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H Nohl

Publications and source records attributed to H Nohl.

At least 73 records · Page 4Linked to original sources
PubMed1990

EPR studies on the oxidation of hydroxyurea to paramagnetic compounds by oxyhemoglobin.

N. Hydroxyurea forms methemoglobin from oxyhemoglobin with concomitant formation of the aminocarbonylaminooxyl radical H2N-CO-NHO., as detected with electron paramagnetic resonance spectroscopy (EPR). This radical could be detected for several hours in a low steady-state concentration. Approximately 1 hr after the reaction had been started, the EPR spectra of two additional paramagnetic intermediates could be detected at low temperature (77 degrees K), a low-spin ferric methemoglobin complex with hydroxyurea (MetHb-NHOH-CO-NH2) and the hemoglobin-nitric oxide adduct (Hb2(+)-NO). The intensities of their EPR spectra increased steadily over the range of more than 64 hr. The low-spin ferric methemoglobin complex was immediately formed when hydroxyurea was dissolved in a methemoglobin whereas the nitric oxide complex was possibly an oxidation product of the MetHb-hydroxyurea adduct. Its oxidative degradation is known to lead to the very toxic compounds nitric oxide and nitrogen dioxide which can therefore contribute to the toxic action of hydroxyurea.

Animals↗
PubMed1990

Is redox-cycling ubiquinone involved in mitochondrial oxygen activation?

In most tissues mitochondria consume more than 90% of cellular oxygen. Although the greatest part of it undergoes tetravalent reduction thereby conserving free energy changes in the form of ATP, a great deal of evidence exists in the literature that also univalently reduced dioxygen is released during respiration. Redox-cycling ubiquinone was considered most frequently to be involved in this univalent e- transfer to oxygen out of sequence however, other components of the respiratory chain could not be excluded. Our investigations on this problem questioned the role of redox-cycling ubiquinone in mitochondrial O2- formation while H2O2 is supposed to accept e- from this source. The paper provides experimental evidence that H2O2 in fact may operate as an oxidant of ubisemiquinone while dioxygen requires protons for such a reaction which are not available in the phospholipid bilayer where ubiquinone undergoes one e- redox-cycling.

Adenosine Triphosphate↗
PubMed1990

Free radical intermediates in the oxidation of N-methylhydroxylamine and N,N-dimethylhydroxylamine by oxyhemoglobin.

Nitroxide radicals have been detected in the methemoglobin formation reaction between oxyhemoglobin and the substituted hydroxylamine compounds, N-methylhydroxylamine and N,N-dimethylhydroxylamine, by ESR spectroscopy. The stability of these nitroxide radicals was considerably higher than that of the NH2O. radical derived from unsubstituted hydroxylamine. Only in the case of N-methylhydroxylamine the detection of the nitroxide radical required the use of a flow system, because the radical was found to undergo a rapid degradation with the concomitant formation of a secondary product, the beta-aminonitroxide CH3NO.CH2NH2. The nitroxide radical derived from N,N-dimethylhydroxylamine and oxyhemoglobin was stable for more than 1 hour. In addition, formation of low-spin iron-(III)-complexes from methemoglobin and excess substituted hydroxylamine was observed in both cases. Neither N-methylhydroxylamine nor N,N-dimethyldroxylamine formed the hemoglobin-nitric oxide complex found with unsubstituted hydroxylamine. Parallels and differences in the reaction path of un-, mono- and disubstituted hydroxylamines are discussed.

Animals↗
PubMed1989

Detection of free radicals as intermediates in the methemoglobin formation from oxyhemoglobin induced by hydroxylamine.

