2006 stock assessment of North Sea plaice using a Bayesian catch-at-age model

Projectverslag over de verbetering van de toestandsbeoordeling van schol en tong. Problemen ronde de onzekerheid en bias in de toestandsbeoordeling en de gegevens die daar voor worden gebruikt zijn onderzocht. Dit verslag betreft de onzekerheid in de toestandsbeoordeling van Noordzee schol aan de hand van een Bayesiaans 'catch at age'- model (vangst per leeftijd)

The biotransformation of isoprene and the two isoprene monoepoxides by human cytochrome P450 enzymes, compared to mouse and rat liver microsomes

The metabolism of isoprene was investigated with microsomes derived from cell lines expressing human CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1, or CYP3A4. The formation of epoxide metabolites was determined by gas chromatographic analysis. CYP2E1 showed the highest rates of formation of the isoprene monoepoxides 3,4-epoxy-3-methyl-1-butene (EPOX-I) and 3,4-epoxy-2-methyl-1-butene (EPOX-II), followed by CYP2B6. CYP2E1 was the only enzyme showing delectable formation of the diepoxide of isoprene, 2-methyl-1,2:3,4-diepoxybutane. Both isoprene nonepoxides were oxidized by CYP2E1 to the diepoxide at similar enzymatic rates. In order to determine the relative role of CYP2E1 in hepatic metabolism, isoprene as well as the two monoepoxides were also incubated with a series of ten human liver microsomal preparations in the presence of the epoxide hydrolase inhibitor cyclohexene oxide. The obtained activities were correlated with activities towards specific substrates for CYP1A2, CYP2A6, CYP2C9, CYP2D6, CYP2E1 and CYP3A. The results were supportive for those obtained with single human P450 enzymes. Isoprene (monoepoxide) metabolism showed a significant correlation with CYP2E1 activity, determined as chlorzoxazone 6-hydroxylation. CYP2E1 is therefore the major enzyme involved in hepatic metabolism of isoprene and the isoprene monoepoxides in vitro. To investigate species differences with regard to the role of epoxide hydrolase in the metabolism of isoprene monoepoxides, the epoxidation of isoprene by human liver microsomes was compared to that of mouse and rat liver microsomes. The amounts of monoepoxides formed as a balance between epoxidation and hydrolysis, was measured in incubations with and without the epoxide hydrolase inhibitor cyclohexene oxide. Inhibition of epoxide hydrolase resulted in similar rates of monoepoxide formation in mouse, rat and man. Without inhibitor, the total amount of monoepoxides present at the end of the incubation period was twice as high for mouse liver microsomes than for rat and even 15 times as high as for human liver microsomes. Thus, differences in epoxide hydrolase activity between species may be of crucial importance for the toxicity of isoprene in the various species

Prediction of isoprene diepoxide levels in vivo in mouse, rat and man from enzyme kinetic data in vitro and physiologically-based pharmacokinetic modeling

The present study was designed to explain the differences in isoprene toxicity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the human situation was attempted. For this purpose, enzyme kinetic parameters Km and Vmax were determined in vitro in mouse, rat and human liver microsomes/cytosol for the cytochrome P450-mediated formation of isoprene mono- and diepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diepoxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences regarding isoprene diepoxide levels were forecasted. Almost similar isoprene diepoxide liver and lung concentrations were predicted in mouse and rat, while predicted levels in humans were about 20-fold lower. However, when interindividual variation in enzyme activity was introduced in the human model, the levels of isoprene diepoxide changed considerably. It was forecasted that in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentrations in the liver increased to similar concentrations as predicted for the mouse. However, the interpretation of the latter finding for human risk assessment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxide concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining the extent of isoprene toxicity. In order to confirm this, in vivo experiments are required in which isoprene epoxide concentrations will be established in rats and mice

Preservar a memória do mestre Schenberg

In the present study, the enzymatic conjugation of the isoprene monoepoxides 3,4 epoxy-3-methyl-1-butene (EPOX-I) and 3,4-epoxy-2-methyl-1-butene (EPOX-II) with glutathione was investigated, using purified glutathione S-transferases (GSTs) of the α, μ, π and θ-class of rat and man. HPLC analysis of incubations of EPOX-I and EPOX-II with [35S]glutathione (GSH) showed the formation of two radioactive fractions for each isoprene monoepoxide. The structures of the EPOX-I and EPOX-II GSH conjugates were elucidated with 1H-NMR analysis. As expected, two sites of conjugation were found for both isoprene epoxides. EPOX-II was conjugated more efficiently than EPOX-I. In addition, the μ and θ class glutathione S-transferases were much more efficient than the α and π class glutathione S-transferases, both for rat and man. Because the μ- and θ-class glutathione S-transferases are expressed in about 50 and 40-90% of the human population, respectively, this may have significant consequences for the detoxification of isoprene monoepoxides in individuals who lack these enzymes. Rat glutathione S-transferases were more efficient than human glutathione S-transferases: rat GST Tl-1 showed about 2.1-6.5-fold higher activities than human GST T1-1 for the conjugation of both EPOX-I and EPOX-II, while rat GST M1-1 and GST M2-2 showed about 5.2-14-fold higher activities than human GST M1a-1a. Most of the glutathione S-transferases showed first order kinetics at the concentration range used (50-2000 μM). In addition to differences in activities between GST-classes, differences between sites of conjugation were found. EPOX-I was almost exclusively conjugated with glutathione at the C4-position by all glutathione S-transferases, with exception of rat GST M1-1, which also showed significant conjugation at the C3-position. This selectivity was not observed for the conjugation of EPOX-II. Incubations with EPOX-I and EPOX-II and hepatic S9 fractions of mouse, rat and man, showed similar rates of GSH conjugation for mouse and rat. Compared to mouse and rat, human liver S9 showed a 25-50-fold lower rate of GSH conjugation