Characterization of a Mouse Model of Oral Potassium Cyanide Intoxication

Potassium cyanide (KCN) is an inhibitor of cytochrome C oxidase causing rapid death due to hypoxia. A well-characterized model of oral KCN intoxication is needed to test new therapeutics under the Food and Drug Administration Animal Rule. Clinical signs, plasma pH and lactate concentrations, biomarkers, histopathology, and cyanide and thiocyanate toxicokinetics were used to characterize the pathology of KCN intoxication in adult and juvenile mice. The acute oral LD50s were determined to be 11.8, 11.0, 10.9, and 9.9 mg/kg in water for adult male, adult female, juvenile male, and juvenile female mice, respectively. The time to death was rapid and dose dependent; juvenile mice had a shorter mean time to death. Juvenile mice displayed a more rapid onset and higher incidence of seizures. The time to observance of respiratory signs and prostration was rapid, but mice surviving beyond 2 hours generally recovered fully within 8 hours. At doses up to the LD50, there were no gross necropsy or microscopic findings clearly attributed to administration of KCN in juvenile or adult CD-1 mice from 24 hours to 28 days post-KCN challenge. Toxicokinetic analysis indicated rapid uptake, metabolism, and clearance of plasma cyanide. Potassium cyanide caused a rapid, dose-related decrease in blood pH and increase in serum lactate concentration. An increase in fatty acid-binding protein 3 was observed at 11.5 mg/kg KCN in adult but not in juvenile mice. These studies provide a characterization of KCN intoxication in adult and juvenile mice that can be used to screen or conduct preclinical efficacy studies of potential countermeasures.

Neurobehavioral and Cardiovascular Effects of Potassium Cyanide Administered Orally to Mice

The Food and Drug Administration Animal Rule requires evaluation of cardiovascular and central nervous system (CNS) effects of new therapeutics. To characterize an adult and juvenile mouse model, neurobehavioral and cardiovascular effects and pathology of a single sublethal but toxic, 8 mg/kg, oral dose of potassium cyanide (KCN) for up to 41 days postdosing were investigated. This study describes the short- and long-term sensory, motor, cognitive, and behavioral changes associated with oral dosing of a sublethal but toxic dose of KCN utilizing functional observation battery and Tier II CNS testing in adult and juvenile mice of both sexes. Selected tissues (histopathology) were evaluated for changes associated with KCN exposure with special attention to brain regions. Telemetry (adult mice only) was used to evaluate cardiovascular and temperature changes. Neurobehavioral capacity, sensorimotor responsivity or spontaneous locomotor activity, and rectal temperature were significantly reduced in adult and juvenile mice at 30 minutes post-8 mg/kg KCN dose. Immediate effects of cyanide included bradycardia, adverse electrocardiogram arrhythmic events, hypotension, and hypothermia with recovery by approximately 1 hour for blood pressure and heart rate effects and by 2 hours for body temperature. Lesions consistent with hypoxia, such as mild acute tubular necrosis in the kidneys corticomedullary junction, were the only histopathological findings and occurred at a very low incidence. The mouse KCN intoxication model indicates rapid and completely reversible effects in adult and juvenile mice following a single oral 8 mg/kg dose. Neurobehavioral and cardiovascular measurements can be used in this animal model as a trigger for treatment.

Accumulation of CD11b⁺Gr-1⁺ cells in the lung, blood and bone marrow of mice infected with highly pathogenic H5N1 and H1N1 influenza viruses

Infection with pathogenic influenza viruses is associated with intense inflammatory disease. Here, we investigated the innate immune response in mice infected with H5N1 A/Vietnam/1203/04 and with reassortant human H1N1 A/Texas/36/91 viruses containing the virulence genes hemagglutinin (HA), neuraminidase (NA) and NS1 of the 1918 pandemic virus. Inclusion of the 1918 HA and NA glycoproteins rendered a seasonal H1N1 virus capable of inducing an exacerbated host innate immune response similar to that observed for highly pathogenic A/Vietnam/1203/04 virus. Infection with 1918 HA/NA:Tx/91 and A/Vietnam/1203/04 were associated with severe lung pathology, increased cytokine and chemokine production, and significant immune cell changes, including the presence of CD11b(+)Gr-1(+) cells in the blood, lung and bone marrow. Significant differential gene expression in the lung included pathways for cell death, apoptosis, production and response to reactive oxygen radicals, as well as arginine and proline metabolism and chemokines associated with monocyte and neutrophil/granulocyte accumulation and/or activation. Arginase was produced in the lung of animals infected with A/Vietnam/1204. These results demonstrate that the innate immune cell response results in the accumulation of CD11b(+)Gr-1(+) cells and products that have previously been shown to contribute to T cell suppression.

Evaluation of Acute Immunotoxicity of Aerosolized Aflatoxin B(1) in Female C57BL/6N Mice

There is evidence for immunotoxicity of aflatoxin B1 (AFB(1)) in chronic animal feeding studies; however, little information is available as to the effects of inhalation exposure. This study evaluated the acute affects of aerosolized AFB(1) on systemic immune function of female C57BL/6N mice following a single aerosol exposure. Mice were exposed in nose-only inhalation tubes to 0, 2.86, 6.59 and 10 mug AFB(1) aerosol/L air for 90 minutes. A negative control group of untreated mice and a positive control group of cyclophosphamide-treated mice were included to account for day to day variation. Three days following exposure, mice were sacrificed and body, liver, lung, thymus and spleen weights, and complete blood counts and white blood cell differentials were measured. Splenocytes were isolated for flow cytometric analysis of CD4(+) and CD8(+) lymphocytes, CD19(+) B-cells and natural killer cells (NK 1.1(+)). The effect of AFB(1) on humoral immunity was assessed by measuring serum anti-keyhole limpet hemocyanin (KLH) IgM levels. Of the tissues examined, only the thymus weight of AFB(1) exposed mice decreased significantly compared to naive mice; however, the decrease was not dose related and was also observed in the 0 AFB(1) aerosol control group. A decrease in the mean white blood cell count of treated vs. naive mice was observed at all dose levels but was clearly not dose related and was statistically significant only in the 0 and 2.86 mug/L groups. Red blood cell and platelet counts and white blood cell differentials were not significantly affected by AFB(1). The number of CD4(+) (helper T-cells), CD8(+) (cytotoxic T-cells) and CD19(+) (B-cells) decreased in spleens of AFB(1) aerosol exposed mice compared to naive mice; however, the decrease was not dose-related and was also observed in the 0 AFB(1) exposure group. Dose-related changes in the CD4(+)/CD8(+) T-lymphocyte ratios were not observed. The IgM response to KLH was not significantly different in AFB(1) compared to naive mice, suggesting that AFB(1) did not effect antigen-specific antibody production. Based on the results of this study, a single AFB(1) inhalation exposure up to 10 mug/L for 90 minutes (CxT = 900 mug .min/L) did not significantly alter the immune parameters measured in this study. The aerosol vehicle (ethanol) and/or stress could have masked subtle AFB(1)-dependent changes in thymus and spleen weights, and in splenic lymphocyte subpopulations. However, for other immunological parameters, such as the IgM response to KLH, there was clearly no significant effect of AFB(1) aerosol exposure.

