrenodoxin

A stable covalent complex was prepared by cross-linking adrenodoxin reductase with adrenodoxin using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The covalent complex was purified extensively until free components were removed completely. The major component of the complex had a molecular weight of 63 kDa, which corresponds to a 1:1 stoichiometric complex between adrenodoxin reductase and adrenodoxin. NADPH-cytochrome c reduction activity of the covalent complex was comparable to that of an equimolar mixture of adrenodoxin reductase and adrenodoxin (native complex), and the NADPH-ferricyanide reduction activity of the complex was equal to that of the native one. In contrast to the native complex, the covalent complex produced much less superoxide upon NADPH-oxidation, and the covalent complex was found to be more stable than the native complex, suggesting that the complex state is more favorable for catalysis. From these results, we conclude that the adrenodoxin molecule does not need to dissociate from the complex during electron transfer from NADPH to cytochrome c.

Immunohistochemical localization of NADPH-cytochrome P450 reductase in human tissues

In an attempt to elucidate the mechanism(s) behind the susceptibility of tissues to the toxic effects of chemicals, NADPH-cytochrome P450 reductase IgG has been used to map the distribution and localization of the cytochrome P450 system in hepatic and extrahepatic human tissues. Employing the Western blot procedure this antibody recognized a single band in human liver microsomes which corresponded in mol. wt to the purified reductase. Immunoreactive NADPH-cytochrome P450 reductase staining was detected in all zones of the liver acinus, with maximal staining in hepatocytes adjacent to the terminal hepatic venules (zone 3). Considerable variation in the intensity of reductase staining was observed in different segments of the gastrointestinal tract. Staining was most intense in the enterocytes of the small intestine, with maximal staining at the tips of the villi. Colonic epithelial cells were variably positive while the rectum was negative. Pancreatic ductal cells were positive whereas exocrine cells were negative. In the lung, reductase was detected in bronchiolar and bronchial epithelial cells, Clara cells and alveolar lining cells. In the kidney, the proximal and distal convoluted tubules, the loops of Henle, the collecting ducts in the medulla and the transitional epithelium all stained positively for reductase. The results demonstrate specific cellular localization of NADPH-cytochrome P450 reductase, and hence the cytochrome P450 system, in human tissues. The differential distribution of the reductase within human tissues may lead to a better understanding of the mechanism underlining site-specific carcinogenesis.

Demonstration of cytochrome reductases in rat liver peroxisomes: biochemical and immunochemical analyses

In this study we utilized the analytical cell fractionation approach in combination with immunoblotting techniques and immunoelectron microscopy to test for the presence of NADPH cytochrome P-450 reductase and NADH cytochrome c (b5) reductase in rat liver peroxisomes. Highly purified peroxisomes from clofibrate-treated rats exhibited both NADPH cytochrome P-450 reductase activity and NADH cytochrome c reductase activity (using cytochrome c as an electron acceptor). These activities were inhibited by the respective reductase antibodies made against the endoplasmic reticulum (ER) enzymes. Immunoblot data in combination with immunoelectron microscopy indicated that the peroxisomal NADPH cytochrome P-450 reductase is localized in the matrix of the organelle and has a subunit of molecular weight similar to that of the ER enzyme, whereas the NADH cytochrome c (b5) reductase is localized in the membranes of the peroxisomes. Again, the subunit molecular weight was similar to that of the ER enzyme. The presence of these reductases in peroxisomes further supports the role of this organelle in bile acid synthesis and cholesterol metabolism.

Sodium cholate extraction of rat liver nuclear xenobiotic-metabolizing enzymes

DNA is the purported target of several carcinogenic and mutagenic agents. Nuclear enzymes which could generate or detoxify reactive metabolites are of major concern. Several such enzymes have been identified within nuclei, but obtaining samples with enriched content or activity is difficult, time-consuming, and uses harsh isolation techniques. Extraction of rat liver nuclear suspensions with cholate-containing buffer results in solubilization of 25-30% of the protein. Linear extraction was obtained for total protein and cytochromes P-450 and b5, NADPH-cytochrome P-450 reductase, NADH-cytochrome b5 reductase, DT-diaphorase, and microsomal-like epoxide hydrolase with specific activities comparable to values reported for isolated nuclear membrane, while the yield was five to ten times greater. Detergent extracts of rat liver nuclei were employed to study the comparative response of microsomal and nuclear enzymes to chemical treatment. While the responses to acute inductive (phenobarbital and 3-methylcholanthrene) and toxic (carbon tetrachloride and dibromochloropropane) treatments were qualitatively similar, an initiation-promotion protocol (diethylnitrosamine with phenobarbital promotion) resulted in divergent responses between the enzymes in the two subcellular fractions. Detergent extracts of nuclei offer an efficient means of recovering xenobiotic-metabolizing enzymes from rat liver nuclei, and have been utilized to demonstrate a differential response of nuclear enzymes during preneoplastic development.

