Induction of a high affinity nitrosamine demethylase in rat liver microsomes by acetone and isopropanol

The effects of acetone and isopropanol on the microsomal monooxygenase system have been investigated to study the role of this enzyme system in the metabolism of nitrosamines. Treatment of rats with acetone or isopropanol (2.5-5 ml/kg, i.g.) causes a 3-4.5-fold enhancement in the NADPH-dependent nitrosodimethylamine demethylase (NDMAd) activity. This is accompanied by only moderate increases in the gross cytochrome P-450 (P-450) content and NADPH-cytochrome c reductase activity. Several other monooxygenase activities were increased to different extents from an 8% increase in aryl hydrocarbon hydroxylase to a 261% increase in ethoxycoumarin O-dealkylase activities. Kinetic analysis indicates that a low Km form of NDMAd (Km = 0.07 mM) is induced by these treatments. In the microsomes of the treated rats, this high affinity form becomes predominant, in contrast to control microsomes which possess at least three Km-values for NDMAd. The treatment also enhances the metabolism of nitrosomethylethylamine, nitrosomethylbenzylamine and nitrosomethylaniline although to lesser extents than with nitrosodimethylamine. Several lines of observations suggest that the enhanced NDMAd is due to the induction of one or more specific P-450 isozyme(s) by pretreatment with acetone or isopropanol: (a) The treatment induces proteins with molecular weights (Mr) of 50 000 and 52 000 which are in the range of known P-450 isozymes. (b) The induction of these proteins and NDMAd activity was inhibited by CoCl2 and cycloheximide. (c) The induced microsomes had a peak at 450.6 nm different from the 450.0 nm peak of control microsomes. When added to the incubation mixture, both acetone and isopropanol inhibit NDMAd activity. Isopropanol is more potent than acetone and is shown to be a competitive inhibitor with a Ki-value of 0.151 mM.     

Studies on the mechanisms of induction of N-nitrosodimethylamine demethylase by fasting,acetone, and ethanol

Previous work has shown that induction of a high-affinity NADPH-dependent nitrosodimethylamine demethylase (NDMAd) in liver microsomes occurs in rats due to fasting, ethanol consumption, and streptozotocin-induced diabetes. Several lines of observations suggest that this is due to the induction of specific cytochrome P-450 isozymes. Induction of P-450 species by ethanol has also been observed by other investigators. Since each of the above altered metabolic states has in common elevated levels of ketone bodies, the possible role of acetone, a known inducer of NDMAd, in the induction of the demethylase activity was investigated. Levels of endogenous acetone in fasted rats correlated (r = 0.72) with a three- to fourfold increase in NDMAd activity. However, a dose-response experiment showed endogenous levels of acetone to be capable of causing at most 40% of the induction in fasted rats. This suggests that other ketone bodies or factors may have contributed to the induction. The induction of NDMAd by ethanol was enhanced by alcohol dehydrogenase inhibitors pyrazole and acetaldehyde oxime, suggesting that ethanol, rather than its metabolites, was responsible for the induction.     

Regulation of cytochrome P-450j, a high-affinity N-nitrosodimethylamine demethylase, in rat hepatic microsomes

Polyclonal antibodies were produced in rabbits against purified cytochrome P-450j isolated from isoniazid-treated adult male rats. The monospecificity of immunoadsorbed antibody to cytochrome P-450j was demonstrated by Ouchterlony double diffusion analyses, enzyme-linked immunosorbent assays, and immunoblots. Immunoquantitation results indicated that rat liver microsomal cytochrome P-450j content decreases between 3 and 6 weeks of age in both the male and female animal. Several xenobiotics, such as Aroclor 1254, mirex, and 3-methylcholanthrene, repressed cytochrome P-450j levels when administered to male rats. Isoniazid, dimethyl sulfoxide, pyrazole, 4-methylpyrazole, and ethanol were inducers of cytochrome P-450j in rat liver although these compounds showed different inducing potencies. Microsomes from adult male rats with chemically induced diabetes also contained elevated levels of cytochrome P-450j compared to untreated animals. Cytochrome P-450j levels were measurable in kidney, whereas this isozyme was barely detectable in lung, ovaries, and testes; however, extrahepatic cytochrome P-450j was inducible by isoniazid. Approximately 80-90% of microsomal N-nitrosodimethylamine demethylation was inhibited by antibody to cytochrome P-450j whether the microsomes were isolated from untreated rats or animals administered inducers or repressors of cytochrome P-450j. The residual catalytic activity resistant to antibody inhibition may be a reflection of the inaccessibility of a certain amount of cytochrome P-450j due to interference by NADPH-cytochrome P-450 reductase based on results obtained with the reconstituted system. There was a good correlation (r2 = 0.87) between cytochrome P-450j content and N-nitrosodimethylamine demethylase activity in microsomes from rats of different ages and treated with various xenobiotics. The evidence presented indicates that cytochrome P-450j is the primary, and perhaps sole, microsomal catalyst of N-nitrosodimethylamine demethylation at substrate concentrations relevant to hepatocarcinogenesis induced by N-nitrosodimethylamine.     