Extrapulmonary tissue responses in cynomolgus macaques (Macaca fascicularis) infected with highly pathogenic avian influenza A (H5N1) virus

The mechanisms responsible for virulence of influenza viruses in humans remain poorly understood. A prevailing hypothesis is that the highly pathogenic virus isolates cause a severe cytokinemia precipitating acute respiratory distress syndrome and multiple organ dysfunction syndrome. Cynomolgus macaques (Macaca fascicularis) infected with a human highly pathogenic avian influenza (HPAI) H5N1 virus isolate (A/Vietnam/1203/2004) or reassortants of human influenza virus A/Texas/36/91 (H1N1) containing genes from the 1918 pandemic influenza A (H1N1) virus developed severe pneumonia within 24 h postinfection. However, virus spread beyond the lungs was only detected in the H5N1 group, and signs of extrapulmonary tissue reactions, including microglia activation and sustained up-regulation of inflammatory markers, most notably hypoxia inducible factor-1α (HIF-1α), were largely limited to this group. Extrapulmonary pathology may thus contribute to the morbidities induced by H5N1 viruses.

Transcriptional responses in spleens from mice exposed to Yersinia pestis CO92

Yersinia pestis is one of the most threatening biological agents due to the associated high mortality and history of plague pandemics. Identifying molecular players in the host response to infection may enable the development of medical countermeasures against Y. pestis. In this study, microarrays were used to identify the host splenic response mechanisms to Y. pestis infection. Groups of Balb/c mice were injected intraperitoneally with 2-257CFU of Y. pestis strain CO92 or vehicle. One group was assessed for mortality rates and another group for transcriptional analysis. The time to death at the 8 and 257CFU challenge doses were 5.0+/-2.3 and 3.8+/-0.4 days, respectively. Gene profiling using Affymetrix Mouse Genome 430 2.0 Arrays revealed no probe sets were significantly altered for all five mice in the low-dose group when compared to the vehicle controls. However, 534 probe sets were significantly altered in the high dose versus vehicle controls; 384 probe sets were down-regulated and 150 probe sets were up-regulated. The predominant biological processes identified were immune function, cytoskeletal, apoptosis, cell cycle, and protein degradation. This study provides new information on the underlying transcriptional mechanisms in mice to Y. pestis infection.

Metabolism of thalidomide in human microsomes, cloned human cytochrome P-450 isozymes, and Hansen's disease patients

Previous in vitro studies in rat microsomal preparations suggested that thalidomide is metabolized by the cytochrome P450 system (CYP). In this study, we examined the extent of thalidomide metabolism by preparations of pooled human microsomes, microsomes containing cloned human CYP isozymes (CYPIA2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4), and Hansen's disease patients. Results indicated that thalidomide was a poor substrate for CYP isozymes. Alteration of incubation buffer, pH, incubation time, and microsome and thalidomide concentrations did not increase the production of any metabolites. Thalidomide also did not inhibit metabolism of CYP-specific substrates and therefore any interactions with other drugs that are metabolized by the same enzyme system are unlikely. Hansen's patients were given a single oral dose of thalidomide (400 mg), and their blood and urine were collected at time points up to 72 hours, processed, and analyzed by tandem mass spectrometry. Although thalidomide was present in the plasma and urine, no metabolites were found in the plasma and very low amounts of the 5-OH thalidomide metabolite were present in the urine. These results suggest that thalidomide does not undergo significant metabolism by human CYP and that clinically important interactions between thalidomide and drugs that are also metabolized by this enzyme system are unlikely. The major route of thalidomide breakdown in humans and animals is through spontaneous hydrolysis with subsequent elimination in the urine.

Disposition of inhaled isobutene in F344/N rats

Isobutene (2-methylpropene) (CAS No. 115-11-7) is a gas widely used in the chemical manufacturing industry. As an aid to planning long-term toxicity studies, research was conducted to determine the effect of exposure concentrations on the absorption and metabolism of isobutene in F344/N rats. Male F344/N rats (11-15 weeks of age) were exposed for 2 hr to 0, 40, 400, or 4000 ppm isobutene, and a time-course evaluation of blood levels of isobutene was performed using headspace analysis methods. Blood levels of isobutene were linearly related to exposure concentrations between 40 and 400 ppm but increased in a supralinear fashion at the highest concentration, suggesting that the capacity of the rats to metabolize isobutene had been exceeded. Total uptake, excretion patterns, and metabolic conversions were studied in rats exposed for up to 6 hr to 0, 2, 40, 400, or 4000 ppm [14C]isobutene. Absorption of the inhaled isobutene was approximately 8% up to 40 ppm isobutene, but decreased at the higher concentrations. The amount of isobutene metabolized per ppm.hr of exposure was also linear up to 40 ppm but decreased at higher concentrations. Over 90% of the absorbed isobutene was metabolized at exposure concentrations up to 400 ppm, but the exposure to approximately 4000 ppm isobutene resulted in approximately 20% of the absorbed dose exhaled as the unmetabolized isobutene. Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol. Based on these studies, linear dose-response relationships would be expected in chronic toxicity studies for exposures up to 40 ppm isobutene. Additional studies would be required to determine if repeated exposures would induce higher metabolic capacities in the exposed rats.

Physiologically based modeling of 2-butoxyethanol disposition in rats following different routes of exposure

2-Butoxyethanol (BE) is widely used as a solvent in coatings and other consumer products and has shown hematotoxicity in laboratory animals. To provide a physiological basis for extrapolating toxicokinetic data observed in rats to humans, a blood flow rate-limited, physiologically based pharmacokinetic model was developed to describe the distribution and metabolism of BE in rats following drinking water, dermal, and inhalation exposures. The major urinary metabolite, butoxyacetic acid, represented 45 to 60% of the absorbed dose in all three routes of exposure. Other identified urinary metabolites in our studies included ethylene glycol and BE-glucuronide. A model formulation of the possible metabolic pathways based on the experimental data was proposed. The amounts of individual urinary metabolites were used to develop the model. Metabolic constants were estimated by fitting the data within the constraints of in vitro measurements. The model explained the change of profiles of urinary metabolites in different exposure routes by taking into account the differences in absorption rate and by incorporating a minor pathway for metabolism by skin. Sensitivity analysis showed that metabolic constants and blood flow rate to liver had a relatively larger influence on the production of urinary metabolites than the organ volume or the partition coefficient for BE.

