The drug and carcinogen metabolism system of rat colon microsomes

Modulation of colonic xenobiotic metabolizing enzymes by feeding bile acids: comparative effects of cholic, deoxycholic, lithocholic and ursodeoxycholic acids

Primary and secondary bile acids such as cholic (CHA), deoxycholic (DCA) and lithocholic (LCA) acids have been shown to increase colon tumorigenesis. It has been suggested that inhibition of xenobiotic metabolizing enzymes such as glutathione S-transferase (GST) and UDP-glucuronyltransferase (UGT) by bile acids may be a factor in the development of colon cancer. While enzyme inhibition has been demonstrated in vitro, it is unclear whether feeding bile acids modulates colonic GST and UGT in vivo. To test this notion, male, Sprague-Dawley rats (n = 100) were assigned to a control (CON) or test diets containing 0.2% CHA, DCA, LCA or ursodeoxycholic acid (UDCA). After 5 weeks, colonic tissue was harvested and used for enzyme and cell proliferation measurements. The response to bile acids varied with the enzyme measured and appeared isoenzyme specific. GST-alpha activity was lower in the bile acid fed groups compared with CON. While GST-mu was lower in the LCA-fed group, GST-pi was lower in the DCA-, CHA- and UDCA-fed groups. Unlike GST, both UGT and NADPH-cytochrome P-450 reductase (CYC) activities were increased by bile acids. The proliferative response of the colonic epithelium varied with the bile acids and was regionally specific. These data demonstrate that feeding bile acids alters the activity of colonic phase I and II enzymes; however, the physiological effect of these enzymatic perturbations is yet to be determined.

Expression and induction of cytochrome P-450 1A1 and P-450 2D subfamily in the rat glioma C6 cell line

The cytochrome P-450 (P-450) monooxygenase system can catalyze the oxidation of a wide variety of endogenous and exogenous compounds, including steroid hormones, fatty acids, drugs and pollutants. The functions of this system are as diverse as the substrates. Though this enzyme system has the highest level of activity in the liver, it is present in other tissues, including brain. In this study, we have established the rat glioma C6 cell line as an in vitro model system to examine the expression and induction of P-450 1A1 and the P-450 2D subfamily. Rat glioma C6 cells were treated with P-450 inducers phenobarbital (PB) or benzo[a]anthracene (BA). The presence of P-450 1A1 and 2D1-5 was detected by reverse transcription followed by polymerase chain reaction (RT-PCR) and confirmed by restriction enzyme digestion. The induction of P-450 1A1 and 2D1-5 was quantified using competitive PCR. Although P-450 2D1-5 do not seem to be affected by PB or BA treatment, tenfold induction of P-450 1A1 mRNA after BA treatment was detected. Western blot analysis of microsomal preparations of glioma C6 cells demonstrated the presence of P-450 1A1 at the protein level. ELISAs showed that BA induces P-450 1A1 proteins 7.3-fold. These experiments provide further evidence that the rat glioma C6 cell line contains an active cytochrome P-450 monooxygenase system which can be induced by P-450 inducers. In summary, we believe the presence of the cytochrome P-450 monooxygenase system in glial cells of the brain may be important in chemotherapy and carcinogenesis of brain tumors.

Phenobarbital and 3-methylcholanthrene treatment alters phase I and II enzymes and the sensitivity of the rat colon to the carcinogenic activity of azoxymethane

It has been hypothesized that cancer risk may be influenced by phase I and II drug-metabolizing enzyme systems. This study attempted to determine the relationship between colon phase I and II enzyme activity and the subsequent induction of aberrant crypt foci (ACF), preneoplastic lesions by azoxymethane (AOM), a colon-specific carcinogen. Phenobarbital (PB) and 3-methylcholanthrene (MC) treatment (prototype hepatic inducers of phase I and II enzymes) provided the framework to study the induction of phase I and II enzymes in the rat colonic mucosa. Following induction for five consecutive days, the animals were given a single injection of AOM. Phase I and II enzymes were determined fluorometrically and spectrophotometrically and ACF were identified microscopically. Phase I and II xenobiotic metabolizing enzymes were induced in the rat colonic mucosa by prototype hepatic inducers. A lower number of ACF and crypt multiplicity was observed in animals induced with MC than in those in the non-induced and PB groups. Altered levels of phase I and II enzymes in the colon during preinitiation stages were associated with modulation in the growth of ACF, putative preneoplastic lesions.

Dietary modulation of human platelet phenolsulphotransferase activity

1. The mammalian phenolsulphotransferase enzymes are known to play a major role in both the detoxification and possibly the activation of pre-carcinogenic phenols and aromatic amines. 2. Vegetable cytosol preparations were tested in vitro for their ability to affect the sulphation of two reference compounds (rho-nitrophenol and dopamine, which are selective substrates for the phenol and monoamine forms of phenolsulphotransferase respectively), and to act as substrates for the enzymes in comparison with the same reference compounds. 3. The majority of cytosols greatly decreased (> 80%) the sulphation of either or both the reference compounds. This effect may have been due to either enzyme inhibition or substrate binding. 4. Whereas some of the cytosols were sulphated under the assay conditions, most were not. Additionally, it was found that a cytosol that decreased the sulphation of the two reference compounds was not necessarily poorly sulphated itself. 5. It is concluded that dietary factors have the potential to play a major role in modulating the sulphation detoxification pathway, and have wide ranging implications with regard to adverse drug reactions.

