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V Ullrich

Publications and source records attributed to V Ullrich.

At least 145 records · Page 8Linked to original sources
PubMed1979

Models for ferric cytochrome P450. Characterization of hemin mercaptide complexes by electronic and ESR spectra.

Hemin coordinated with mercaptide sulfur as fifth ligand and various sixth ligands were investigated as models for cytochrome P450 in its native ferric low-spin state and its ligand complexes. Mixing the hemin with its ligands below -60 degrees C prevented the reduction of the hemin by mercaptide and made it possible to characterize each sample both by electronic and ESR spectra. Excess of mercaptide formed hemin-dimercaptide complexes with hyperporphyrin spectra with two Soret bands around 380 and 370 nm. The second mercaptide could be exchanged by other ligands with hydroxyl, phosphine, thioether, isocyanide, amine, imidazole, and pyridine groups. The comparison of these spectral data with cytochrome P450 substantiates mercaptide as the fifth ligand and makes a hydroxyl group a more likely candidate for the native sixth ligand than an imidazole group.

Binding Sites↗
PubMed1979

Model systems for the coordination chemistry of cytochrome P-450.

Studies on model complexes have supported the presence of a mercaptide as the fifth ligand of cytochrome P-450 monooxygenases. When alcohol or thiol ligands are added to the sixth coordination position of a five-coordinated 4-nitrobenzene thiolate complex of FeIII protoporphyrin IX dimethyl ester chloride low spin complexes with optical and EPR-spectra very similar to cytochrome P-450 are obtained. From a comparison with all ligands of cytochrome P-450 and the model complexes it is concluded that a hard ligands must occupy the sixth coordination position of cytochrome P-450. An imidazole group is less likely, also in view of the ligand field parameters. The significance of the fifth and sixth ligand of cytochrome P-450 is discussed with respect to the monooxygenase mechanism.

Cytochrome P-450 Enzyme System↗
PubMed1979

Oxycytochrome P-450: its breakdown to superoxide for the formation of hydrogen peroxide.

Four different experimental studies are described which were designed to evaluate the role of oxycytochrome P-450 in the formation of superoxide anions and hydrogen peroxide. The use of lipophilic copper chelates with superoxide dismutase like activity revealed that the primary site of interaction of these agents is related to the inhibition of the flavoprotein. NADPH-cytochrome P-450 reductase. Measurements of the proton assisted nucleophilic displacement of superoxide from oxycytochrome P-450 by high concentrations of sodium azide indicated an increase in the rate of hydrogen peroxide formation concomitant with the inhibition of the N-demethylation of ethylmorphine. Studies on the effect of NADH on the rate of hydrogen peroxide formation during NADPH oxidation by liver microsomes failed to reveal a stimulatory or synergistic effect in a manner analogous to results obtained during the cytochrome P-450 dependent oxidation of substrates such as ethylmorphine. These results suggest that hydrogen peroxide formation may not require the reduction of oxycytochrome P-450 to peroxycytochrome P-450. Measurements of the reduction of succinylated cytochrome c using purified cytochrome P-450 and the flavoprotein, NADPH-cytochrome P-450 reductase, directly demonstrate the formation of superoxide anions. It is concluded that oxycytochrome P-450 may decompose to generate hydrogen peroxide.

Animals↗
PubMed1977

Multiple forms of cytochrome P450 in the microsomal monooxygenase system.

The microsomal monooxygenase system is characterized by its broad substrate specificity which includes endogenous substrates as well as lipophilic drugs and chemicals. From in vitro investigations it was known that the relative reactivities and the pattern of products varied greatly with species, sex, age, diet or pretreatment with drugs of the animal. The suggestion that this was possibly due to a variety of cytochrome P450 enzymes rather than a single monooxygenase was recently confirmed by the isolation of several cytochrome P450 species with different although overlapping substrate specificities. In view of the consequences of a genetic and environment-dependent pattern on monooxygenases for drug metabolism and drug-mediated toxicity the methods of a quantitative assessment of the various forms are discussed.

Animals↗
PubMed1977

The reactivities of tyrosine and tryptophan residues in lipid-bound cytochrome b5.