Four distinct paramagnetic intermediates could be observed in the reaction between oxyhemoglobin and hydroxylamine using ESR spectroscopy. The radical species exhibited different stability properties thus different techniques were required for their detection. Two of them were identified as the hydronitroxide radical (NH2O.) and the hemoglobin-nitric oxide complex (Hb2+-NO). The third one is a low-spin iron-(III)-complex, possibly the methemoglobin-hydroxylamine adduct. A fourth paramagnetic species was detected only in the absence of the iron chelator DETAPAC thus indicating that free iron ions were responsible for the formation of this intermediate. The same species was observed when a Fenton system was used to generate the radicals. This species was identified as being the Fe(NO)2X2 complex described in the literature (X = inorganic anions such as OH- or PO3-(4). The identification of the radical intermediates detected in the hydroxylamine-induced methemoglobin formation contributes to a more detailed understanding of the reaction sequence.

Animals↗
PubMed1989

2,3,7,8, tetrachlorodibenzo-p-dioxin induces oxygen activation associated with cell respiration.

We have investigated the influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bioenergetic functions of isolated heart-mitochondria. Electron transfer and energy conservation activities were found to be decreased in the presence of very low amounts of the polychlorinated biphenyl compound (1.5 nmol/mg mitochondrial protein). The effect was greatest when substrates for complex I were used. In this case coupling of oxidative phosphorylation to respiration was drastically diminished, essentially at the expense of state 3 respiration, and P/O values were found around 2 instead of 3. Succinate-related energy conservation remained practically unaffected in the presence of TCDD, suggesting an interference of the toxic compound at coupling site I. SOD plus catalase were found to protect energy-linked respiration from the effect of dioxin indicating the involvement of superoxide radicals and H2O2 in the development of the observed phenomena. The present contribution provides experimental evidence on the formation of these oxygen species in the presence of TCDD. Furthermore, the site of action of TCDD is demonstrated and discussed in relation to the oxygen radical formation observed.

Animals↗
PubMed1988

Identification of the site of adriamycin-activation in the heart cell.

Based on the assumption, the selective cardiotoxicity of anthraquinone antibiotics is due to peculiarities concerning their metabolism in the heart, we have investigated the exogenous NADH oxidoreductase, a heart-specific enzyme recently described (H. Nohl, Eur. J. Biochem. 169, 585 1987) for its possible role in the development of cardiotoxic effects. Cytosolic anthraquinones have direct access since the enzyme was shown to be associated with the cytosolic face of the inner mitochondrial membrane. Redox properties, kinetic data and the poor substrate selectivity suggest the exogenous NADH-oxidoreductase to be involved in the activation of cellular anthraquinones. According to this concept, a direct single electron-shuttle from exogenous NADH to the anthraquinone adriamycin was demonstrated by the detection of adriamycin-semiquinone-related ESR signals. Activation of adriamycin to its semiquinone state at the expense of NADH was also observed with the solubilized NADH-oxidoreductase of heart mitochondria. Microsomal activation of adriamycin was found to result from contaminating exogenous NADH-oxidoreductase of heart mitochondria attached to microsomal membrane fractions. Based on these findings, it was concluded that adriamycin activation in heart cells is due to the existence of the heart specific exogenous NADH-oxidoreductase. Considering the physiological function of this enzyme, activation of cellular adriamycin also appears to be regulated by metabolic changes of cytosolic NADH/NAD ratios.

Animals↗
PubMed1987

Demonstration of the existence of an organo-specific NADH dehydrogenase in heart mitochondria.

Experimental evidence is presented showing the existence of an NADH-consuming enzyme in heart mitochondria, in addition to the NADH--ubiquinone oxidase of complex I. In contrast to the latter, the novel enzyme is accessible from the extramitochondrial space. Removal of the outer membranes from intact mitochondria had no influence on exogenous NADH consumption, indicating its location at the cytosolic face of the inner membrane. The enzyme could be solubilized from this membrane and purified by sedimentation through preformed sucrose gradients. Liver mitochondria exhibited no oxidation of external NADH, suggesting that the enzyme is organo-specific. The "exogenous NADH dehydrogenase" of heart mitochondria was found to introduce reducing equivalents into the respiratory chain before the rotenone block, indicating that the enzyme is associated with complex I. The enzyme was also demonstrated to be involved in electron flow from the respiratory chain to exogenous electron acceptors, including NAD+. This permitted us to elicit the existence of an energy-dependent reversed electron flow from complex II to complex I. The redox shuttle established by the novel enzyme could be of significance for the regulation of cellular NADH and the metabolic activation of foreign compounds such as adriamycin.