Role of beta-carotene on the changes in activity patterns and levels of biotransforming enzymes in transplantable murine lymphoma

The differential levels of induction of hepatic microsomal cytochrome P-450 (cyt. P-450), UDP-glucuronyl transferase (UDPGT) and cytosolic glutathione-S-transferase (GST) activities were evaluated over various periods of time, following tumor transplantation in male Swiss albino mice in the presence and absence of beta-carotene supplementation in their basal diet (100 mg/kg). An increase in the total hepatic microsomal cytochrome P-450 and UDP-glucuronyl transferase and cytosolic GSH-transferase activities (1.5 to 2 fold) occurred during the later stage of tumor progression (22 +/- 2 days onwards). However, beta-carotene supplementation throughout the study increased or decreased the random activity trends of the above markers significantly (P < 0.05- < 0.01). Finally, beta-carotene supplementation could enhance the survival of the host bearing lymphoma by almost 2-fold (50-60 days) over and above the lymphoma controls (30-35 days).

Expression analysis of the mixed function oxidase system in rat brain by the polymerase chain reaction

Metabolism of therapeutic drugs in the body by the mixed function oxidase system is an important consideration in the analysis of a drug's effectiveness. P450-dependent metabolism within the brain of a neuro-specific drug may affect the drug's course of action. To determine whether cytochrome P450 was expressed in brain, RNA was isolated from the whole brains of rats treated with a variety of known hepatic P450 inducers, including amitriptyline, imipramine, isosafrole, phenobarbital, and beta-naphthoflavone. The RNA was analyzed for the presence of P450 isozymes by the PCR technique. Differential expression of P450IA1, P450IIB1, P450IIB2, P450IID, and P450IIE1 was detected in the brain samples, depending on the treatment. Cytochrome P450 reductase expression was also detected in the brain samples, giving strong evidence that the brain contains a competent mixed function oxidase system under all conditions studied.

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of microsomal enzyme inducers upon UDP-glucuronic acid concentration and UDP-glucuronosyltransferase activity in the rat intestine and liver

This study was conducted to evaluate UDP-glucuronosyl-transferase (UDP-GT) activity, UDP-glucuronic acid (UDP-GA) concentration, and UDP-glucose (UDPG) concentration in the rat intestine and liver following oral administration of butylated hydroxyanisole (BHA), benzo[a]pyrene (BaP), 3-methylcholanthrene (3MC), phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), or trans-stilbene oxide (TSO). Microsomal UDP-GT activity was assayed in vitro with acetaminophen (AA), harmol (HA), and 1-naphthol (NA) as the aglycones. Intestinal HA and AA glucuronidation were enhanced by BHA, BaP, and TSO, whereas 3MC, PB, PCN, and TCDD augmented hepatic HA-glucuronide formation and BHA, PB, PCN, TCDD, and TSO significantly increased hepatic AA glucuronidation. All inducing agents except PB and PCN markedly increased both intestinal and hepatic NA glucuronidation. PB, PCN, and TCDD paradoxically decreased intestinal glucuronidation of AA and HA. A similar effect upon hepatic glucuronidation was not observed with any of the agents studied. Hepatic UDP-GA concentration was increased significantly by all inducers studied except PCN and TCDD, whereas hepatic UDPG concentration was increased only by BHA. In the intestine, significant increases in UDP-GA concentration were produced only by BHA and BaP, which also elevated intestinal UDPG. These results demonstrate that microsomal enzyme inducers evoke different effects upon intestinal and hepatic glucuronidation. These differences are manifested with regard to induced changes in UDP-GT activity as well as treatment-induced alterations in UDP-GA content. Thus, the present study further underscores the marked variance of intestinal and hepatic xenobiotic glucuronidation.

Reconstitution of the brain mixed function oxidase system: purification of NADPH-cytochrome P450 reductase and partial purification of cytochrome P450 from whole rat brain

NADPH-cytochrome P450 reductase was purified to apparent homogeneity and cytochrome P450 partially purified from whole rat brain. Purified reductase from brain was identical to liver P450 reductase by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blot techniques. Kinetic studies using cerebral P450 reductase reveal Km values in close agreement with those determined with enzyme purified from rat liver. Moreover, the brain P450 reductase was able to function successfully in a reconstituted microsomal system with partially purified brain cytochrome P450 and with purified hepatic P450c (P450IA1) as measured by 7-ethoxycoumarin and 7-ethoxyresorufin O-deethylation. Our results indicate that the reductase and P450 components may interact to form a competent drug metabolism system in brain tissue.

Characterization of cytochrome P450-dependent dimethylhydrazine metabolism in human colon microsomes

Polyclonal antibodies to components of the rat liver cytochrome P450 system were used to examine the composition and function of the microsomal cytochrome P450-dependent monooxygenase system of human colonic mucosal cells. Anticytochrome P450 reductase antibody gave a strong band of immunocross-reactivity in human colon microsomes at the same molecular weight level as purified cytochrome P450 reductase from rat liver, as well as hepatic microsomes isolated from untreated or phenobarbital-treated rats. These results demonstrate the presence of cytochrome P450 reductase in human colon cells. Similarly, cytochromes P450 IIB1 and IIA1 also appear to be present in Western blots of human colon microsomes. These antibodies, as well as antibodies to reductase and cytochrome b5, inhibit dimethylhydrazine metabolism in human colon microsomes to varying degrees. These data argue for a functional P450-dependent drug metabolism system in colon capable of activating/metabolizing the colon-specific model carcinogen, 1,2-dimethylhydrazine.