Disposition of polycyclic aromatic hydrocarbons in the respiratory tract of the beagle dog. II. The conducting airways

Physiological models have predicted that the lipophilicity of solutes such as polycyclic aromatic hydrocarbons (PAHs) will delay clearance from the respiratory tract. This clearance consists of a delayed penetration of the mucous lining layer (MLL), allowing mucociliary clearance, followed by a slow penetration of PAHs through walls of the conducting airways. To test this prediction, mucociliary clearance and retention in the mucosa of PAHs deposited in the conducting airways of the Beagle dog were measured. Mucociliary clearance of particles and dissolved PAHs was measured by instilling onto the MLL in a main stem bronchus or the distal trachea small volume of saline containing either dissolved benzo(a)pyrene (BaP) or phenanthrene (Phe), or a suspension of particulate solvent green (SG) or macroaggregated albumin (MAA). Sequential lavage of the mucous-retained materials followed the instillations. Retention of BaP in the airway walls of the bronchial tree was studied by instilling the hydrocarbon in an ethanol/saline solution at precise locations of the upper bronchial tree, and measuring the concentration of BaP and its major metabolites in the tissues. Results indicated that mucociliary clearance of SG and MAA particles in the trachea of the Beagle dog occurred at average rates of 27-30 mm/min. Of the two solutes, only the highly lipophilic BaP was sufficiently retained within the MLL to be transported with the mucociliary escalator. In addition, a fraction of the lipophilic materials cleared at a very rapid rate, in excess of 90 mm/min. This may indicate that one monolayer of pulmonary surfactant at the air interface is spreading out of the lungs on top of the MLL ata faster rate than mucociliary clearance. However, despite the protective properties of the MLL, fractions of BaP penetrating to the bronchial epithelium had a clearance half-time in the range of 1.4 hr, a period during which considerable metabolism of the PAH occurred. This long retention indicates a diffusion-limited uptake of BaP by the airways, and underscores the potential for local toxicity of highly lipophilic toxicants in the bronchial epithelium.

Species differences in urinary butadiene metabolites; identification of 1,2-dihydroxy-4-(N-acetylcysteinyl)butane, a novel metabolite of butadiene

1,3-Butadiene (BD) is used in the manufacture of styrene-BD and polybutadiene rubber. Differences seen in chronic toxicity studies in the susceptibility of B6C3F1 mice and Sprague-Dawley rats to BD raise the question of how to use the rodent toxicology data to predict the health risk of BD in humans. The purpose of this study was to determine if there are species differences in the metabolism of BD to urinary metabolites that might help to explain the differences in the toxicity of BD. The major urinary metabolites of BD in F344/N rats, Sprague-Dawley rats, B6C3F1 mice, Syrian hamsters, and cynomolgus monkeys were identified as 1,2-dihydroxy-4-(N-acetylcysteinyl)-butane (I) and the N-acetylcysteine conjugate of BD monoxide [1-hydroxy-2-(N-acetylcysteinyl)-3-butene] (II). These mercapturic acids are formed by addition of glutathione at either the double bond (I) or the epoxide (II) respectively. When exposed to approximately 8000 p.p.m. of BD for 2 h, the mice excreted 3-4 times as much metabolite II as I, the hamster and the rats produced approximately 1.5 times as much metabolite II as I, while the monkeys produced primarily metabolite I. The ratio of formation of metabolite I to the total formation of the two mercapturic acids correlated well with the known hepatic epoxide hydrolase activity in the different species. These data suggest that (i) the availability of the monoepoxide for conjugation with glutathione is highest in the mouse, followed by the hamster and the rat, and is lowest in the monkey; and (ii) the epoxide availability is inversely related to the hepatic activity of epoxide hydrolase, the enzyme that removes the epoxide by hydrolysis. The ratio of the two mercapturic acids in human urine following BD exposure may indicate the pathways of BD metabolism in humans and may aid in the determination of the most appropriate animal model for BD toxicity.

Effect of dose on the disposition of methoxyethanol, ethoxyethanol, and butoxyethanol administered dermally to male F344/N rats

The glycol ethers methoxyethanol (ME), ethoxyethanol (EE), and butoxyethanol (BE) are widely used in industrial and household products. Rodent studies indicate the ME and EE are potentially toxic compounds causing teratogenic, fetotoxic, hematotoxic, and testicular effects. Exposure of rodents to high concentrations of BE resulted in anemia due to hemolysis of blood cells, leukopenia, hemoglobinuria, and liver and kidney damage. The purpose of this study was to determine the uptake, metabolism, and excretion of dermally administered glycol ethers as a function of the externally applied dose. Three different amounts of the 14C-labeled glycol ethers (450-4000 mumole/kg) were applied to same-sized areas on the clipped backs of F344/N rats, and nonoccluded percutaneous absorption was measured. The rates of excretion of the 14C-labeled parent compound and metabolites by different routes were measured, as well as the amount of 14C remaining in the carcass. Within the dose range studied, the absorption and metabolism of these three glycol ethers by F344/N rats was linearly related to the dermally applied dose. The absorption of all three glycol ethers was approximately 20-25%, regardless of the chain length of the alkyl group or the dose administered. The majority of the absorbed dose was excreted in the urine. Feces and exhaled CO2 represented minor routes of excretion. The alkoxyacetic acid was a major metabolite for all three glycol ethers. The formation of small amounts of ethylene glycol indicated cleavage of the ether bond.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of exposure concentration on the disposition of inhaled butoxyethanol by F344 rats

The glycol ethers are a class of solvents widely used due to their range of vapor pressures and miscibility in aqueous and organic media. Butoxyethanol (BE) causes anemia and lowered hematocrits in rats due to direct hemolysis of red blood cells. Exposure to BE is most likely to occur by dermal contact or by inhalation. In this paper, we report the uptake, metabolism, and excretion of BE following 6-hr exposure at different inhaled concentrations. The uptake and metabolism of BE were essentially linear up to 438 ppm. The majority of the inhaled butoxy-[14C]ethanol was eliminated in the urine with butoxyacetic acid (BAA) being the major urinary metabolite, accompanied by lesser amounts of ethylene glycol and BE glucuronide. A small proportion (5-8%) of the retained BE was exhaled as 14CO2. Most (greater than 80%) of the [14C]BE-derived material in blood was in the plasma. BAA was the major metabolite of BE in plasma. Ratios of ethylene glycol to BAA in plasma were higher than those in urine. The BE-derived 14C in plasma rapidly became associated with the acid-precipitable (protein) fraction, probably due to binding of metabolites to proteins or incorporation of the BE metabolites into the carbon pool. These results indicate that, in rats, overall metabolism of BE to BAA, the hemolytic metabolite, was linearly related to the exposure concentration up to a concentration that caused severe toxicity (438 ppm). Assuming that the toxicity of inhaled BE is directly proportional to the formation of BAA, the toxicity of inhaled BE can be expected to be linearly related to the exposure concentration up to exposure concentrations that cause mortality.

Metabolism of [14C]benzene by cynomolgus monkeys and chimpanzees

Rodent bioassays indicate that B6C3F1 mice are more sensitive to the carcinogenicity of benzene than are rats. The urinary profile of benzene metabolites is different in rats vs mice. Mice produce higher proportions of hydroquinone conjugates and muconic acid, indicators of metabolism via pathways leading to putative toxic metabolites, than do rats. In both species, metabolism to hydroquinone and muconic acid is favored at low concentrations of benzene, indicating that these pathways are easily saturated. These species differences in the metabolism of benzene make it difficult to predict the health risk to humans and how this risk varies with dose. For this reason, the metabolism of [14C]benzene by cynomolgus monkeys and chimpanzees, animals phylogenetically closer to humans than rodents, was studied. Monkeys were dosed ip with 5, 50, or 500 mg [14C]benzene/kg body wt. Urine was collected for up to 24 hr following exposure and was analyzed for benzene metabolites. The proportion of the administered 14C excreted in the urine of monkeys decreased from approximately 50 to 15% as the dose increased. Phenyl sulfate was the major urinary metabolite. The proportion of hydroquinone conjugates and muconic acid in the monkey's urine decreased as the dose increased. The proportion of catechol conjugates was not affected by dose. The proportion of these metabolites in the urine was quite variable from animal to animal, but the proportion of muconic acid was consistently much lower in the monkey than in the mouse or rat. Three chimpanzees were administered 1 mg [14C]benzene/kg body wt, iv; essentially all of the injected 14C was recovered in the urine. Of the total urinary metabolites, 79% were accounted for by phenyl conjugates and less than 15% by hydroquinone conjugates or muconic acid. Catechol conjugates were not detected. The metabolism of benzene appeared to be qualitatively similar but quantitatively different in the species studied. The mouse, the sensitive rodent species, forms the highest levels of hydroquinone conjugates and muconic acid and the chimpanzee, the lowest. In all animal species studied for the effect of dose on benzene metabolism, as the dose decreased, a larger proportion of the benzene metabolites was represented by hydroquinone conjugates and muconic acid.