Characterization of ethanol-inducible human liver N-nitrosodimethylamine demethylase

Through the use of monospecific antibodies directed against hepatic cytochrome P-450j, an enzyme induced in rats treated with ethanol or isoniazid, we have purified from human liver the related cytochrome P-450 termed HLj. HLj resembles rat P-450j and P-450 LM3a, the homologous cytochrome in rabbit liver, in its NH2-terminal amino acid sequence, in being in highest concentration in liver microsome samples prepared from two patients intoxicated by ethanol and one patient given isoniazid, and in catalyzing the metabolic activation of the procarcinogen N-nitrosodimethylamine. Furthermore, each of nine human liver RNA samples contained a species of mRNA hybridizable to a cloned HLj cDNA. We conclude that HLj is related by structure, function, and some regulatory characteristics to rat P-450j and rabbit P-450 LM3a, cytochromes critical for metabolism of several clinically relevant cytotoxic and carcinogenic agents.     

Induction of rat hepatic drug metabolizing enzymes by dimethylcyclosiloxanes

Low molecular weight dimethylcyclosiloxanes (DMCS) are important precursors in the synthesis of polydimethysiloxane polymers widely used in industry, and in medical and personal care products. The objective of this study was to characterize the ability of two DMCS, octamethylcyclosiloxane (D4) and decamethylcyclopentasiloxane (D5) to induce drug metabolizing enzymes in rats. Male and female Sprague-Dawley rats were administered 1, 5, 20, or 100 mg/kg D4 or D5 in corn oil daily by gavage for 4 days. Changes in the levels of activity and/or immunoreactivity of CYP1A1/2, CYP2B1/2, CYP3A1/2 and NADPH cytochrome P450 reductase in liver microsomes were examined. Significant increases were observed in the liver to body weight ratio in female rats administered either D4 or D5 at doses > or = 20 mg/kg. Increases in the liver to body weight ratio were observed in male rats treated with > or = 100 mg/kg D5 but not with D4. Relatively large increases in CYP2B1/2 enzymatic activity and immunoreactive protein were observed with increasing concentrations of both D4 and D5. Significant increases in 7-pentoxyresorufin O-depentylase (PROD) activity were also detected in male and female rats given D4 at doses > or = 5 mg/kg. D5 increased PROD activity in male rats at doses > or = 20 mg/kg and in female rats at doses > or = 5 mg/kg. 7-Ethoxyresorufin O-deethylase (EROD) activity was increased in both male and female rats receiving > or = 20 mg/kg D4 or > or = 5 mg/kg D5; however, no changes were detected in CYP1A1/2 immunoreactive protein in rats of either sex. D4 and D5 caused significant increases in CYP3A1/2 immunoreactive protein in only male rats treated with 100 mg/kg of either compound. However, significant increases were detected in CYP3A1/2 immunoreactive protein in female rats at D4 doses > or = 20 mg/kg and D5 doses > or = 5 mg/kg. Induction of NADPH cytochrome P-450 reductase immunoreactive protein was observed with D4 in female rats and in both male and female rats with D5. Induction of CYP2B/1/2, CYP3A1/2 and NADPH cytochrome P450 reductase was observed in rats treated with 50 mg/kg phenobarbital by intraperitoneal injection. Maximal CYP2B induction detected with D4 was approximately 50% of the increase observed with phenobarbital. In summary, D4 and D5 induced CYP2B1/2 in adult rat liver in a manner similar to that observed with phenobarbital; however, differences were observed between D4 and D5 in their ability to induce CYP3A1/2 and NADPH cytochrome P450 reductase. Female rats were more sensitive to the inductive properties of low doses of both DMCS than male rats whereas male rats were more responsive to phenobarbital induction.     

