A simulation model describing the metabolism of inhaled and ingested selenium compounds

Organ distribution and retention of the trace element selenium were studied in rats after inhalation of selenious acid and elemental selenium aerosols. A simulation model of selenium metabolism was developed from data obtained. Although the rate of absorption of these two compounds into blood was different, once absorbed both chemical forms behaved identically. This indicates that both compounds joined the same selenium metabolic pool. The simulation model was extended to estimate the levels of selenium in people inhaling selenium compounds normally found in urban atmospheres and in individuals exposed to selenium compounds in the workplace. Predictions of the model for uptake and organ retention of selenium from the environment were compared to measured organ burdens in humans. Inhalation of selenium compounds at levels normally present in urban atmospheres does not contribute significantly to human body burdens of selenium. The most probable source of selenium in human tissues is due to ingestion of selenium in food. However, selenium present in the workplace, in concentrations at the timeweighted threshold limit value, may pose an inhalation hazard to at least some of the workers.     

Human fatality associated with Pacific ciguatoxin contaminated fish

Ciguatera is a food poisoning identified as the principal risk factor in the consumption of tropical fish in Oceania. The syndrome, which follows ingestion of ciguatoxin-contaminated ciguateric fishes, is characterised by an array of gastrointestinal and neurological features. In this report we examine forensic samples associated with a human fatality using a ³H-brevetoxin binding assay and reversed-phase HPLC/MS and HPLC/MS/MS. Three Pacific ciguatoxins (P-CTX) were detected in the implicated fish flesh sample by LC-MS/MS, implicating multiple P-CTXs in the fatal case. Additionally, ciguatoxin was identified in a liver sample obtained at post-mortem. The level of ciguatoxin detected (0.14 ppb P-CTX-1 equivalents by binding assay) indicated that at least 10% of the ingested P-CTX-1 remained in the human liver 6 days after the toxic fish was consumed. This study confirms the potential of tropical reef fish to accumulate sufficient P-CTX to be lethal to humans, especially if the liver and viscera are consumed as part of the meal.     

Human variation in CYP-specific chlorpyrifos metabolism

Chlorpyrifos, an organophophorothioate insecticide, is bioactivated to the neurotoxic metabolite, chlorpyrifos-oxon (CPO) by cytochromes P450 (CYPs). To determine the variability in chlorpyrifos bioactivation, CPO production by human liver microsomes from 17 individual donors was compared relative to phenotype and genotype. CPO production varied over 14-fold between individuals in incubations utilizing 20 μM chlorpyrifos as substrate, while CPO production varied 57-fold in incubations with 100 μM chlorpyrifos. For all but two samples, the formation of the less toxic metabolite, 3,5,6-trichloro-2-pyridinol (TCP), was greater than CPO production. TCP production varied 9-fold in incubations utilizing 20 μM chlorpyrifos as substrate and 19-fold using 100 μM chlorpyrifos. Chlorpyrifos metabolism by individual human liver microsomes was significantly correlated with CYP2B6, CYP2C19 and CYP3A4 related activity. CPO formation was best correlated with CYP2B6 related activity at low (20 μM) chlorpyrifos concentrations while CYP3A4 related activity was best correlated with CPO formation at high concentrations (100 μM) of chlorpyrifos. TCP production was best correlated with CYP3A4 activity at all substrate concentrations of chlorpyrifos. The production of both CPO and TCP was significantly lower at a concentration of 20 μM chlorpyrifos as compared to 100 μM chlorpyrifos. Calculations of percent total normalized rates (% TNR) and the chemical inhibitors ketoconazole and ticlopidine were used to confirm the importance of CYP2B6, CYP2C19, and CYP3A4 for the metabolism of chlorpyrifos. The combination of ketoconazole and ticlopidine inhibited the majority of TCP and CPO formation. CPO formation did not differ by CYP2B6 genotype. Individual variations in CPO production may need to be considered in determining the risk of chlorpyrifos poisoning.     