Biochemical and morphologic responses of rat nasal epithelia to hyperoxia

While performing its functions in olfaction, modification of inspired air, and protection of the lower respiratory tract from high concentrations of potentially harmful inhalants, the nasal mucosa can be injured by a number of inhalants. In this study, F344/N male rats were exposed to filtered air or hyperoxia (85 or 87% oxygen), 24 hr/day, 7 days/week, for 1 (acute exposure) or 11 (chronic exposure) weeks. There were distinct differences between the different epithelial regions examined in replicative and morphologic responses as well as altered enzyme activities in response to oxygen exposure. Neither acute nor chronic hyperoxic exposure caused degenerative, necrotizing, or inflammatory changes in any of the nasal epithelial examined. Hyperoxia-induced hypertrophy, but not hyperplasia, of the non-ciliated cuboidal (NCC) epithelium occurred after both acute and chronic exposure. Cell replication was increased in portions of the NCC and respiratory epithelia after acute hyperoxia exposure. There were significant increases, compared to controls, in the specific activity of glucose-6-phosphate dehydrogenase in the nasal turbinates, maxilloturbinates, and lateral wall epithelium (NCC epithelium), the nasal septum (respiratory epithelium), and the ethmoturbinates (olfactory epithelium), and in the specific activity of glutathione peroxidase in the NCC epithelium and ethmoturbinates after acute hyperoxia exposure. The specific activity of cytochrome P450-dependent monooxygenase-catalyzed O-deethylation of 3-cyano-7-ethoxycoumarin was significantly decreased, compared to controls, in the NCC epithelium. These results suggest that hyperoxia exposure induces morphologic and biochemical alterations in nasal epithelia which appear to be protective responses of certain cell types to hyperoxia.

Toxicokinetics of inhaled 1,3-butadiene in monkeys: comparison to toxicokinetics in rats and mice

1,3-Butadiene is a potent carcinogen in mice and a weaker carcinogen in rats. People are exposed to butadiene through its industrial use--largely in rubber production (over 3 billion pounds of butadiene were produced in 1989)--and because it is common in the environment, occurring in cigarette smoke, gasoline vapor and in the effluents from fossil fuel incineration. Epidemiological studies have provided some evidence for butadiene carcinogenicity in people. Differences in the uptake and metabolism of inhaled butadiene between rodents and primates, including people, might be reflected in differences in its toxicity. In order to compare uptake and metabolism in primates to that in rodents--for which data were already available--we exposed cynomolgus monkeys (Macaca fascicularis) to 14C-labeled butadiene at concentrations of 10.1, 310 or 7760 ppm for 2 hr. Exhaled air and excreta were collected during exposure and for 96 hr after exposure. The uptake of butadiene as a result of metabolism was much lower in monkeys than in rodents. For equivalent inhalation exposures, the concentrations of total butadiene metabolites in the blood were 5-50 times lower in monkey than in the mouse, the more sensitive rodent species, and 4-14 times lower than in the rat. If the toxicokinetics of butadiene in people is more like that of the monkey than that of rodents, then our data suggest that people will receive lower doses of butadiene and its metabolites than rodents following equivalent inhalation exposures to butadiene. This has important implications for assessing the risk to humans of butadiene exposure based on animal studies.

An active feedback system for isotonic studies of smooth muscle

A feedback system used to perform isotonic studies of smooth muscle is presented. This system is capable of applying a constant force to muscle samples regardless of their contractile activities. The force applied to the tissue is controlled using a proportional integral control system that drives a linear motor. The device is integrated into a sucrose gap tissue bath apparatus where measurements of displacement and electrical activity are also possible. The frequency of canine colonic smooth-muscle electrical oscillations is positively related to applied force.

Effect of repeated benzene inhalation exposures on benzene metabolism, binding to hemoglobin, and induction of micronuclei

Metabolism of benzene is thought to be necessary to produce the toxic effects, including carcinogenicity, associated with benzene exposure. To extrapolate from the results of rodent studies to potential health risks in man, one must know how benzene metabolism is affected by species, dose, dose rate, and repeated versus single exposures. The purpose of our studies was to determine the effect of repeated inhalation exposures on the metabolism of [14C]benzene by rodents. Benzene metabolism was assessed by characterizing and quantitating urinary metabolites, and by quantitating 14C bound to hemoglobin and micronuclei induction. F344/N rats and B6C3F1 mice were exposed, nose-only, to 600 ppm benzene or to air (control) for 6 hr/day, 5 days/week for 3 weeks. On the last day, both benzene-pretreated and control animals were exposed to 600 ppm, 14C-labeled benzene for 6 hr. Individual benzene metabolites in urine collected for 24 hr after the exposure were analyzed. There was a significant decrease in the respiratory rate of mice (but not rats) pretreated with benzene which resulted in lower levels of urinary [14C]benzene metabolites. The analyses indicated that the only effects of benzene pretreatment on the metabolite profile in rat or mouse urine were a slight shift from glucuronidation to sulfation in mice and a shift from sulfation to glucuronidation in rats. Benzene pretreatment also had no effect, in either species, on formation of [14C]benzene-derived hemoglobin adducts. Mice and rats had similar levels of hemoglobin adduct binding, despite the higher metabolism of benzene by mice. This indicates that hemoglobin adduct formation occurs with higher efficiency in rats. After 1 week of exposure to 600 ppm benzene, the frequency of micronucleated, polychromatic erythrocytes (PCEs) in mice was significantly increased. Exposure to the same level of benzene for an additional 2 weeks did not further increase the frequency of micronuclei in PCEs. These results indicate that repeated exposures to benzene, such as might be encountered by humans as a result of occupational or environmental exposures, are not likely to change or increase benzene metabolism.

Simultaneous measurement of electrical activity from two colonic smooth muscle layers using a dual sucrose gap apparatus

An apparatus using the sucrose gap technique is presented. With this apparatus simultaneous measurements of contractile and intracellular electrical activity from the two smooth muscle layers of the colon are made. An "L-shaped" muscle preparation consisting of a leg from the circular muscle layer and a leg from the longitudinal muscle layer is used. A theoretical discussion of the device's operation is presented. Finally, experimental results that validate the theory are included.