Histone demethylase LSD1 is a folate-binding protein

Methylation of lysine residues in histones has been known to serve a regulatory role in gene expression. Although enzymatic removal of the methyl groups was discovered as early as 1973, the enzymes responsible for their removal were isolated and their mechanism of action was described only recently. The first enzyme to show such activity was LSD1, a flavin-containing enzyme that removes the methyl groups from lysines 4 and 9 of histone 3 with the generation of formaldehyde from the methyl group. This reaction is similar to the previously described demethylation reactions conducted by the enzymes dimethylglycine dehydrogenase and sarcosine dehydrogenase, in which protein-bound tetrahydrofolate serves as an accepter of the formaldehyde that is generated. We now show that nuclear extracts of HeLa cells contain LSD1 that is associated with folate. Using the method of back-scattering interferometry, we have measured the binding of various forms of folate to both full-length LSD1 and a truncated form of LSD1 in free solution. The 6R,S form of the natural pentaglutamate form of tetrahydrofolate bound with the highest affinity (K(d) = 2.8 μM) to full-length LSD1. The fact that folate participates in the enzymatic demethylation of histones provides an opportunity for this micronutrient to play a role in the epigenetic control of gene expression.     

Hypocholesterolemic effect of rice protein is due to regulating hepatic cholesterol metabolism in adult rats

Aging is one of major risk factors for developing hypercholesterolemia. To elucidate the cholesterol-lowering mechanism exerted by rice protein (RP), the effects on hepatic cholesterol outputs and cholesterol metabolism related enzymes were investigated in adult rats, which were fed by casein (CAS) and RP without cholesterol in diets. After 2 weeks of feeding, the significant cholesterol-lowering effect was observed in adult rats fed by RP compared to CAS. The hepatic total- and VLDL-cholesterol secretions into circulation were significantly depressed in RP group, whereas biliary outputs of bile acids and cholesterol were effectively stimulated by RP-feeding, causing an increase in fecal sterol excretion compared to CAS. As a result, the apparent cholesterol absorption was significantly inhibited by RP. RP-feeding significantly increased the activity and gene expression of cholesterol 7α-hydroxylase, whereas acyl-CoA:cholesterol acyltransferase-2 activity and gene expression were significantly decreased by RP as compared with CAS. Neither activity nor gene expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase of RP did differ from CAS in the liver. The present study demonstrates that rice protein can prevent hypercholesterolemia through modifying hepatic cholesterol metabolism under cholesterol-free dietary condition. The findings suggest that hypocholesterolemic action induced by rice protein is attributed in part to the inhibition of cholesterol absorption during the adult period.     

Induction of hepatic tyrosine aminotransferase mRNA by protein synthesis inhibitors.

Several protein synthesis inhibitors were as effective as the inducers hydrocortisone or cyclic AMP in elevating rat liver tyrosine aminotransferase mRNA levels when assayed in the wheat germ cell-free translational system. Cycloheximide, emetine, or puromycin increased this mRNA activity 6- to 7-fold within 4 h after in vivo administration. No increase in total hepatic mRNA levels or tryptophan oxygenase mRNA was found after treatment with these protein synthesis inhibitors. Furthermesults suggest that a short lived protein may specifically regulate the level of functional hepatic tyrosine aminotransferase mRNA or that ongoing translation of this mRNA is required for its degradation.     

Melatonin reduces rat hepatic macromolecular damage due to oxidative stress caused by delta-aminolevulinic acid

Delta-aminolevulinic acid, precursor of heme, accumulates in a number of organs, especially in the liver, of patients with acute intermittent porphyria. The potential protective effect of melatonin against oxidative damage to nuclear DNA and microsomal and mitochondrial membranes in rat liver, caused by delta-aminolevulinic acid, was examined. Changes in 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels, an index of DNA damage, and alterations in membrane fluidity (the inverse of membrane rigidity) and lipid peroxidation in microsomal and mitochondrial membranes, as indices of damage to lipid and protein molecules in membranes, were estimated. Measurements were made in rat liver after a 2 week treatment with delta-aminolevulinic acid (40 mg/kg b.w., every other day). To test the potential protective effects of melatonin, the indole was injected (i.p. 10 mg/kg b.w.) 3 times daily for 2 weeks. 8-OHdG levels and lipid peroxidation in microsomal membranes increased significantly whereas microsomal and mitochondrial membrane fluidity decreased as a consequence of delta-aminolevulinic acid treatment. Melatonin completely counteracted the effects of delta-aminolevulinic acid. Melatonin was highly effective in protecting against oxidative damage to DNA as well as to microsomal and mitochondrial membranes in rat liver and it may be useful as a cotreatment in patients with acute intermittent porphyria.     