Cytochrome b5-mediated redox cycling of estrogen

Previously, we have demonstrated microsomal cytochrome P450-catalyzed redox cycling of estrogens. In this study, we investigated the role of cytochrome b5 in redox cycling in order to obtain a full understanding of enzymatic contributions to redox reactions of estrogens. Pure cytochrome P450c and hydrogen peroxide or cumene hydroperoxide oxidized diethylstilbestrol (DES) to diethylstilbestrol-4',4"-quinone (DES Q). This oxidation by H2O2 was doubled by addition of cytochrome b5 to cytochrome P450c (molar ratio of 1:4), but did not proceed with cytochrome b5 alone. The stimulation by cytochrome b5 of the cytochrome P450c-catalyzed oxidation of DES to DES Q occurred via modulation of the Vmax of cytochrome P450c rather than of the Km. DES Q was reduced to DES by purified cytochrome b5 and NADH-dependent cytochrome b5 reductase. Pretreatment of microsomes with an antibody to cytochrome b5 reductase inhibited microsomal NADH-dependent reduction of DES Q to DES by 55%. Cytochrome b5 likely participates in the oxidation of DES to DES Q by interacting with cytochrome P450c and in the reduction of DES Q to DES by interacting with cytochrome b5 reductase. Thus, the study demonstrates that cytochrome b5 plays an active role in biological oxidation and reduction reactions.

Tissue-specific induction of the carcinogen inducible cytochrome P450 isoform, P450IAI, in colonic epithelium

The epithelial cells of the gastrointestinal tract have the capacity to engage in biotransformation of ingested chemicals. A principal component of phase I metabolism of xenobiotics is the family of hemeproteins referred to as cytochrome(s) P450. The presence of cytochrome P450 isoforms was examined by Western blot analysis in the epithelial cells of the colon and proximal small intestine of male rats following oral administration with either beta-naphthoflavone or phenobarbital. The appearance of beta-naphthoflavone-inducible cytochrome P450IAI was observed in the colon and small intestine. The appearance of this cytochrome P450 isoform was concurrent with increases (up to 150-fold) in cytochrome P450-related O-deethylation of 7-ethoxycoumarin and 7-ethoxyresorufin in both tissues. Following administration of phenobarbital, cytochrome P450IIBI was identified immunochemically in the small intestine. However, this isozyme could not be detected in colon. These data suggest that the epithelial cells of the proximal small intestine respond to beta-naphthoflavone and phenobarbital in a manner similar to the liver, whereas colonic epithelial cells may have a greater capacity to respond to P450IAI-type inducers such as beta-naphthoflavone. Evidence exists that differences in cytochrome P450 isozyme composition can affect the ultimate metabolic fate of ingested chemicals, including carcinogens, and thus a role for colonic P450-dependent monooxygenase activity in the biogenesis of cancer in this tumor-susceptible tissue is suggested.

Human colon tumor cell line LS174T drug metabolizing system

The effects of culture variables on the specific content and activity of various enzymes of the drug metabolizing system were assessed in colon tumor cell line LS174T. The NADH reduced cytochrome b5 (cyt b5)4 spectrum of these cells was similar to rat liver cyt b5. When released from the membrane by trypsin and concentrated, the cyt b5 was found to cross react with rabbit antibody to rat liver cyt b5 and human liver cyt b5. The enzyme activities were found stable over limited cell passages with control values of 0.03 and 0.13 mumol/min/mg protein for NADPH and NADH cytochrome c (cyt c) reducing activity, 0.05 nmol cyt b5 and 0.013 nmol cytochrome P450 per milligram of microsomal protein. Phenobarbital/hydrocortisone showed a consistent, but not always significant increase in the NADPH and NADH cyt c reduction and benzanthracene an increase in the NADH cyt c reducing activity and cyt b5 content. Griseofulvin lowered the NADH cyt c reducing activity. Delta-aminolevulinic acid (0.5 mM) caused a significant decrease in the specific activity of all enzymes, as judged by a student's t test, with a p less than 0.001.

Effect of dietary lipids on hepatic and intestinal monooxygenases in mice

The effect of dietary lipids on hepatic and intestinal monooxygenases was studied by feeding C57BL/6N mice (for 2 wks) diets containing 5% and 23.5% (wt/wt) olive oil or corn oil. At the end of the feeding period, we measured arylhydrocarbon hydroxylase (AHH) activity in S9 preparations from liver, small intestine, and colon; and, using the same S9 preparations from the liver, we observed the activation of the following three dietary promutagens: 2-amino-3-methylimidazo(4,5-f)quinoline, 2-amino-3,8-dimethylimidazo(4,5-f) quinoxaline, and 2-amino-6-methyldipyrido(1,2-a:3',2'-d)imidazole. The results showed that high-fat diets increased hepatic AHH activity both in corn oil and olive oil diets compared with the low-fat diets; also, a 5% corn oil diet had significantly higher AHH activity compared with the 5% olive oil diet. AHH activity was, respectively, 48.6 +/- 5.1 and 79.5 +/- 11.4 pmol 3OH-benzo[a]pyrene formed/mg/min in the 5% and 23.5% olive oil diets and 66.1 +/- 5.1 and 83.9 +/- 12.2 in the 5% and 23.5% corn oil diets; values are means +/- SE, n = 16. The results also showed a significant increase in the ability of hepatic S9 fractions from animals on high-fat diets to activate promutagens in the Salmonella/plate test. On the contrary, AHH activity in the small intestine and colon was not affected by the fat content of the diet.

Cytochrome P450 activity and distribution in the human colon mucosa

Enzymatic activity associated with the mixed-function oxidase system was determined in microsomes prepared from the mucosal cells extracted from normal human colons. A high activity toward nitrogen oxidation reactions was observed. 1,2-Dimethylhydrazine, a colon-specific carcinogen, was metabolized at a higher rate in vitro by human colon microsomes as compared with the rat, and exhibited a km ten-fold lower, 1.03 mmol/l versus 9.68 mmol/l, respectively. This activity was inhibited by classic cytochrome P450 inhibitors; 70% inhibition was achieved using 70 mmol/l metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), 20 mmol/l; SKF-525A (diethylaminoethyl-2,-2-diphenylvalerate HCl), or 350 mumol/l n-octylamine. These data suggest the presence of a stable, active mixed-function oxidase system in the human colon mucosa which has a preferential activity toward nitrogenous compounds and provides a mechanism for the activation of carcinogens. Its distribution in the colon appears to parallel the reported incidence of human colonic carcinomas.