Dibromochloropropane (DBCP) effects on the reproductive function of the adult male rat

In order to evaluate the effect of dipromochloropropane (DBCP) on reproductive function, groups of male adult Wistar rats were injected subcutaneously with different doses of DBCP. A strictly dose-dependent effect resulting in histological alteration principally toward the seminiferous tubules was observed. Sperm count and sperm motility showed azoospermia or oligoasthenospermia with a significative recovery in the lower dose of DBCP treated rats. Among the enzymatic activities evaluated as biochemical markers of testicular function: LDH was not appreciably affected by the DBCP treatment while GGT and NADPH-cyt P450-reductase significantly enhanced suggesting an induction of the detoxification processes.

Immunochemical detection of cytochrome P450C-M/F and NADPH-cytochrome P450 reductase in rat liver and kidney

The localization and distribution of NADPH-cytochrome P450 reductase and cytochrome P450C-M/F were investigated immunohistochemically in the liver and the kidney of untreated rats employing both an unlabelled antibody peroxidase-antiperoxidase method and a peroxidase labelled primary antibody technique. In both immunohistochemical procedures, the reductase and P450C-M/F were detected in hepatocytes throughout the liver. In contrast, the reductase and P450C-M/F in the kidney were only detectable in the proximal tubule cells.

Beta adrenoreceptor in vascular smooth muscle with special reference to subcellular localization and number of binding sites

The binding of monoiodinated cyanopindolol (ICYP) to membranes from canine and rat aortas, mesenteric arteries and mesenteric nerves has been studied. Binding of ICYP was shown by differential centrifugation and sucrose gradient purification as well as digitonin treatment to be directly related to the content of arterial muscle plasma membrane assessed by marker enzymes. The Kd values obtained were all in the range of 10 to 30 pM but maximum binding site (Bmax) values varied depending upon the extent of contamination by nonplasmalemmal membranes. The more purified plasma membrane fractions have Bmax many times higher than those reported in the literature. Although purified membrane fractions all had ICYP binding characterized by a single binding site with similar Kd values, binding to some crude fractions was complex. In addition, plasma membranes for mesenteric nerves, removed carefully by dissection in our study, were shown to contain ICYP binding sites with a similar Kd and Bmax values to those in arterial muscle membrane. Using arterial muscle purified membranes after removal of nerves by dissection, Bmax values from mesenteric arteries were demonstrated to be much larger than those from aorta in both species. Although the necessity of purifying the membranes depends on the objectives of a given experiment, our result strongly suggest that measurement of changes in receptor densities requires an analytical approach which provides plasma membrane-enriched fractions derived from a single cell type and purified to a known and comparable degree, especially when changes in density related to experimental or disease processes are being investigated.

Target inactivation analysis applied to determination of molecular weights of rat liver proteins in the purified state and in microsomal membranes

In principle, target inactivation analysis provides a means of determining the molecular weights (Mr) and states of aggregation of proteins in native environments where they are functionally active. We applied this irradiation technique to the rat liver microsomal membrane proteins: cytochrome b5, epoxide hydrolase, flavin-containing monooxygenase, NADH-ferricyanide reductase, NADPH-cytochrome P-450 reductase, and seven different forms of cytochrome P-450. Catalytic activities, spectral analysis of prosthetic groups, and sodium dodecyl sulfate-polyacrylamide electrophoresis/peroxidase-coupled immunoblotting were used to estimate apparent Mr values in rat liver microsomal membranes. Except in one case (cytochrome P-450PCN-E), the estimated Mr corresponded most closely to that of a monomer. Purified cytochrome P-450PB-B, NADPH-cytochrome P-450 reductase and epoxide hydrolase were also subjected to target inactivation analysis, and the results also suggested monomeric structures for all three proteins under these conditions. However, previous hydrodynamic and gel-exclusion results clearly indicate that all three of these proteins are oligomeric under these conditions. The discrepancy between target inactivation Mr estimates and hydrodynamic results is attributed to a lack of energy transfer between monomeric units. Thus, while P-450PCN-E may be oligomeric in microsomal membranes, target inactivation analysis does not appear to give conclusive results regarding the states of aggregation of these microsomal proteins.