Human phenylalanine monooxygenase and thioether metabolism

Objectives The substrate specificity of wild‐type human phenylalanine monooxygenase (wt‐hPAH) has been investigated with respect to the mucoactive drug, S‐carboxymethyl‐L‐cysteine and its thioether metabolites. The ability of wt‐hPAH to metabolise other S‐substituted cysteines was also examined. Methods Direct assays of PAH activity were by HPLC with fluorescence detection; indirect assays involved following disappearance of the cofactor by UV spectroscopy. Key findings wt‐hPAH catalysed the S‐oxygenation of S‐carboxymethyl‐L‐cysteine, its decarboxylated metabolite, S‐methyl‐L‐cysteine, and both their corresponding N‐acetylated forms. However, thiodiglycolic acid was not a substrate. The enzyme profiles for both phenylalanine and S‐carboxymethyl‐L‐cysteine showed allosteric kinetics at low substrate concentrations, with Hill constants of 2.0 and 1.9, respectively, for the substrate‐activated wt‐hPAH. At higher concentrations, both compounds followed Michaelis–Menten kinetics, with non‐competitive substrate inhibition profiles. The thioether compounds, S‐ethyl‐L‐cysteine, S‐propyl‐L‐cysteine and S‐butyl‐L‐cysteine were all found to be substrates for phenylalanine monooxygenase. Conclusions Phenylalanine monooxygenase may play a wider role outside intermediary metabolism in the biotransformation of dietary‐derived substituted cysteines and other exogenous thioether compounds.     

The metabolism of phenol and substituted phenols in zebra fish

1. The metabolism of five phenols in zebra fish was studied after uptake from the medium. The results showed no qualitative differences to other Cyprinid fish species, only the oxidation rate seemed to be lower.

2. Phenyl glucuronide, phenyl sulphate, and quinol sulphate were identified as metabolites of phenol.

3. Identified metabolites of 2-cresol were 2-cresyl glucuronide, 2-cresyl sulphate, and 2-hydroxybenzoic acid in trace amounts.

4. Only the glucuronide and sulphate conjugates were detected as metabolites of 4-nitrophenol, 4-chlorophenol, and pentachlorophenol.     

The metabolism of phenol and substituted phenols in zebra fish

1. The metabolism of five phenols in zebra fish was studied after uptake from the medium. The results showed no qualitative differences to other Cyprinid fish species, only the oxidation rate seemed to be lower. 2. Phenyl glucuronide, phenyl sulphate, and quinol sulphate were identified as metabolites of phenol. 3. Identified metabolites of 2-cresol were 2-cresyl glucuronide, 2-cresyl sulphate, and 2-hydroxybenzoic acid in trace amounts. 4. Only the glucuronide and sulphate conjugates were detected as metabolites of 4-nitrophenol, 4-chlorophenol, and pentachlorophenol.     

感冒灵颗粒中对乙酰氨基酚在人体的代谢特征研究

目的研究感冒灵颗粒中对乙酰氨基酚(APAP)在人体的代谢特征。方法 12名健康志愿者单次口服感冒灵颗粒后,收集尿液,采用LC-MS/MS法测定尿液中4种代谢物(葡萄糖醛酸结合物、硫酸结合物、半胱氨酸结合物和乙酰半胱氨酸结合物)的浓度,与常规剂量比较代谢产物的排泄总量及排泄比例。结果单次口服感冒灵颗粒(含APAP 200 mg)后24 h内,4种代谢物总排泄量占给药剂量的(49.1±16.74)%,毒性代谢物排泄率与总代谢物排泄率的比值为(7.7±0.08)%,与常规剂量相比,4种代谢物总排泄率降低,毒性代谢物排泄率与总代谢物排泄率的比值不变。结论感冒灵颗粒中小剂量APAP毒性代谢途径比例未发生改变,而毒性代谢物绝对量呈非线性地显著降低。提示感冒灵颗粒组方可能具有较好的安全性。     

Human microsomal N-oxidative metabolism of cocaine.

The N-oxidative metabolism of cocaine has been described previously in some detail in the mouse, and is associated with hepatotoxic effects of cocaine in this species. As part of an effort to determine whether this metabolism-dependent toxicity is relevant to clinical reports of liver injury from cocaine, the in vitro N-oxidative metabolism of cocaine was examined in microsomal suspensions obtained from human liver specimens. Human hepatic microsomal suspensions were capable of metabolizing cocaine to norcocaine, and norcocaine to N-hydroxynorcocaine, which is the sequential oxidative metabolism observed in mice to lead to toxic metabolite formation. Additionally, incubation of 4-[3H]cocaine with human hepatic microsomal suspensions resulted in the formation of a metabolite that bound irreversibly to microsomal protein. Each of these reactions was inhibited or abolished by gassing the incubation mixture with carbon monoxide, omitting NADPH, or adding the cytochrome P-450 inhibitors SKF 525-A (50 microM) or n-octylamine (3 mM). Competing microsomal esterase activity reduced the apparent rate of N-demethylation of cocaine by 50%, but had much less effect on the N-hydroxylation of norcocaine. In general, apparent KM values for both the N-demethylation and N-hydroxylation reactions in human liver microsomal suspensions were greater than those observed in comparable incubations using mouse hepatic microsomal suspensions, whereas Vmax rates were lower. The extent of irreversible (i.e. nonextractable) binding of cocaine following its metabolism in vitro was comparable between human and mouse microsomal suspensions, however. Similarities in the N-oxidative metabolism of cocaine suggest that humans and mice share the same bioactivation mechanism of cocaine-induced liver injury.     