Formation of nuclear anomalies in rat intestine by benzidine and its biliary metabolites

Administration of benzidine (BZ) by intraperitoneal injection (i.p.) to rats (0-100 mg/kg) produced, after 24 h, a dose-dependent formation of nuclear anomalies (micronuclei, pyknotic and karyorrhectic nuclei) in intestinal epithelial cells analysed both in isolated cell suspensions and in the intestinal crypts in tissue sections. When bile collected (0-4 h) from rats treated with BZ (150 mg/kg, i.p.) was infused into the duodenum of recipient rats, nuclear anomalies were observed in mucosal epithelial cells, after 24 h, with a similar distribution to that in rats given BZ by i.p. injection. The formation of nuclear anomalies in the intestine is in accord with the intestinal carcinogenic effect of BZ and is, at least partially, dependent on exposure of epithelial cells to biliary metabolites of BZ.

An improved method for determination of colonic mucosal proliferation: uptake of tritiated thymidine under hyperbaric oxygen conditions

In vitro uptake of tritiated thymidine under normal oxygen tension in standard tissue culture conditions (95% O2 and 5% CO2, 1 atm pressure) may only approximate the in vivo concentration of oxygen delivered to the colon epithelium in situ. Using standard O2 pressure conditions, however, generally good results are obtained. Incubation of tissues under slightly hyperbaric oxygen conditions has been reported to facilitate the uptake of thymidine in vitro and could permit greater sensitivity in calculating labeling indices. In this study we have compared measurements of colonic cellular proliferation by quantifying labeled cells in mouse colon by measuring DNA synthesis in rat colon exposed in vivo or in vitro to tritiated thymidine. In vitro incubations were performed in normal and hyperbaric oxygen conditions. We conclude that the hyperbaric oxygen technique markedly improves the uptake of thymidine in the colon and affords greater sensitivity in determining actual colonic labeling profiles.

An isolated perfused model for the study of colonic metabolism and transport

The murine colonic perfusion model allows for the examination of absorption, metabolism, and portal transfer by the colon under physiologic conditions. This model was characterized by the use of four radiolabeled compounds: estradiol and Vitamin D3, both physiologically active circulating steroid compounds, and benzo[a]pyrene and N-acetylaminofluorene, xenobiotic carcinogens of the aromatic hydrocarbon and aromatic amide classes, respectively. Hemodynamic parameters and oxygen consumption of the preparation were stable throughout perfusion. Estradiol and N-acetylaminofluorene entered the portal vein at a rate of 2% of the lumenal dose per hour. Benzo[a]pyrene crossed at 0.4% of the lumenal dose per hour. The rate of transfer of Vitamin D3 was negligible. Analysis of the lumenal label revealed only substrate. In all experiments less than 0.02% of the applied substrate remained in the tissue compartment. Analysis of the vascular perfusate demonstrated evidence for sulfates of estradiol and N-acetylaminofluorene. Three conjugate classes were found associated with benzo[a]pyrene, constituting 42% of the portal label. Hydrolysis data suggests the presence of double conjugates of benzo[a]pyrene involving glutathione. In the case of aromatic hydrocarbons, conjugation, particularly thioether formation, implies hydroxylation and epoxide formation. For sulfation an N-acetylamino-fluorene ring or N-hydroxylation is required. The latter process could allow for the delivery of highly carcinogenic N-O sulfates to the liver.

Changes in phase I and phase II biotransformation with age in male Fischer 344 rat colon: relationship to colon carcinogenesis

Using male Fischer 344 rats classified as young (2-4 months), middle aged (12-14 months), and old (22-25 months), the activities of several Phase I and Phase II biotransformation pathways in the large intestine were investigated, including benzo[a]pyrene hydroxylase (BPOH), alcohol dehydrogenase (ADH), glutathione S-transferase (GST), glutathione peroxidase (GSH-PX), beta-glucuronidase (BG), and microsomal and nuclear glucuronyltransferase (UDPGT). Levels of oxidized (GSSG) and reduced (GSH) glutathione and uridine 5'-diphosphoglucuronic acid (UDPGA) were also measured. BPOH increased 33% in old rats, while ADH and BG activity remained unchanged with age. Nuclear UDPGT remained unchanged with age, whereas form I of GSH-PX declined slightly in old rats. GST, microsomal UDPGT, and form II of GSH-PX declined by 38, 37 and 44%, respectively, in old rats. The decrease in GST and microsomal UDPGT was also significant in middle aged rats. Levels of colonic GSH, GSSG and UDPGA were found to be unchanged with age. These in vitro data suggest the possibility that if reactive intermediates are generated to the same extent in old rats as in young rats, decreased detoxification mechanisms in the old rat may increase susceptibility of the colon to actions of chemical carcinogens.

Induction of cytochrome P-450 in the rabbit eye by phenobarbital, as detected immunohistochemically

The present study was aimed to demonstrate that cytochrome P-450 was induced by drug administration in particular tissues in the eye. Phenobarbital was used as an inducer. We showed immunohistochemically that cytochrome P-450 was induced in the cornea, conjunctiva and ciliary epithelium of rabbits after four days of intraperitoneal administration of phenobarbital at a dose of 80 mg per kg per day. We did not detect significant immunofluorescence in other ocular tissues. Prolonged administration caused degeneration of the ciliary epithelium, but not pathological change was seen in other ocular tissues. In this case, immunofluorescence was not detected in the ciliary epithelium but in the cornea, conjunctiva and lens.