Electrical and mechanical interactions between the muscle layers of canine proximal colon

Electrical and mechanical interactions between the two smooth muscle layers of canine colon have been studied using a dual sucrose gap apparatus. Muscle samples were dissected into an L-shape, with one leg cut in the circular direction and the other cut in the longitudinal direction. Longitudinal muscle was removed from the circular leg and circular muscle was removed from the longitudinal leg. The bend of the L contained both layers. The activity of the two layers was studied simultaneously under basal conditions, after stimulation by neostigmine and carbachol, and in the presence of tetrodotoxin. Interactions were more common after stimulation and were marked by modification of one layer's mechanical and electrical activity during increased activity in the other layer. Two patterns were commonly observed. First, during a burst of membrane potential oscillations and spike potentials in the longitudinal layer, slow waves in the circular layer developed spike potentials and some slow waves were also prolonged. Second, during a slow-wave cycle in the circular layer, the amplitude of membrane potential oscillations in the longitudinal layer was increased with an associated increase in the incidence of spike potentials. These interactions were associated with contractions of increased strength, which were similar in both layers. All interactions continued after nerve-conduction blockade by tetrodotoxin.

Disposition of three glycol ethers administered in drinking water to male F344/N rats

The glycol ethers 2-methoxyethanol (ME), 2-ethoxyethanol (EE), and 2-butoxyethanol (BE) are widely used solvents in industrial and consumer applications. The reproductive, teratogenic, and hematotoxic effects of the glycol ethers are due to the alkoxyacetic acid metabolites of these compounds. The effect of alkyl group length on disposition of these three glycol ethers was studied in male F344/N rats allowed access for 24 hr to 2-butoxy[U-14C]ethanol, 2-ethoxy[U-14C]ethanol, or 2-methoxy[U-14C]ethanol in drinking water at three doses (180 to 2590 ppm), resulting in absorbed doses ranging from 100 to 1450 mumols/kg body wt. Elimination of radioactivity was monitored for 72 hr. The majority of the 14C was excreted in urine or exhaled as CO2. Less than 5% of the dose was exhaled as unmetabolized glycol ether. Distinct differences in the metabolism of the glycol ethers as a function of alkyl chain length were noted. For BE 50-60% of the dose was eliminated in the urine as butoxyacetic acid and 8-10% as CO2; for EE 25-40% was eliminated as ethoxyacetic acid and 20% as CO2; for ME 34% was eliminated as methoxyacetic acid and 10-30% as CO2. Ethylene glycol, a previously unreported metabolite of these glycol ethers, was excreted in urine, representing approximately 10, 18, and 21% of the dose for BE, EE, and ME, respectively. Thus, for longer alkyl chain lengths, a smaller fraction of the administered glycol ether was metabolized to ethylene glycol and CO2. Formation of ethylene glycol suggests that dealkylation of the glycol ethers occurs prior to oxidation to alkoxyacetic acid and, as such, represents an alternate pathway in the metabolism of these compounds that does not involve formation of the toxic acid metabolite.

Effect of exposure concentration, exposure rate, and route of administration on metabolism of benzene by F344 rats and B6C3F1 mice

To determine the effect of exposure concentration and the route of administration on benzene metabolism, male F344/N rats and B6C3F1 mice were orally exposed to 1, 10, and 200 mg benzene/kg, and by inhalation for 6 hr to 5, 50, and 600 ppm benzene vapor. The effect of different exposure rates on the metabolism of benzene was determined by exposing rodents over different time intervals to the same total amount of benzene [constant concentration X time factor (C X T) = 300 ppm.hr]. Water-soluble metabolites constituted greater than 90% of the metabolite dose to the tissues and were used as a measure of the metabolism of benzene via different pathways. Water-soluble metabolites were measured in the blood, urine, liver, lung, and bone marrow from animals killed following oral exposures and during and following inhalation exposures. The total "dose" to the tissue of individual metabolites was determined by the area under the curve (AUC). The results indicated a shift in metabolism from putative toxification pathways to detoxification pathways as the exposure concentration or oral dose increased. In mice, hydroquinone glucuronide and muconic acid (markers of toxification metabolic pathways) represented a greater percentage of the administered dose at low doses than at high doses. At high doses, phenylglucuronide and prephenylmercapturic acid (detoxification products) increased as a percentage of the administered dose. This same metabolic shift was observed in rats, except that hydroquinone glucuronide was a minor metabolite of benzene at all concentrations. The AUC of phenylsulfate (detoxification pathway) was proportional to the exposure concentration in both species. Within the range of C X T factors studied, the rate of the inhalation exposure to benzene did not affect the AUC of metabolites in tissues of rats; however, a high dose rate (600 ppm 0.5 hr) in mice caused a shift in metabolism to phenyl conjugates. The comparison of oral and 6-hr inhalation exposures indicated that, in terms of metabolite dose to tissues, there is no simple relationship between these two routes of administration. An oral dose and an inhalation exposure concentration which produce an equal dose of one metabolite produce very different doses of another metabolite. These studies demonstrated a species difference in benzene metabolism, as well as a metabolic shift in benzene metabolic pathways as the exposure concentration was increased.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of dose, dose rate, route of administration, and species on tissue and blood levels of benzene metabolites

Studies were completed in F344/N rats and B6C3F1 mice to determine the effect of dose, dose rate, route of administration, and rodent species on formation of total and individual benzene metabolites. Oral doses of 50 mg/kg or higher saturated the capacity for benzene metabolism in both rats and mice, resulting in an increased proportion of the administered dose being exhaled as benzene. The saturating air concentration for benzene metabolism during 6-hr exposures was between 130 and 900 ppm. At the highest exposure concentration, rats exhaled approximately half of the internal dose retained at the end of the 6-hr exposure as benzene; mice exhaled only 15% as benzene. Mice were able to convert more of the inhaled benzene to metabolites than were rats. In addition, mice metabolized more of the benzene by pathways leading to the putative toxic metabolites, benzoquinone and muconaldehyde, than did rats. In both rats and mice, the effect of increasing dose, administered orally or by inhalation, was to increase the proportion of the total metabolites that were the products of detoxification pathways relative to the products of pathways leading to putative toxic metabolites. This indicates low-affinity, high-capacity pathways for detoxification and high-affinity, low-capacity pathways leading to putative toxic metabolites. If the results of rodent studies performed at high doses were used to assess the health risk at low-dose exposures to benzene, the toxicity of benzene would be underestimated.