Induction of glutathione-S-transferase isoenzymes by protein A in rat liver

The administration of Protein A, a cell wall protein of Staphylococcus aureus Cowan I cells, causes an induction of glutathione-s-transferase in rat liver. Proteins, cross reactive with anti human glutathione-s-transferase, acidic (pi), basic (alpha, and neutral (mu) isoenzymes, are induced by 5.8, 2.2 and 6.15 fold respectively. The induction of glutathione -s-transferases, at least in part, might play a role in manifestation of therapeutic properties of Protein A.     

Induction of rat hepatic mixed function oxidases by aromatic amines and its relationship to their bioactivation to mutagens

Hepatic microsomal mixed-function oxidase activities were determined in rats pretreated with the aromatic amines 2-aminoanthracene, 2-naphthylamine or 4-aminobiphenyl. All three amines stimulated the O-deethylation of ethoxyresorufin (cytochromes P-448) but none had any effect on the p-hydroxylation of aniline. 2-Aminoanthracene and 4-aminobiphenyl also stimulated the NADPH-dependent reduction of cytochrome c and 2-naphthylamine inhibited the N-demethylation of benzphetamine. Hepatic preparations from animals pretreated with 2-aminoanthracene were more efficient in converting this carcinogen to mutagens while in contrast pretreatment with Aroclor 1254 caused a marked decrease in mutagenicity. 4-Aminobiphenyl also enhanced its own activation but Aroclor-pretreated preparations were the most effective. The latter preparations were also more efficient than controls in activating 2-naphthylamine to mutagens. It is concluded that 4-aminobiphenyl and 2-aminoanthracene enhance their own activation at least partly, by inducing the synthesis of cytochromes P-448.     

Elevation of hepatic epoxide hydratase activity by ethoxyquin is due to increased synthesis of the enzyme

Feeding of the antioxidant ethoxyquin to rats leads to an increase of epoxide hydratase activity in liver microsomes. The apparent half life of the increase is 3–4 days. Elevation of epoxide hydratase activity is also obtained by intraperitoneal treatment of mice with ethoxyquin. This elevation is prevented by concomitant treatment with cycloheximide. When radiolabelled leucine is incorporated into microsomal protein by liver cell fractions from either ethoxyquin-fed or untreated rats, gel electrophoresis reveals that ethoxyquin feeding increases incorporation into epoxide hydratase. These results suggest that the elevation of epoxide hydratase activity by ethoxyquin is due to increased biosynthesis of the enzyme, i.e. enzyme induction.     

The trithorax-group protein Lid is a histone H3 trimethyl-Lys4 demethylase

Recent studies have demonstrated that histone methylation can be dynamically regulated through active demethylation. However, no demethylase specific to histone H3 trimethyl-Lys4 (H3K4me3) has been identified. Here we report that the Drosophila melanogaster protein 'little imaginal discs' (Lid), a JmjC domain-containing trithorax group protein, can demethylate H3K4me3. Consistent with its genetic classification, Lid positively regulates Hox gene expression in S2 cells.     

Purification, stabilization, and characterization of rat hepatic triglyceride lipase

Hepatic triglyceride lipase (H-TGL) was purified to near homogeneity from heparin-containing rat liver perfusates with the following column chromatography steps: heparin-Sepharose affinity chromatography, anion-exchange chromatography on DEAE-Sephacel, and gel filtration on Ultrogel AcA 34. A final specific activity of 45,000 μmol fatty acid/mg/h was obtained with an overall 31% recovery of catalytic activity. The heparin-Sepharose step resulted in a 20-fold purification, while the DEAE and gel filtration steps led to further purification with complete recovery of activity. An extensive survey of various detergents as potential stabilizers of H-TGL activity led to the selection of Triton N-101 for use in the column buffers of the DEAE and gel filtration steps. Relative to initial H-TGL activity upon dilution in buffer without detergent, recoveries between 90 and 100% were consistently obtained with Triton N-101-containing buffers following a 24-h incubation at 20°C. In contrast after a 24-h incubation at 20°C those control samples lacking detergent were at least 95% inactivated. The highly purified H-TGL exhibited a single major band by sodium dodecyl sulfate-electrophoresis. The use of DEAE chromatography and stabilization of H-TGL with Triton N-101 are the improvements in purification that resulted in an 8-fold enhancement in specific activity relative to the highest previous report of purification from rat liver perfusates.     