Purified cytochrome b5 from rabbit liver microsomes was bound to liposomes prepared from microsomal lipids. Tyrosyl and tryptophyl side chains of the protein were modified by water-soluble reagents and the reactivities of these amino acid residues in the liposome-bound cytochrome b5 were compared to those of the free protein. At pH 13, 80% of the tyrosines in lipid-free cytochrome b5 ionized immediately, whereas in the lipid-bound protein only 65% ionized within the first minute. In contrast, acetylation with acetylimidazole resulted in the conversion of all 5 tyrosine groups of lipid-free as well as lipid-bound cytochrome b5 into O-acetylated derivatives, which upon treatment with hydroxylamine were completely deacetylated. Reaction with N-bromosuccinimide revealed that only 60% of the 4 tryptophan residues present in cytochrome b5 were accessible to the reagent in the lipid-bound protein, although all tryptophans could be modified in lipid free cytochrome b5. It was concluded that the two tyrosines in the region linking the protein to the membrane are not shielded by lipid bilayer but that of the three tryptophans in the same region one is completely buried in the membrane, whereas the remaining two tryptophans may be both partly exposed to the solvent or alternatively, one may be partially and the other completely exposed.

Animals↗
PubMed1977

Spectral evidence for a liver mitochondrial cytochrome P450 involved in bile acid metabolism.

Optical difference spectroscopy of liver mitochondria has revealed the presence of a cytochrome P450 species by its ligand reactions with carbon monoxide, metyrapone and diethylphenylphosphine. Its concentration of 0.15 nmol/mg mitochondrial protein is high enough to be detectable by ESR also. A microsomal contamination of the mitochondria could be excluded. Mitochondrial cytochrome P450 forms an enzyme-substrate complex with 5beta-cholestane-3alpha, 7alpha, 12alpha-triol with Ks value very similar to the Km value of the 26-hydroxylation of this substrate. This supports the existence in liver mitochondria of a cytochrome P450-dependent 26-monooxygenase for bile acid precursors, as previously postulated by us on the basis of a photochemical action spectrum.

Animals↗
PubMed1977

[The role of cytochrome P450 in the activation of drugs (author's transl)].

Unspecific microsomal monooxygenases have been found in many organisms of different developmental stages. In higher organisms liver is the main organ of drug metabolism but smaller intestine, lung and skin also show this activity. The corresponding membrane-bound enzyme system could be isolated by modern chromatographic techniques and was found to consist of a reductase and a series of cytochrome P450 enzymes. Each of these cytochromes has a different, but with other forms overlapping substrate specificity. The steady-state concentrations of the various forms is regulated by induction with drugs and foreign compounds. The unspecificity of the systems is also reflected in the varying pattern of metabolites. In general stable and more polar metabolites are formed by the monooxygenation reaction, but reactive and unstable products may also appear, e.g. N-hydroxy compounds, 1,2-diphenols, epoxides and a new class of compounds which have been characterized as carbenes.

Adult↗
PubMed1976

Membrane effects on drug monooxygenation activity in hepatic microsomes.

The temperature dependence of drug monooxygenation in phenobarbital-induced rat liver microsomes has been investigated. With 7-ethoxycoumarin as a substrate the activity of the microsomes could be measured down to 0 degrees C by the increase in fluorescence of the dealkylated reaction product 7-hydroxycoumarin (umbelliferone). Arrhenius plots of the activities at various temperatures between 0 degrees C and 45 degrees C showed a break in the activation energy around 20 degrees C. Addition of deoxycholate or high concentrations of glycerol, known to solubilize membrane-bound enzymes, abolished the break of the activation energy. Cholesterol, incorporated into the microsomal membrane in amounts equimolar to the microsomal phospholipid content led to a decrease of the activation energy at low temperatures and to an increase at higher temperatures, resulting in a loss of the break. The activity of microsomal NADPH-cytochrome c reductase with the water -soluble electron acceptor dichlorophenolindophenol showed no discontinuity in the Arrhenius plot. In addition the cumene hydroperoxide-mediated and cytochrome P-450-dependent O-dealkylation of 7-ethoxycoumarin proceeded without a break in the activation energy. It is concluded that phospholipid phase transitions affect the electron transfer from the reductase to cytochrome P-450.

Animals↗