Differences in the pathways for metabolism of benzene in rats and mice simulated by a physiological model

Studies conducted by the National Toxicology Program on the chronic toxicity of benzene indicated that B6C3F1 mice were more sensitive to the carcinogenic effects of benzene than were F344 rats. A physiological model was developed to describe the uptake and metabolism of benzene in rats and mice. Our objective was to determine if differences in toxic effects could be explained by differences in pathways for benzene metabolism or by differences in total uptake of benzene. Compartments incorporated into the model included liver, fat, a poorly perfused tissue group, a richly perfused tissue group, an alveolar or lung compartment and blood. Metabolism of benzene was assumed to take place only in the liver and to proceed by four major competing pathways. These included formation of hydroquinone conjugates (HQC), formation of phenyl conjugates (PHC), ring-breakage and formation of muconic acid (MUC), and conjugation with glutathione with subsequent mercapturic acid (PMA) formation. Values for parameters such as alveolar ventilation, cardiac output, organ volumes, blood flow, partition coefficients, and metabolic rate constants were taken from the literature. Model simulations confirmed that during and after 6-hr inhalation exposures mice metabolized more benzene on a mumole per kilogram body weight basis than did rats. After oral exposure, rats metabolized more benzene than mice at doses above 50 mg/kg because of the more rapid absorption and exhalation of benzene by mice. Model simulations for PHC and PMA, generally considered to be detoxification metabolites, were similar in shape and dose-response to those for total metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

A physiological model for simulation of benzene metabolism by rats and mice

Studies conducted by the National Toxicology Program on the chronic toxicity of benzene indicated that B6C3F1 mice are more sensitive to the toxic effects of benzene than are F344 rats. A physiological model was developed to describe the uptake and metabolism of benzene in rats and mice and to determine if the observed differences in toxic effects could be explained by differences in the pathways for metabolism of benzene or by differences in uptake of benzene. Major pathways for elimination of benzene included metabolism to hydroquinone glucuronide or hydroquinone sulfate, phenyl glucuronide or phenyl sulfate, muconic acid, and prephenyl mercapturic acid or phenyl mercapturic acid. Model simulations for total benzene metabolized and for profiles of benzene metabolites were conducted for oral or inhalation exposure and compared to data for urinary excretion of benzene metabolites after exposure of rats and mice to [14C]- or [3H]-benzene by inhalation or gavage. Results for total amount of benzene metabolized, expressed per kilogram body weight, indicated that for inhalation exposure concentrations up to 1000 ppm, mice metabolized at least two to three times as much benzene as did rats. Simulations of oral exposure to benzene resulted in more benzene metabolized per kilogram body weight by rats at oral exposures of greater than 50 mg/kg. Patterns of metabolites formed after either route of exposure were very different for F344/N rats and B6C3F1 mice. Rats primarily formed the detoxification metabolite, phenyl sulfate. Mice formed hydroquinone glucuronide and muconic acid in addition to phenyl sulfate. Hydroquinone and muconic acid are associated with pathways leading to the formation of the putative toxic metabolites of benzene. Metabolic rate parameters, Vmax and Km, were very different for hydroquinone conjugate and muconic acid formation compared to formation of phenyl conjugates and phenyl mercapturic acids. Putative toxication pathways could be characterized as high affinity, low capacity whereas detoxification pathways were low affinity, high capacity. Model simulations suggested that for both rats and mice at lower exposure concentrations hydroquinone and muconic acid represented a larger fraction of the total benzene metabolized than at higher exposure concentrations where detoxification metabolites were predominant. Preferential production of a putative toxic metabolite at low exposure concentrations may have important implications in risk assessment for benzene.

A toxikinetic model for simulation of benzene metabolism

People exposed to benzene, an important industrial solvent and a common pollutant, can develop aplastic anemia and leukemia. The objectives of this study were to develop a physiological model for the metabolism of benzene, based on studies in laboratory animals, and to use this model to predict benzene metabolism in people to concentrations near the current permissible exposure limits. Model simulations predicted that for 8-h inhalation exposures to below 10 ppm, hydroquinone metabolites would predominate. Hydroquinone is associated with pathways leading to the formation of the putative toxic metabolite, benzoquinone. Lower levels of muconic acid, a marker for the putative toxic metabolite, muconaldehyde, were predicted. At concentrations above 10 ppm, detoxification metabolites such as the phenyl conjugates predominate. Predictions of benzene metabolism in humans based on our physiological model may have important implications for risk assessment. Because there may be preferential production of a putative toxic metabolite at low exposure concentrations, linear extrapolation of toxicity observed at high concentrations may underestimate risk at low exposure concentrations.

Effect of repeated benzene inhalation exposures on subsequent metabolism of benzene

Benzene is a known human leukemogen and animal carcinogen. To better assess the risks associated with benzene exposure, it would be helpful to determine whether repeated inhalation exposures would affect the metabolism of benzene. The purpose of these experiments was to determine if exposure of F344 rats and B6C3F1 mice to 600 ppm benzene, 6 h/day, 5 days/week for 3 weeks, would affect the subsequent in vivo metabolism of inhaled [14C]benzene.

Disposition of inhaled 1-chloro-2-propanol in F344/N rats

Propylene chlorohydrins, of which 1-chloro-2-propanol (1-CP) is a constituent, used as intermediates in the manufacture of propylene oxide and have been identified as potential air pollutants. The objective of these studies was to determine whether changes in the inhaled exposure concentration would affect the disposition of 1-CP in rats. In addition, experiments were conducted to identify the carbon atom of 1-CP that is metabolized to CO2. Rats were exposed nose-only to [14C]1-CP for 6 hr to 8.3 +/- 1.0 ppm (26.1 +/- 3.2 micrograms/liter air) or 77 +/- 4 ppm (245 +/- 13 micrograms/liter air) (mean +/- SE). There were two major routes of elimination of 14C, urinary and exhalation of CO2, which together accounted for about 80% of the total 14C in excreta and carcass. Half-times for elimination of 14C in urine as 14CO2 were between 3 and 7 hr with no effect of exposure concentration on the elimination half-times for either route. After the end of exposure, kidneys, livers, trachea, and nasal turbinates contained high concentrations of [14C]1-CP equivalents at both exposure concentrations (30-50 nmol 14C/g tissue for the 8 ppm exposure level and 200-350 nmol 14C/g tissue for the 80 ppm exposure level). Elimination of 14C from tissues was biphasic with about 50% of the material in a tissue being rapidly eliminated with a half-time of 1 to 3 hr and the remaining material slowly eliminated with a half-time of 40 to 80 hr. There was no effect of exposure concentration on elimination half-times in tissues. Major metabolites detected in urine and tissues (liver, kidney, and lung) were N-acetyl-S-(hydroxypropyl)cysteine and/or S-(2-hydroxypropyl)-cysteine. Little unmetabolized 1-CP (less than 1%) was detected in analyzed tissues or urine. We propose a metabolic scheme in which the major pathway for metabolism of 1-CP is to CO2 (which is exhaled) and to cysteine conjugates and mercapturic acids that are excreted in the urine. Both carbon-2 and carbon-3 are metabolized in part to CO2.

A reverse isotope dilution method for determining benzene and metabolites in tissues

A method utilizing reverse isotope dilution for the analysis of benzene and its organic soluble metabolites in tissues of rats and mice is presented. Tissues from rats and mice that had been exposed to radiolabeled benzene were extracted with ethyl acetate containing known, excess quantities of unlabeled benzene and metabolites. Butylated hydroxytoluene was added as an antioxidant. The ethyl acetate extracts were analyzed with semipreparative reversed-phase HPLC. Isolated peaks were collected and analyzed for radioactivity (by liquid scintillation spectrometry) and for mass (by UV absorption). The total amount of each compound present was calculated from the mass dilution of the radiolabeled isotope. This method has the advantages of high sensitivity, because of the high specific activity of benzene, and relative stability of the analyses, because of the addition of large amounts of unlabeled carrier analogue.