N-demethylation of N-nitrosodimethylamine catalyzed by purified rat hepatic microsomal cytochrome P-450: isozyme specificity and role of cytochrome b5

Metabolism of the potent hepatocarcinogen N-nitrosodimethylamine (NDMA) was evaluated in reconstituted monooxygenase systems containing each of 11 purified rat hepatic cytochrome P-450 isozymes. The reaction has an absolute requirement for cytochrome P-450, NADPH-cytochrome P-450 reductase, and NADPH, as well as a partial dependence on dilauroylphosphatidylcholine. Of the cytochrome P-450 isozymes evaluated, only cytochrome P-450j, purified from livers of ethanol- or isoniazid-treated rats, had high catalytic activity for the N-demethylation of NDMA. At substrate concentrations of 0.5 and 5 mM, rates of NDMA metabolism to formaldehyde catalyzed by cytochrome P-450j were at least 15-fold greater than the rates obtained with any of the other purified isozymes. At the pH optimum (approximately 6.7) for the reaction, the Km,app and Vmax were 3.5 mM and 23.9 nmol/min/nmol cytochrome P-450j, respectively. With hepatic microsomes from ethanol-treated rats, which contain induced levels of cytochrome P-450j, the Km,app and Vmax were 0.35 mM and 3.9 nmol/min/nmol cytochrome P-450, respectively. Inclusion of purified cytochrome b5 in the reconstituted system containing cytochrome P-450j caused a six-fold decrease in Km,app (0.56 mM) of NDMA demethylation with little or no change in Vmax (29.9 nmol/min/nmol cytochrome P-450j). Trypsin-solubilized cytochrome b5, bovine serum albumin, or hemoglobin had no effect on the kinetic parameters of the reconstituted system, indicating a specific effect of intact cytochrome b5 on the Km,app of the reaction. These results demonstrate high isozyme specificity in the metabolism of NDMA to an ultimate carcinogen and further suggest an important role for cytochrome b5 in this biotransformation process.     

Production of acetone and conversion of acetone to acetate in the perfused rat liver

The utilization of millimolar concentrations of [2-14C]acetone and the production of acetone from acetoacetate were studied in perfused livers from 48-h starved rats. We devised a procedure for determining, in a perfused liver system, the first-order rate constant for the decarboxylation of acetoacetate (0.29 +/- 0.09 h-1, S.E., n = 8). After perfusion of livers with [2-14C]acetone, labeled acetate was isolated from the perfusion medium and characterized as [1-14C]acetate. No radioactivity was found in lactate or 3-hydroxybutyrate. After 90 min of perfusion with [2-14C]acetone, the specific activity of acetate was 30 +/- 4% (n = 13) of the initial specific activity of acetone. We conclude that, in perfused livers from 2-day starved rats, acetone metabolism occurs for the most part via free acetate.     

The lysine-specific demethylase 1 is a novel substrate of protein kinase CK2

Protein kinase CK2 is a pleiotropic serine/threonine kinase responsible for the generation of a substantial proportion of the human phosphoproteome. CK2 is generally found as a tetramer with two catalytic, α and α′ and two non catalytic β subunits. CK2α C-terminal tail phosphorylation is regulated during the mitotic events and the absence of these phosphosites in α′ suggests an isoform specialization. We used a proteomic approach to identify proteins specifically phosphorylated by a CK2α phosphomimetic mutant, CK2αT344ET360ES362ES370E (CK2α4E), in human neuroblastoma SKNBE cellular extract. One of these proteins is lysine-specific demethylase 1 (LSD1 or KDM1A), an important player of the epigenetic machinery. LSD1 is a FAD-dependent amine oxidase and promotes demethylation of lysine 4 and lysine 9 of mono- and di-methylated histone H3. We found that LSD1 is a new substrate and an interacting partner of protein kinase CK2. Three CK2 phosphosites, (Ser131, Ser137 and Ser166) in the N-terminal region of LSD1 have been identified. This domain is found in all chordates but not in more ancient organisms and it is not essential for LSD1 catalytic event while it could modulate the interaction with CK2 and with other partners in gene repressing and activating complexes. Our data support the view that the phosphorylation of the N-terminal domain by CK2 may represent a mechanism for regulating histone methylation, disclosing a new role for protein kinase CK2 in epigenetics.