Showing papers by "Franz Oesch published in 1984"

Characterization, localization and regulation of a novel phenobarbital-inducible form of cytochrome P450, compared with three further P450-isoenzymes, NADPH P450-reductase, glutathione transferases and microsomal epoxide hydrolase.

Abstract: Two cytochromes P450 (PB1 and PB2) have been isolated from the livers of rats treated with phenobarbital. PB2 (mol. wt. 53 500) is novel and is the first example of a phenobarbital-inducible enzyme with a Soret peak at 447 nm. Using an enzyme-linked immunosorbent assay, some immunochemical and structural similarities were observed between these cytochromes. PB1 and PB2 were induced by phenobarbital, Aroclor 1254, trans-stilbene oxide and to a lesser extent by isosafrole. Immunohistochemical localization of these proteins in the liver of untreated rats showed PB1 to be localized in a large area and PB2 in a narrow range of cells around the central vein. This demonstrates the heterogeneity of hepatocytes even within the centrilobular area and indicates that the synthesis of these two proteins is regulated differently although both are induced by the same agent, phenobarbital. Two 3-methylcholanthrene inducible cytochromes MC1 (mol. wt. 54 500) and MC2 (mol. wt. 57 000) were present at very low levels, MC2 mostly in the periportal region but also diffusely distributed throughout the lobule including some centrilobular cells, MC1 concentrated in the centrilobular region. The localization of two major groups of glutathione transferases (GST's) was also different. 'C' type proteins (Yb Yb') and microsomal epoxide hydrolase (EH), were concentrated around the central vein, whereas the 'B' type proteins (Ya Yc) and cytochrome P450 reductase were distributed in a larger area of this region. Thus, the localization was different for some members of the same enzyme family, whilst similarities in the localization existed across the border of the families: (i) PB2, MC1, EH and GST 'C' type proteins were concentrated in a narrow area around the central vein; (ii) PB1 and GST 'B' type proteins occupied a large centrilobular area; (iii) MC2 levels were very low, predominantly periportal but also diffusely distributed throughout the lobule. Treatment of the animals with inducers increased the staining intensity and in several cases extended the areas of cells containing these proteins over the adjacent zone without fundamentally altering their distributions. However, treatment with beta-naphthoflavone led to a shift of MC1 to the periportal area. This suggests that the expression of these proteins in certain cells is not an irreversible quality of differentiation but depends on the degree of suppression and derepression of regulatory components.(ABSTRACT TRUNCATED AT 400 WORDS)

Existence of multiple forms of microsomal epoxide hydrolases with radically different substrate specificities

Abstract: Evidence for the existence in rat and rabbit liver of two microsomal epoxide hydrolases with radically different substrate specificities was obtained, one with a broad specificity (EHb), whilst the other catalyzed the hydrolysis of cholesterol 5 alpha,6 alpha-oxide (EHch), a reaction taken as diagnostic since it was not observed with pure fractions of EHb. The two enzymes were physically separated by immunoprecipitation using antibodies which had been raised against EHb purified to apparent homogeneity. The substrate specificity of the two enzymes is radically different and mutually complementary. Cholesterol 5 alpha,6 alpha-oxide has a trisubstituted oxirane ring. All epoxides of this nature tested to date were not, or very poor, substrates of EHb. The two enzymes can also effectively be discriminated by inhibitors, in that 5 alpha,6 alpha-imino-5 alpha-cholestane-3 beta-ol potently inhibits EHch but not EHb whilst 1,1,1-trichloropropene oxide has the opposite specificity. The cytosolic EH did not significantly contribute to the catalysis of the hydrolysis of cholesterol 5 alpha,6 alpha-oxide.

Dynamics of the localization of drug metabolizing enzymes in tissues and cells.

Abstract: metabolism. In addition, various inducers affect the liver weight, the number of hepatocytes, the cellular content of endoplasmic reticulum, and the phospholipid composition of this organelle (DePierre et al., 1981). Many inducers also increase the rates of synthesis of DNA, RNA, protein and haem (DePierre et al., 1981). Of course, in all cases a rationale for these effects can be suggested (see Table 2). Indeed, since all the different metabolic processes in a cell are more or less interdependent, it was somewhat naive of us to believe that xenobiotics could induce drug-metabolizing enzymes without having profound effects on other aspects of cellular metabolism as well.

Significance of Various Enzymes in the Control of Mutagenic and Carcinogenic Metabolites Derived from Aromatic Structures

Abstract: One important early contribution to the control of chemical carcinogenesis is provided by the enzyme pattern responsible for the generation and disposition of reactive metabolites. Especially well studied is the important group of enzymes responsible for the control of reactive epoxides. Many natural as well as man-made foreign compounds, including pharmaceuticals, possess olefinic or aromatic double bonds. Such compounds can be transformed to epoxides by microsomal monooxygenases present in very many mammalian organs. By virtue of their electrophilic reactivity such epoxides may spontaneously react with nucleophilic centers in the cell and thus covalently bind to DNA, RNA, and protein. Such alterations of critical cellular macromolecules may disturb the normal biochemistry of the cell and lead to cytotoxic, allergenic, mutagenic, and/or carcinogenic effects. Whether such effects will be manifested depends on one hand on the chemical reactivity as well as other properties (geometry, lipophilicity) of the epoxide in question. On the other hand, enzymes controlling the concentration of such epoxides are another important contributing factor. Several microsomal monooxygenases exist differing in activity and substrate specificity. With respect to large substrates, some monooxygenases preferentially attack at one specific site different from that attacked by others. Some of these pathways lead to reactive products, others are detoxification pathways. Moreover, enzymes metabolizing such epoxides represent a further determining factor. These enzymes include epoxide hydrolases and glutathione transferases. These enzymes do not play a pure inactivating role, but can in some cases also act as coactivating enzymes. Enzymes involved in biosynthesis and further metabolism of epoxides differ in quantity and sometimes also in substrate specificity between organs, developmental stages, sexes, and animal species.(ABSTRACT TRUNCATED AT 250 WORDS)

A facile microsynthesis of 14C-labelled picene

Abstract: Microscale Wittig olefination of commercially available [7-14C]-benzal dehyde of high specific radioactivity with aryl bromomethyl phosphonium salt and subsequent oxidative photocyclisation affords pure 14C-labelled picene in efficient yield.