One‐pot synthesis of [14C]arsenobetaine bromide

A one‐pot synthesis of [¹⁴C]arsenobetaine from 2‐dimethylarsinoylacetic acid is described. Cation‐ and anion‐exchange columns are used in the purification of the compound and its conversion to the bromide salt. Copyright © 2004 John Wiley & Sons, Ltd.     

Human placental glutathione S-transferase-mediated metabolism of methyl parathion

The ability of human placental glutathione S-transferase (GSHTr) to metabolize methyl parathion (MeP) was examined. MeP was found to be a substrate for both partially purified pre-term and highly purified term placental GSHTr. The characterization of the reaction by high performance liquid chromatography revealed the presence of desmethyl parathion (DesMeP) as the sole metabolite. Term placental GSHTr activity towards MeP ranged from 2.22 to 3.53 nmoles DesMeP formed X min-1 X mg-1 while an activity of 0.60 to 1.12 nmoles DesMeP formed X min-1 X mg-1 was observed with the pre-term placental enzyme. The absence of the O-dearylation reaction by pre-term and term placental GSHTr represents a major species- and/or tissue-specific difference.     

Human metabolism and metabolic interactions of deployment-related chemicals

It has been suggested that chemicals and, more specifically, chemical interactions, are involved as causative agents in deployment-related illnesses. Unfortunately, this hypothesis has proven difficult to test, because toxicological investigations of deployment-related chemicals are usually carried out on surrogate animals and are difficult to extrapolate to humans. Other parts of the problem, such as the definition of variation within human populations and the development of methods for designating groups or individuals at significantly greater risk, cannot be carried out on surrogate animals, and the data must be derived from humans. The relatively recent availability of human cell.fractions, such as microsomes, cytosol, etc., human cells such as primary hepatocytes, recombinant human enzymes, and their isoforms and polymorphic variants has enabled a significant start to be made in developing the human data needed. These initial studies have examined the human metabolism by cytochrome P450, other phase I enzymes, and their isoforms and, in some cases, their polymorphic variants of compounds such as chlorpyrifos, carbaryl, DEET, permethrin, and pyridostigmine bromide, and, to a lesser extent, other chemicals from the same chemical and use classes, including solvents, jet fuel components, and sulfur mustard metabolites. A number of interactions at the metabolic level have been described both with respect to other xenobiotics and to endogenous metabolites. Probably the most dramatic have been seen in the ability of chlorpyrifos to inhibit not only the metabolism of other xenobiotics such as carbaryl and DEET but also to inhibit the metabolism of steroid hormones.     

Genotoxicity testing of arsenobetaine, the predominant form of arsenic in marine fishery products

Marine fishery products may contain high levels of arsenic, mainly in the form of organic arsenic compounds. Arsenobetaine has been identified as the predominant form occurring in marine fishery products. The potential initiating and promoting capacities of this compound were therefore investigated in vitro. In the Salmonella typhimurium assay, no mutagenicity was observed in strains TA97, TA98 and TA100 without activation or after addition of a liver-enzyme fraction or gut-flora extract. The compound was also negative in the forward mutation assay of the HGPRT gene and in the test for sister chromatid exchanges in V79 Chinese hamster cells. No inhibition of metabolic co-operation between V79 Chinese hamster cells was observed at arsenobetaine concentrations up to 10 mg/ml. In addition, arsenobetaine had no synergistic or antagonistic effects on the action of the positive controls benzo[a]pyrene and tetradecanoylphorbol-13-acetate.     