Drug metabolism in rat colon: resolution of enzymatic constituents and characterization of activity

A direct demonstration of the basis of mixed function oxidase activity in rat colonic mucosa was achieved by resolution of microsomes into two components, cytochrome P-450 and cytochrome P-450 reductase, which on recombination with phosphatidylcholine catalyzed hydroxylation of benzo[alpha]pyrene and benzphetamine. Reconstitution of hydroxylation activity requires both the cytochrome P-450 component and the cytochrome P-450 reductase component in addition to phospholipid. Omission of either of the protein components or the phospholipid component reduces the activity almost to background levels. The kinetic parameters (Km values) for the reconstituted system suggest that the colonic mucosal system is quite similar to the liver microsomal system in its catalytic capacity as well as in its enzymic composition. The purified colon components substitute for their liver counterparts reasonably well, again consistent with the argument that the colon mucosal mixed function oxidase system is analogous to the liver system.

Distributions and properties of cytochromes P-450 and cytochrome P-450 reductase from rat colon mucosal cells

Cytochrome P-450 reductase and cytochrome P-450 fractions have been separated and partially purified from colonic mucosal microsomes of rat pretreated with phenobarbital or beta-naphthoflavone. Colonic cytochrome P-450 reductase has a molecular weight of 76,000. The Km values of colonic cytochrome P-450 reductase for the artificial electron acceptors cytochrome c, ferricyanide, and dichlorophenolindophenol and the electron donor NADPH are 6, 50, 11 and 11 microM, respectively. Immunochemical techniques identified the presence of beta-naphthoflavone Forms 1, 4 and 5 after beta-naphthoflavone treatment but beta-naphthoflavone Forms 1 and 4 and phenobarbital Form 1 after phenobarbital treatment.

Role of reactive oxygen in bile salt stimulation of colonic epithelial proliferation

Our previous studies had suggested a link between bile salt stimulation of colonic epithelial proliferation and the release and oxygenation of arachidonate via the lipoxygenase pathway. In the present study, we examined the role of reactive oxygen versus end products of arachidonate metabolism via the cyclooxygenase and lipoxygenase pathways in bile salt stimulation of rat colonic epithelial proliferation. Intracolonic instillation of 5 mM deoxycholate increased mucosal ornithine decarboxylase activity and [3H]thymidine incorporation into DNA. Responses to deoxycholate were abolished by the superoxide dismutase mimetic CuII (3,5 diisopropylsalicylic acid)2 (CuDIPS), and by phenidone or esculetin, which inhibit both lipoxygenase and cyclooxygenase activities. By contrast, indomethacin potentiated the response. Phenidone and esculetin suppressed deoxycholate-induced increases in prostaglandin E2 (PGE2), leukotriene B4 (LTB4), and 5, 12, and 15-hydroxyeicosatetraenoic acid (HETE), whereas CuDIPS had no effect. Indomethacin suppressed only PGE2. Deoxycholate (0.5-5 mM) increased superoxide dismutase sensitive chemiluminescence 2-10-fold and stimulated superoxide production as measured by cytochrome c reduction in colonic mucosal scrapings or crypt epithelium. Bile salt-induced increases in chemiluminescence were abolished by CuDIPS, phenidone, and esculetin, but not by indomethacin. Intracolonic generation of reactive oxygen by xanthine-xanthine oxidase increased colonic mucosal ornithine decarboxylase activity and [3H]thymidine incorporation into DNA approximately twofold. These effects were abolished by superoxide dismutase. The findings support a key role for reactive oxygen, rather than more distal products of either the lipoxygenase or cyclooxygenase pathways, in the stimulation of colonic mucosal proliferation by bile salts.

Aging affects the drug metabolism systems of rat liver, kidney, colon and lung in a differential fashion

Microsomes prepared from the liver, lungs, colon and kidney cortex of Sprague Dawley rats of ages 2, 4, 10, 24 and 78 weeks were assessed for hydroxylation activity with the substrate benzo[alpha]pyrene. Liver microsomal activity declined after reaching a peak of activity at 10 weeks. The hydroxylation of benzo[alpha]pyrene by colon, kidney and lung microsomes, however, either remained the same or decreased only slightly. During the age range examined inducibility of hydroxylation activity by beta-naphthoflavone decreased with age in liver but actually increased with age in the extrahepatic tissues. Although phenobarbital did not elicit any increases in liver, kidney or lung, it increased substantially benzo[alpha]pyrene hydroxylation activity in colon microsomes of 78 week old rats. Total cytochrome P-450 content was induced at all age groups in all tissues by beta-naphthoflavone and in all tissues except lung by phenobarbital. Induction of cytochrome P-450 in kidney by phenobarbital was only observed in 24 and 78 week old rats. These data suggest an increased role for extrahepatic activation of benzo[alpha]pyrene with aging. In contrast to total content of cytochrome P-450, the beta-naphthoflavone inducible amount of Form 5 which has a high turnover number for benzo[alpha]pyrene, declined by 55% in liver between 2 weeks and 78 weeks while it increases dramatically in all extrahepatic tissues (from 80 to 138%).

Fatty acid stimulated N-demethylation of 1,2-dimethylhydrazine and tetramethylhydrazine by rat colonic mucosa