Distribution of microsomal monooxygenases in the rabbit respiratory tract

The distribution of microsomal cytochrome P-450 isozymes 2, 4, 5, and 6 and the pulmonary FAD-containing monooxygenase was determined in 10 different anatomical regions of the respiratory tract using immunoblot analysis and enzymatic assays. Cytochrome P-450 isozymes 2 and 5 and the FAD-containing monooxygenase were detected by immunoblotting in all of the pulmonary and nasal samples, although levels in nasal tissues were generally much lower than those levels found in the lung. Cytochrome P-450 isozyme 4, which is generally not present in extrahepatic tissues, was detected in nasal ethmoturbinates and mucosa. Different isozymes of cytochrome P-450 appear to be responsible for the N-demethylation of benzphetamine in lung as compared with nasal tissues. Isozyme 2 is responsible for the N-demethylation of benzphetamine in the lung, whereas another isozyme, possibly isozyme 3a is responsible for N-demethylation in nasal tissues. The presence of isozyme 3a in nasal samples was indicated by the presence of high rates of aniline hydroxylation.

Differences in the metabolism and disposition of inhaled [3H]benzene by F344/N rats and B6C3F1 mice

Benzene is a potent hematotoxin and has been shown to cause leukemia in man. Chronic toxicity studies indicate that B6C3F1 mice are more susceptible than F334/N rats to benzene toxicity. The purpose of the studies presented in this paper was to determine if there were metabolic differences between F344/N rats and B6C3F1 mice which might be responsible for this increased susceptibility. Metabolites of benzene in blood, liver, lung, and bone marrow were measured during and following a 6-hr 50 ppm exposure to benzene vapor. Hydroquinone glucuronide, hydroquinone, and muconic acid, which reflect pathways leading to potential toxic metabolites of benzene, were present in much greater concentrations in the mouse than in rat tissues. Phenylsulfate, a detoxified metabolite, and an unknown water-soluble metabolite were present in approximately equal concentrations in these two species. These results indicate that the proportion of benzene metabolized via pathways leading to the formation of potentially toxic metabolites as opposed to detoxification pathways was much higher in B6C3F1 mice than in F344 rats, which may explain the higher susceptibility of mice to benzene-induced hematotoxicity and carcinogenicity.

Effects of the respiratory tract on inhaled materials

Inhalation exposure is often compared to intravenous or oral routes of administration with regard to the biological fate of inhaled materials. Such comparisons, however, overlook the contribution of respiratory tract enzymes to the metabolic fate and toxicity of inhaled materials. The effect of respiratory tract metabolism on the toxicity of inhaled materials is thought to be substantial for many compounds for the following reasons. (1) High concentrations of xenobiotic metabolizing enzymes occur in the nose and substantial concentrations occur in the lung. (2) The respiratory tract tissues are the first exposed to inhaled materials and are exposed to the highest concentrations (barring tissue specific uptake). (3) The products of respiratory tract metabolism may have different toxicities from those of hepatic metabolism. (4) Tissues at risk to toxic metabolites formed in the respiratory tract are different from those formed in the liver. These four reasons for concluding that respiratory tract metabolism may influence the toxicity of inhaled materials are backed by a solid body of expanding experimental data. Therefore, a complete assessment of the fate of inhaled materials should include assessment of potential contributions of respiratory tract metabolism.

A high pressure liquid chromatographic method for the separation and quantitation of water-soluble radiolabeled benzene metabolites

The glucuronide and sulfate conjugates of benzene metabolites as well as muconic acid and pre-phenyl- and phenylmercapturic acids were separated by ion-pairing HPLC. The HPLC method developed was suitable for automated analysis of a large number of tissue or excreta samples. p-Nitrophenyl [14C]glucuronide was used as an internal standard for quantitation of these water-soluble metabolites. Quantitation was verified by spiking liver tissue with various amounts of phenylsulfate or glucuronides of phenol, catechol, or hydroquinone and analyzing by HPLC. Values determined by HPLC analysis were within 10% of the actual amount with which the liver was spiked. The amount of metabolite present in urine following exposure to [3H]benzene was determined using p-nitrophenyl [14C]glucuronide as an internal standard. Phenylsulfate was the major water-soluble metabolite in the urine of F344 rats exposed to 50 ppm [3H]benzene for 6 h. Muconic acid and an unknown metabolite which decomposed in acidic media to phenylmercapturic acid were also present. Liver, however, contained a different metabolic profile. Phenylsulfate, muconic acid, and pre-phenylmercapturic acids as well as an unknown with a HPLC retention time of 7 min were the major metabolites in the liver. This indicates that urinary metabolite profiles may not be a true reflection of what is seen in individual tissues.

Effect of dose on the absorption and excretion of [14C]benzene administered orally or by inhalation in rats and mice

The effect of dose on the absorption and excretion of [14C]benzene was studied using 13-week old male F344/N rats, Sprague-Dawley rats, and B6C3F1 mice. Gastrointestinal absorption of benzene administered by gavage was greater than 97% in these species for doses between 0.5 and 150 mg benzene/kg body wt. At oral doses below 15 mg/kg, greater than 90% of the 14C excreted was in the urine as nonethylacetate extractable material. Above 15 mg/kg, in both rats and mice, an increasing percentage of the administered benzene was exhaled unmetabolized, suggesting saturation of metabolic pathways. Above 50 mg/kg, total metabolites (as determined by 14C in the urine, feces, and carcass after 2 days) were not linearly related to administered dose. Total metabolites per unit body weight was equal in F344/N rats and B6C3F1 mice at gavage doses up to 50 mg/kg; however, total metabolites in mice did not increase at higher doses. For inhalation exposures, the percentage of inhaled benzene that was absorbed and retained during a 6-hr exposure decreased from 33 +/- 6% (mean +/- standard deviation) to 15 +/- 9% in rats, and from 50 +/- 15 to 10 +/- 2% in mice as the exposure concentration was increased from approximately 26 to 2600 micrograms/liter (10 to 1000 ppm at 615 Torr, 23 degrees C). Total metabolite formation was exponentially related to the benzene exposure concentration with one-half the maximal amount of metabolite formation occurring at 220 micrograms/liter (84 ppm) for B6C3F1 mice and 650 micrograms/liter (250 ppm) for F344/N rats. Total metabolites were higher in mice than in rats at any of the vapor concentrations used due mainly to the higher amount inhaled by mice. Saturation of overall metabolism in mice but not in rats at high doses by both routes of administration indicates species differences in metabolism of benzene.

Formation of hydrogen peroxide and N-hydroxylated amines catalyzed by pulmonary flavin-containing monooxygenases in the presence of primary alkylamines

In atypical reaction, incubation of purified rabbit pulmonary flavin-containing monooxygenase with certain primary alkylamines results in the oxidation of NADPH and the formation of hydrogen peroxide. In addition, significant amounts of N-hydroxylated primary amine are also generated, as determined by colorimetric assay and GC/MS analysis of n-octylamine metabolites. Similar reactions appear to be catalyzed by the mouse pulmonary enzyme. In contrast, incubation of primary alkylamines with hepatic flavin-containing monooxygenases from rabbit, mouse, or pig does not result in NADPH oxidation or metabolism. Another effect of primary alkylamines is marked activation of the mouse pulmonary and pig hepatic flavin-containing monooxygenases with some substrates. The structural requirements for primary alkylamines to elicit NADPH oxidation by the rabbit pulmonary enzyme or to activate the mouse pulmonary and pig hepatic enzymes are identical. This indicates that different flavin-containing monooxygenases probably have a conserved alkylamine-binding site of defined specificity. In the case of the rabbit pulmonary enzyme, this binding may occur very close to or at the catalytic site resulting in some N-hydroxylation of the alkylamine.