Stability of drug metabolizing enzymes during the incubation conditions of the liver microsomal assay with non-induced and induced mouse liver S-9 fractions

The purpose of this work was to study the relative activities and stabilities of phase-I and phase-II drug metabolizing enzymes in incubation mixtures used in vitro genotoxicity testing in order to optimize the conditions of the assay, increase sensitivity and eliminate false negative results. Cytochrome P-450, NADPH-cytochrome P-450 (cytochrome c) reductase activity and various phase-I and phase-II enzyme activities of the drug-metabolizing system were determined in incubation mixtures used in liver microsomal assays. The behaviour of aminopyrine N-demethylase and p-nitroanisole O-demethylase activities as phase-I markers have been reported previously. Other activities measured were glutathione S-transferase, glutathione S-epoxide transferase and epoxide hydrase, and lipid peroxidation (LP) was determined. The experiments were carried out on liver S9 fractions derived from non-induced mice or mice induced with sodium phenobarbital (PB), and/or beta-naphthoflavone (beta-NF). The phase-II enzymes were much more stable (70-90% residual activity) than phase-I enzyme activities (35-60%) in all conditions tested. The residual cytochrome P-450 was approximately 70% stable and the remaining activity of NADPH-cytochrome c-reductase about 80%, indicating that this latter enzyme does not limit the rate of the monoxygenase system in these conditions. Phase-II enzymes were induced to a smaller extent (about 2 times) than in phase-I enzymes (5-6 times) by beta-NF + PB. NADPH-cytochrome c-reductase behaved as phase-II enzymes in this respect as well as for stability. LP was appreciably higher in non-induced than in induced animals. Treatment with the beta-NF + PB mixture, however, showed that induced enzymes were more stable than those obtained by simple induction with either beta-NF or PB alone. These results lead to the conclusion that prolonged incubation times in mutagenicity assays are unnecessary when considering the relative stabilities of the various phase-I and phase-II enzyme activities in the drug-metabolizing system.

Antisera against estrogen synthetase from human placental microsomes. Antibody characterization and cross-reactivity studies in other organs

Antibodies were obtained against both protein components of the cytochrome P-450 enzyme system responsible for estrogen biosynthesis, estrogen synthetase (aromatase), from human placental microsomes. The antiserum against the NADPH-cytochrome P-450 reductase component (antiserum denoted RE-DFBIV) gave a single major band at the Mr of the authentic enzyme by immunoblotting after electrophoretic separation of SDS-solubilized microsomes and inhibited both the reductase and aromatase activities in human placental and endometrial microsomes (Tseng, L. and Bellino, F.L. (1985) J. Steroid Biochem. 22, 555-557) and in homogenates of cultured aromatase-stimulated human endometrial stromal cells and human ovarian microsomes. The antiserum against the cytochrome P-450 component of aromatase (antiserum denoted P45FBIII) also gave a single band at the Mr of the authentic protein by immunoblotting after electrophoresis, and inhibited aromatase activity in homogenates of human placental microsomes, ovarian and decidual particulate fractions and cultured aromatase-stimulated endometrial stromal cells. This antiserum had no effect on NADPH-cytochrome c reductase activity in any of the systems studied. We conclude that these antiserum preparations separately recognize the NADPH-cytochrome P-450 reductase and cytochrome P-450 components of aromatase in human placenta, ovary, decidua and endometrium. Epitopes on these aromatase component proteins involved in enzyme activity are shared among these various human tissue sources.

Induction of the hepatic microsomal cytochrome P-450 system by trialkyl phosphorothioates in rats

Single i.p. doses of O,O,O-triethyl phosphorothioate [OOO-Et(S)], one of the suicide substrates for cytochrome P-450, caused a rapid increase of NADPH-cytochrome c reductase activity in rat liver microsomes. The increase was dose dependent but did not coincide with the recovery from the inhibition of drug-metabolizing activities. There was no change of Km value of the reductase in the induced state. The co-administration of cycloheximide repressed the stimulatory effect of OOO-Et(S), suggesting that a de novo synthesis of enzyme protein may be responsible for the increase in activity. Of four homologous tri-n-alkyl esters tested, the triethyl compound was the most effective at 24 and 48 hr after administration. Triethyl phosphate, the oxygen analog of OOO-Et(S), also caused an increase of the reductase activity, but carbon disulfide had no influence on this activity. Although O,O,S-triethyl phosphorodithioate [OOS-Et(S)] and its n-alkyl homologs also caused the inhibition of drug-metabolizing activities and the increase of the reductase activity, the recovery and the stimulation of enzyme activity were different from that of O,O,O-tri-n-aklyl phosphorothioates.