A fatty acid stimulated, NADPH-independent pathway for the N-demethylation of 1,1-dimethylhydrazine (1,1-DMH) with the generation of HCHO was demonstrated in 10,000 g soluble fractions of colonic mucosal homogenates. Tetramethylhydrazine and, to a lesser extent, aminopyrine, but not 1,2-DMH or methylhydrazine, were also substrates for this reaction. Isolated superficial colonic epithelial cells metabolized 1,1-DMH at a faster rate than proliferative epithelial cells. Indomethacin, an inhibitor of cyclooxygenase activity, and 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of both cyclooxygenase and lipoxygenase activities, suppressed HCHO production from 1,1-DMH by 50 and 80%. However, in the presence of indomethacin or ETYA, arachidonate hydroperoxide stimulated HCHO formation. This suggested a peroxidative mechanism for 1,1-DMH metabolism, related in part to prostaglandin synthesis. A possible role for lipoxygenase activity in mediating 1,1-DMH metabolism was suggested by the ability of linoleate, which did not increase prostaglandin synthesis, to stimulate 1,1-DMH metabolism and by the fact that ETYA was more effective than indomethacin as an inhibitor of 1,1-DMH metabolism. The fatty acid stimulated pathway for N-demethylation was clearly distinct from the mixed function oxidase activities. NADPH did not stimulate 1,1-DMH metabolism to HCHO. 7,8-Benzoflavone or SKF-525A, inhibitors of cytochrome P-450, and methimazole, an inhibitor of N-demethylation catalyzed by the hepatic microsomal FAD-containing monooxygenase, did not suppress HCHO formation. To the extent that 1,1-DMH and tetramethylhydrazine reach the colon unchanged, the results suggest that fatty acid stimulated cooxidation pathways in colonic mucosa may contribute to the metabolism of these agents. Metabolism by superficial cells which are destined to slough may be an important defense mechanism against the toxic and carcinogenic actions of these hydrazines in colon.

The effects of fractionated thermally oxidized corn oil on drug-metabolizing enzyme systems in the rat

Corn oil samples were heated, with aeration, to 210 degrees C for a total of 5 hr. Both fresh and oxidized samples were urea-fractionated and the individual fractions were administered to male Sprague-Dawley rats by gastric intubation. The effects of urea adducts, adduct-free fractions, non-saponifiable fractions and unfractioned fresh and thermally oxidized oil samples on hepatic, intestinal and colonic drug-metabolizing enzymes were determined. The treatments had no significant effects on hepatic or intestinal drug-metabolizing or mixed-function oxidase activities. There was a significant (P less than 0.05) increase in colonic UDP-glucuronyltransferase activity in rats treated with thermally oxidized corn oil, while the non-saponifiable fraction of the same sample decreased (P less than 0.1) the activity of this enzyme. There was also a significant increase in the activity of colonic benzo[a]pyrene hydroxylase in rats treated with the non-adduct fraction or with urea adducts of the thermally oxidized corn oil. These data suggest the colon as a possible specific site for the alteration of mixed-function oxidase activities by products of thermally oxidized oils.

Quantification of cytochrome P-450-dependent cyclohexane hydroxylase activity in normal and neoplastic reproductive tissues

It is well established that liver microsomal cytochrome P-450 participates in steroid metabolism and probably also in the metabolism of anti-oestrogens such as tamoxifen (Nolvadex). Thus it is possible that variations in cytochrome P-450 levels may influence the responsiveness of human breast and endometrial carcinomas to endocrine therapy. Therefore a simple sensitive spectrophotometric assay for determining levels of cytochrome P-450-dependent cyclohexane hydroxylation activity in breast and uterine microsomes (microsomal fractions) has been developed. Cyclohexane was chosen as a substrate because of the relatively high levels of cyclohexane hydroxylase activity in tumour microsomes and because cyclohexane serves as a substrate for several forms of cytochrome P-450. As previously described [Senler, Dean, Pierce & Wittliff (1985) Anal. Biochem. 144, 152-158], a direct method utilizing isotope-dilution/gas chromatography-mass spectrometry was also developed in order to confirm the results of the spectrophotometric assay. The average activity (cyclohexane-dependent NADPH oxidation) for 139 human breast-tumour microsome preparations was 1.34 nmol/min per mg, which is in the range of that found in untreated mammalian liver (1-3 nmol/min per mg). Also, high enzyme activity was demonstrated in human ovary, normal uterus as well as uterine leiomyomas. Endocrine status appeared to influence enzyme levels, in that mammary tissue from virgin rats contained significantly (P less than 0.025) higher amounts of activity than did tissues from either pregnant or lactating rats. Furthermore, carbon monoxide, as well as an antibody against rat liver cytochrome P-450, completely inhibited NADPH oxidation by breast-carcinoma microsomes. These results strengthen our hypothesis that tumours with high levels of cytochrome P-450 may have a reduced response to additive endocrine therapy.

Procedures for measuring cytochrome P-450-dependent hydroxylation activity in reproductive tissues

Cytochrome P-450 is present in the endoplasmic reticulum at varying concentrations in almost all tissues. However, the existence and role of cytochrome P-450 in normal and neoplastic reproductive tissues has not been clearly demonstrated. Our interest lies in the possibility that variations in cytochrome P-450 levels may influence the responsiveness of breast and endometrial carcinomas to endocrine therapy. This may be of particular importance with agents such as tamoxifen where hydroxylation reactions are known to alter therapeutic activities. Therefore, a simple, sensitive spectrophotometric assay for determining levels of cytochrome P-450-dependent cyclohexane hydroxylase activity in breast and uterine microsomes has been developed. Cyclohexane was chosen as a substrate because of the relatively high levels of cyclohexane hydroxylase activity in tumor microsomes and because cyclohexane serves as a substrate for several forms of cytochrome P-450. In order to confirm the results of the spectrophotometric assay, a direct method utilizing isotope dilution gas chromatography/mass spectrometry (GC/MS) has been developed for detecting low levels of the hydroxylated product, cyclohexanol. By employing a stable isotopically labeled analog of cyclohexanol (cyclohexanol-d12), good agreement was demonstrated between the simple, indirect method (measuring NADPH oxidation at 340 nm) and the more complex, direct method (measuring cyclohexanol formation) utilizing GC/MS. The agreement of results obtained using these two techniques indicates that they are equally valid measures of NADPH-dependent cyclohexane hydroxylase activity. The use of the spectrophotometric method is proposed for rapid, multiple assays such as in the clinical setting, reserving GC/MS analysis for use as a research tool.