Beneficial effects of fish oil on glucose metabolism in spontaneously hypertensive rats

1. Increased interest in fish oil led us to examine their metabolic effects in spontaneously hypertensive rats, which have been reported to have glucose intolerance. 2. Rats were divided into three groups: (i) a control group fed standard rat laboratory chow; (ii) a lard group fed a high-fat diet containing 20% lard; and (iii) a fish oil group fed a high-fat diet containing 20% fish oil for 14 weeks. 3. Systolic blood pressure and fasting blood glucose were markedly increased in the lard group, whereas in the fish oil group they were only transiently increased at the beginning and decreased to levels seen in the control group. 4. Intraperitoneal glucose tolerance test demonstrated that fish oil reversed the impairment of glucose disposal found in the lard group. However, plasma insulin levels were raised transiently at 30 min in the fish oil group compared with the control group. 5. Insulin secretion from pancreatic islets stimulated with glucose in vitro was also enhanced by fish oil. 6. These results lead us to conclude that fish oil improves glucose tolerance by enhancing insulin secretion from pancreatic beta-cells.     

Nickel toxicity on carbohydrate metabolism of a freshwater fish, Colisa fasciatus

Effects of sublethal concentration (0.8 of LC50 96 h) of nickel sulphate (NiSO4 X 7H2O) on muscle glycogen and blood lactic acid levels of a freshwater teleost, Colisa fasciatus, at time intervals from 3-96 h have been recorded. Muscle glycogen levels showed a general decrease at all time intervals compared with controls, with a maximum decrease of 35.2% at 96 h (P less than 0.001). Blood lactic acid showed an increase at all stages attaining a maximum of 78.12% at 12 h (P less than 0.001) except at 24 h where, for reasons not clearly understood, a distinct fall was recorded. The muscle glycogenolysis accompanied by blood hyperlacticaemia is probably the result of some direct action of nickel on the fish. The severe stress conditions caused by nickel-induced pathological changes in the gills and blood may also be responsible for these metabolic changes.     

Presystemic branchial metabolism limits di-2-ethylhexyl phthalate accumulation in fish

Despite the high lipophilicity of di-2-ethylhexyl phthalate (DEHP), fish do not extensively accumulate this ubiquitous environmental contaminant. Experiments with rainbow trout (Salmo gairdneri) fitted with an indwelling cannula showed that the majority of [14C]DEHP did not reach the systemic circulation of the fish, but was present in the exposure water as metabolites. Pharmacokinetic analysis, using a compartmental model that included the gill as a separate metabolic compartment, indicated that DEHP was extensively metabolized as it diffused from water to blood. Isolated perfused gill arches of trout metabolized DEHP in the exposure bath to monoethylhexyl phthalate, demonstrating the ability of the gill to prevent DEHP entry into the fish. The relationship between metabolic clearance and tissue perfusion further suggests that metabolism in the gill can play an important role in determining the accumulation and toxicity of organic chemical pollutants in fish.     

Human hepatic microsomal metabolism of delta 1-tetrahydrocannabinol.

Hepatic microsomal metabolism of delta 1-tetrahydrocannabinol (THC) has been extensively studied in many rodent species, but there have been few reports describing such metabolism in humans. Because several THC metabolites are known to be pharmacologically active, identifying the P-450 subfamilies responsible for their formation is of clinical importance. We have found that, in addition to catalyzing the formation of significant amounts of 7-hydroxy-THC, hepatic microsomes from nine human livers also formed 6 beta-hydroxy-THC at approximately the same rate. In addition, 1 alpha,2 alpha-epoxyhexahydrocannabinol (EHHC) was formed at approximately one-third the rate of 7-hydroxy- and 6 beta-hydroxy-THC, and small amounts of 6 alpha-hydroxy- and 6-keto-THC were also found. Immunoinhibition studies with antibodies raised against human hepatic P-450 2C9, or a mouse hepatic P-450 isozyme belonging to the P-450 3A subfamily, revealed that P-450 2C9 catalyzed the formation of 7-hydroxy-THC, whereas P-450 3A catalyzed the formation of 6 beta-hydroxy-THC, EHHC, and the relatively minor metabolites. In contrast, antibodies raised against human P-450 2C8 had no affect on human microsomal THC hydroxylation. Excellent correlations were found between hepatic microsomal P-450 2C9 and 3A content and 7-hydroxy- and 6 beta-hydroxy-THC formation, respectively. In addition, purified P-450 2C9 catalyzed the formation of 7-hydroxy-THC at a 7-fold higher rate than that observed with microsomes. Microsomal 7-hydroxy-THC formation varied less than 5-fold between the livers, suggesting that this activity is normally expressed and probably not subject to environmental influences.(ABSTRACT TRUNCATED AT 250 WORDS)