Identification of distinct hepatic and pulmonary forms of microsomal flavin-containing monooxygenase in the mouse and rabbit

The flavin-containing monooxygenase has been purified from mouse and rabbit lung microsomes and shown to be distinct from the flavin-containing monooxygenase found in the liver of the same species. The mouse and rabbit lung monooxygenases have a unique ability to N-oxidize the primary aliphatic amine, n-octylamine, commonly included in microsomal incubations to inhibit cytochrome P-450. In the mouse lung, this compound not only serves as a substrate but is also a positive effector of metabolism. The mouse and rabbit lung enzymes have unusual pH optimum, near 9.8, compared to the liver enzymes which have peaks near pH 8.8. Using antibodies raised in goats, Ouchterlony immunodiffusion analysis indicates that the liver and lung proteins are immunochemically dissimilar.

Characterization of the purified microsomal FAD-containing monooxygenase from mouse and pig liver

The FAD-containing monooxygenase (FMO) has been purified from both mouse and pig liver microsomes by similar purification procedures. Characterization of the enzyme from these two sources has revealed significant differences in catalytic and immunological properties. The pH optimum of mouse FMO is slightly higher than that of pig FMO (9.2 vs. 8.7) and, while pig FMO is activated 2-fold by n-octylamine, mouse FMO is activated less than 20%. Compounds, including primary, secondary and tertiary amines, sulfides, sulfoxides, thiols, thioureas and mercaptoimidazoles were tested as substrates for both the mouse and pig liver FMO. Km- and Vmax-values were determined for substrates representative of each of these groups. In general, the mouse FMO had higher Km-values for all of the amines and disulfides tested. Mouse FMO had Km-values similar to those of pig FMO for sulfides, mercaptoimidazoles, thioureas, thiobenzamide and cysteamine. Vmax-values for mouse FMO with most substrates was approximately equal, indicating that as with pig FMO, breakdown of the hydroxyflavin is the rate limiting step in the reaction mechanism. Either NADPH or NADH will serve as an electron donor for FMO, however, NADPH is the preferred donor. Pig and mouse FMOs have similar affinity for NADPH (Km = 0.97 and 1.1 microM, respectively) and for NADH (Km = 48 and 73 microM, respectively). An antibody, prepared by immunizing rabbits with purified pig liver FMO, reacts with purified pig liver FMO but not with mouse liver FMO, indicating structural differences between these two enzymes. This antibody inhibited pig FMO activity up to 60%.

Purification of the flavin-containing monooxygenase from mouse and pig liver microsomes

The microsomal flavin-containing monooxygenase has been purified from mouse and pig liver utilizing Cibacron-Blue Sepharose, Procion-Red agarose, and 2'5'-ADP Sepharose. The enzymes had a final specific activity of 1200 and 954 nmol/min/mg protein from mouse and pig liver respectively. The enzyme from both mouse and pig liver displayed typical flavoprotein spectra and appeared homogeneous by denaturing polyacrylamide gel electrophoresis.

Formation of beta-hydroxyisovalerate by an alpha-ketoisocaproate oxygenase in human liver

Previously we demonstrated the occurrence of a soluble dioxygenase in rat liver which converts alpha-ketoisocaproic acid (the keto acid analog of leucine) to beta-hydroxyisovaleric acid. Herein we show that human liver contains a similar soluble enzyme which converts alpha-ketoisocaproate to beta-hydroxisovaleric acid. We suggest this enzyme functions as a "safety valve" in liver to help prevent excessive accumulation of alpha-ketoisocaproate.

Purification and characterization of an alpha-ketoisocaproate oxygenase of rat liver

Rat liver contains a cytosolic alpha-ketoisocaproate oxygenase which oxidatively decarboxylates and hydroxylates alpha-ketoisocaproate to form beta-hydroxyisovalerate. This oxygenase was purified to near homogeneity. The oxygenase is unstable during purification, unless 5% monothioglycerol is added. The purified enzyme is stable in the presence of 5% monothioglycerol for 3 weeks at 4 degrees C and at least 10 weeks at -80 degrees C. The molecular weight of the alpha-ketoisocaproate oxygenase as determined to be 46,000 and 51,000 using denaturing and nondenaturing conditions, respectively, indicating a monomer. The alpha-ketoisocaproate oxygenase requires Fe2+; other metal ions did not replace Fe2+. Ascorbate activates the enzyme at subsaturating levels of Fe2+, by regenerating Fe2+. The activity is markedly affected by the type of buffer used. For example, the oxygenase activity increased 2- to 3-fold when 0.1 M maleate was used. Iron chelators, such as ADP and EDTA, are inhibitory. The ratio of decarboxylation of 1 mM alpha-[1-14C] ketoisocaproate (as measured by 14CO2 release) to decarboxylation of 1 mM alpha-[1-14C]keto-gamma-methiolbutyrate is 1.0 for all purification fractions, indicating that a single enzyme catalyzes the decarboxylation of both substrates. The apparent Km and Vmax values of the alpha-ketoisocaproate oxygenase using optimized assay conditions are 0.32 mM and 130 nmol/min/mg of protein for alpha-ketoisocaproate and 1.9 mM and 247 nmol/min/mg of protein for alpha-keto-gamma-methiolbutyrate. The principal product of the purified alpha-ketoisocaproate oxygenase, using alpha-ketoisocaproate as a substrate, is beta-hydroxyisovalerate, although small amounts of a compound, which has the chromatographic properties of isovalerate, are also produced.

The mechanism of alpha-ketoisocaproate oxygenase. Formation of beta-hydroxyisovalerate from alpha-ketoisocaproate

A soluble alpha-ketoisocaproate oxygenase from rat liver catalyzes the decarboxylation and hydroxylation of alpha-ketoisocaproate to form beta-hydroxyisovalerate. The source of oxygen (O2 or H2O) enzymatically incorporated into beta-hydroxyisovalerate was investigated using 18O2 and H218O. Greater than 92% of the carboxyl groups of beta-hydroxyisovaleric acid contained 1 18O atom from 18O2 and 15% of the beta-hydroxyl oxygens of beta-hydroxyisovaleric acid contained 18O from 18O2. Since some oxygen of the beta-hydroxyl group is derived from O2 and since others have shown a rapid H2O in equilibrium ROH exchange for similar reactions, we conclude that both of the oxygens of beta-hydroxyisovaleric acid are derived from O2 and that exchange of water oxygen with the beta-hydroxyl group of beta-hydroxyisovaleric acid must occur with an intermediate of the reaction. Thus, the alpha-ketoisocaproate oxygenase would be a dioxygenase. A mechanism consistent with the 18O experiments and other properties of the enzyme is proposed.