Differences between small and large intestine and liver in the inducibility of microsomal enzymes in response to stimulation by phenobarbitone and betanaphthoflavone in the diet

Rats were fed either sodium phenobarbitone (PB) or betanaphthoflavone (BNF) for seven days. Deethylation of 7-ethoxyresorufin ( 7ERR ) and 7-ethoxycoumarin ( 7EC ) was measured in small and large intestine and liver, and cytochrome P-450 in liver. Our semi-purified diet was shown to produce minimal levels of intestinal deethylation activity. BNF was added to the semi purified diet and fed at levels from 0.1 to 100 mg BNF/kg of diet. Significant (P less than 0.05) induction of deethylation in small intestine was seen at all dose levels, ranging from 2-fold at 0.1 mg/kg diet to greater than 100-fold at 100 mg/kg diet. A 3-fold increase was also seen in the large intestine at 50 mg/kg. A significant increase in hepatic deethylation was only seen at 100 mg/kg. PB was administered in drinking water at 50, 100 and 1000 mg PB/l. Significant (P less than 0.05) induction of hepatic deethylation was seen at all dose levels, ranging from 2-fold at 50 mg/l to 5-fold at 1000 mg/l. Hepatic cytochrome P450 was also increased. No significant increase in intestinal deethylation was seen at any of the doses used.

The potent induction of intestinal heme oxygenase by the organotin compound, bis(tri-n-butyltin)oxide

Oral administration of bis(tri-n-butyltin)oxide, an important organotin biocidal agent, produces a substantial elevation in heme oxygenase activity when measured at 16 hours in rat small intestine. An apparent Km for hemin of 100 microM is the same in both control and the organotin-induced 9,000 X g supernatant preparations. Concomitant with elevated heme oxygenase activity there occurs a substantial reduction in benzo(a)pyrene hydroxylase activity (approximately 20% of controls) and cytochrome P-450 concentration (approximately 60% of controls). These perturbations of heme metabolism in intestinal epithelium of the rat define an important new toxicological effect of organotins and raise the possibility that concurrent oral ingestion of environmental pollutants can directly affect the cytochrome P-450-dependent metabolism of other chemicals in the intestine.

Aldrin epoxidase and dimethylhydrazine demethylase activities in tumorous and non-tumorous tissue of the human colon and rectum

High activities of aldrin epoxidase and dimethylhydrazine demethylase were found in human colon and rectum mucosa, the first being as high as in human liver biopsy specimens. Comparisons between tumorous (adenocarcinomas) and non-tumorous tissues of the same individuals revealed loss of activities in tumorous specimens. The presence of epoxidizing enzymes and demethylation of the organ-specific colon carcinogen dimethylhydrazine (DMH) in the intestinal muscosa of tumor-bearing patients implicate them with chemical carcinogenesis also in humans.

Induction of colon mucosal beta-glucuronidase production as a mechanism for 1,2-dimethylhydrazine colon carcinogenesis

Although early studies in germ-free rats showed almost complete dependence on dimethylhydrazine (DMH) colon carcinogenesis upon the presence of colon bacteria, no adequate explanation was given for the 20% tumor incidence observed in germ-free animals. Bacterial activation of liver microsomal products releasing active proximate carcinogens has been the accepted reason for the exquisite specificity DMH has for the colon. Recent work, including the present study, show the colon mucosa is capable of metabolizing carcinogens and activating conjugating forms metabolized in the liver independent of the intestinal microflora. Mucosal beta-glucuronidase production was assayed in coded, scraped mucosa samples from the duodenum/jejunum, ileum, right colon, and left colon of normal and DMH-treated rats. Normal mucosal beta-glucuronidase production was highest in the left colon followed by the right colon, duodenum, and ileum, respectively. Enzyme production in the left colon was significantly increased 24 hours after injection of 25 mg/kg body weight DMH. No elevation was seen in other mucosal samples. Metabolism of DMH to oxidated forms conjugated to glucuronic acid is well established. Thus, this study offers a possible role for carcinogen, induction of a metabolic enzyme in its target tissue.

Patterns of prostaglandin synthesis and degradation in isolated superficial and proliferative colonic epithelial cells compared to residual colon

Prostaglandin (PG) synthesis and degradation were examined in different regions (epithelial versus non-epithelial structures) of the rat distal colon by both HPLC analysis of [14C] arachidonate (AA) metabolites and by specific radioimmunoassays. Intact isolated colonic epithelial cells synthesized mainly PGF2 alpha and TXA2, as monitored from the formation of its stable degradation product TXB2 (PGF2 alpha greater than TXB2 greater than 6-keto-PGF1 alpha, the stable degradation product of PGI2 = PGD2 = PGE2 = 13,14-dihydro-15-keto-PGF2 alpha). The profile of PG products of isolated surface epithelial cells was identical to that of proliferative epithelial cells. However, generation of PGs by surface epithelium was 2 to 3-fold higher than by proliferative cells both basally and in the presence of a maximal stimulating concentration (0.1 mM) of AA. The latter implied a greater synthetic capacity of surface epithelium, rather than differences due to endogenous AA availability. The major sites of PG synthesis in colon clearly resided in submucosal structures; the residual colon devoid of epithelial cells accounted for at least 99% of the total PGs produced by intact distal colon. The profile of AA metabolites formed by submucosal structures also differed markedly from that of the epithelium. The dominant submucosal product was PGE2. PGE2 and its degradation product 13,14-dihydro-15-keto-PGE2 accounted for 63% of the PG products formed by submucosal structures (PGE2 much greater than PGD2 greater than 13,14-dihydro-15-keto-PGE2 greater than PGF2 alpha = TXB2 = 6-keto-PGF1 alpha greater than 13,14-dihydro-15-keto-PGF2 alpha). By contrast, epithelial cells, and particularly surface epithelium, contributed disproportionately to the PG degradative capacity of colon, as assessed from the metabolism of either PGE2 or PGF2 alpha. When expressed as a percentage, epithelial cells accounted for 71% of total colonic PGE2 degradative capacity but only 23% of total colonic protein. Approximately 15% of [3H] PGE2 added to the serosal side of everted colonic loops crossed to the mucosal side intact. Thus, at least a portion of the PGE2 formed in the submucosa reaches, and thereby can potentially influence functions of the epithelium.

Metabolism of the carcinogen 1,2-dimethylhydrazine by isolated human colon microsomes and human colon tumor cells in culture

Human colon microsomes catalyze the metabolism of the model colon carcinogen 1,2-dimethylhydrazine. Activity appears to be distributed in a gradient towards the lower end of the colon. Highest activities were observed for microsomes prepared from the descending segment of the colon with the transverse segment exhibiting lower activities, while the ascending segment showed the lowest rate of metabolism. Dimethylhydrazine metabolism in each segment is inhibited significantly by inhibitors of the cytochrome P-450-dependent mixed function oxidase system. Microsomes prepared from a human colon tumor cell also catalyze the metabolism of 1,2-dimethylhydrazine. Metabolic activity in the cell line can be induced two-fold by treatment of cells with phenobarbital and three-fold by treatment of the cells with phenobarbital plus hydrocortisone. These results show that human colon activates 1,2-dimethylhydrazine and suggest that the human colon may be capable of activating other carcinogens in situ.

Effect of dietary carrageenan and pectin on the reduction of nitro-compounds by the rat caecal microflora

Rats were fed either a basal purified diet, or that diet supplemented with 50 g/kg pectin or iota carrageenan for 50 days, and caecal microbial nitroreductase activity determined using p-nitrobenzoic acid, p-nitrophenol, 2,4-dinitrotoluene, nitrofurantoin and metronidazole as substrates. Both pectin and carrageenan increased the weight of caecal contents, and pectin also increased the number of bacteria per caecum. In contrast, carrageenan decreased the caecal bacterial population. Pectin significantly increased the rate of reduction of metronidazole and the rate of conversion of p-nitrobenzoic acid to p-aminobenzoic acid, while carrageenan significantly decreased the rate of reduction of every compound studied. The results demonstrate that microbial reduction of the nitro-group may be altered by diet, although the response found with one nitro-compound may differ from that seen with another substrate.

Microsomal ethanol oxidation in the colonic mucosa of the rat. Effect of chronic ethanol ingestion

A microsomal ethanol oxidizing system (MEOS) is present in the colonic mucosa of the rat. This MEOS metabolizes ethanol to acetaldehyde at the physiological pH of 7.4. Alcohol dehydrogenase or catalase are not involved in the reaction. The Michaelis Menten constant of the reaction is 13.7 +/- 0.3 mM and the maximal velocity is 219 +/- 30 pmoles acetaldehyde/mg microsomal protein X min. Bacterial ethanol metabolism does not contribute to the acetaldehyde production in the colonic MEOS. Chronic ethanol consumption has no effect on colonic MEOS activity. In addition, chronic ethanol ingestion does not affect colonic microsomal NADPH-cytochrome-c-reductase nor benzo(a) pyrene hydroxylase activity.

Metabolic activation of DMH by colonic microsomes: a process influenced by type of dietary fat

A study was conducted to determine whether 1,2-dimethylhydrazine (DMH), a potent colon carcinogen, is activated to a more potent mutagen by the drug-metabolizing system of the colonic mucosa and to determine the extent to which this metabolism is modified by lipids in the diet. DMH-treated rats fed a diet enriched with 10% corn oil exhibited markedly elevated colonic enzyme levels for mutagen production. This diet also produced the greatest number of animals with colon tumors, when compared with diets containing other levels and sources of lipid. The potential risk of a diet in which unsaturated fat is the sole source of lipid is underlined.

Alteration of the mutagenicity of human fecal extracts by hepatic microsomal enzymes

Human fecal extracts obtain substances mutagenic to Salmonella typhimurium TA 100. The mutagenicity of these extracts can be reduced enzymatically by inducible mammalian microsomal enzymes. Liver homogenates from rats administered the polychlorinated biphenyl mixture of Aroclor 1254 and corn oil were both effective, relative to the quantity of microsomal protein available in the enzyme preparation.

Colon carcinogenesis: an overview

The mechanisms for activation of procarcinogens and some of the factors that affect these reactions are considered. It is proposed that the nucleus of the colonic cell may actually be responsible for the formation of ultimate carcinogenic forms of exogenous procarcinogens. The contributions of the intestinal microbial flora to activation and detoxification of the carcinogens are discussed, and the suggestion that inactive glucuronides of procarcinogens are hydrolyzed to proximate carcinogens by the microbial flora is proposed. Finally, the two-stage mechanism for carcinogenesis is discussed; large bowel cancer is the focal point.

Role of colonic cytochrome P-450 in large bowel carcinogenesis

Rat colon mucosa microsomes contain a competent mixed function oxidase system that hydroxylates the N-methyl drugs benzphetamine and ethylmorphine, the O-alkyl drugs p-nitroanisole and p-nitrophenetole and the polycyclic carcinogen benzo[alpha]pyrene. The colon system's hydroxylation activities can be selectively induced by pretreatment with phenobarbital or beta-naphthoflavone and can be selectively inhibited by SKF-525A or 7,8-benzoflavone. The colon microsomal system has been solubilized with the non-ionic detergent Renex 690 and resolved by column chromatography into its components cytochrome P-450 and cytochrome P-450 reductase. Colon cytochrome P-450 and cytochrome P-450 reductase can be recombined to reconstitute hydroxylation activity. The colon system is also able to activate carcinogens to mutagenic metabolites as demonstrated in the Ames test system. In addition, the activity of the colon system is markedly increased by pretreatment with gastrointestinal hormones.