Animals↗
PubMed1987

A novel superoxide radical generator in heart mitochondria.

Experimental evidence is presented demonstrating the existence of a potent O2.- source in heart mitochondria. The novel O2.- generator is more active than any other known mitochondrial O2.- generator and also exhibits a higher affinity for molecular oxygen. In contrast to mitochondrial O2.- sources reported previously [(1974) FEBS Lett. 42, 68-72; (1978) Eur. J. Biochem. 82, 563-567], the O2.- generator described in this paper is not involved in energy-linked respiration. Superoxide radicals from this source require NADH to initiate their generation, and the radicals formed are released entirely into the extramitochondrial space. NADH-related O2.- generation was also observed with the solubilized exogenous NADH oxidoreductase of heart mitochondria, an enzyme recently described [(1987) Eur. J. Biochem., submitted]. This finding together with the lack of an NADH-dependent O2.- source in liver mitochondria suggests that the novel O2.- generator and the exogenous NADH oxidoreductase of heart mitochondria are identical.

Animals↗
PubMed1986

The mitochondrial site of superoxide formation.

Ubiquinone and cytochrome b566 have both been postulated to cause mitochondrial O2 formation by autoxidation of their reduced forms. The present investigation was made to evaluate capabilities of the two candidates to transfer electrons to molecular oxygen out of sequence of the normal pathway of respiration. The results show that electron transfer from ubisemiquinone to oxygen depends on the availability of protons. In agreement with this finding autoxidation of redox cycling ubiquinone could not be observed due to its location in an aprotic environment of the mitochondrial membrane. However, O2 release from mitochondria was found to be related to the existence of low potential cytochrome b566. The transfer of this b type cytochrome to more positive values caused a concomitant decrease and finally inhibition of univalent electron transfer to oxygen out of sequence. Our findings suggest a role of cytochrome b 566 in mitochondrial O2 formation. A contribution by ubiquinone is unlikely as long as protons are deprived from penetrating into the domain where ubiquinone is operating.

Aerobiosis↗
PubMed1983

OH.-generation by adriamycin semiquinone and H2O2; an explanation for the cardiotoxicity of anthracycline antibiotics.

Anthracycline-induced cardiomyopathy is still a matter of discussion. The many mechanisms proposed cannot explain a selective sensitivity of the heart to these antitumor drugs. The present paper provides experimental evidence which shows that heart tissue has special biochemical conditions which favour an anthracycline-catalysed electron shuttle to H2O2. This results in the generation of highly reactive OH.-radicals, instead of O-.2-radicals, which are expected to be formed in tissues also supplemented with anthracycline-activating microsomal enzyme systems.

Animals↗
PubMed1982

[Biological and gerontological significance of oxygen].

The phylogenetic "discovery" of oxygen as the terminal electron acceptor of respiration may be regarded as a prerequisite of the development to higher forms of life. The rationale for this assumption is based on the considerable higher energy gain associated with aerobic respiration which allowed to go new pathways in metabolism. However, oxygen constitutes both a benefit and a threat to living things because aerobic respiration involves the formation of dangerously reactive intermediates. These intermediates have been recognized to play a key role in many pathogenetic events and postulated to contribute to the biological process of aging. Organisms which have availed themselves of the benefit of aerobic respiration, developed a series of synergistic defenses against its danger. Perturbation of oxygen homeostasis occurs when formation of toxic oxygen intermediates exceeds the capacity of protective mechanisms. A sudden event which leads to such an imbalance may cause the manifestation of different types of maladies. Our own investigations on rat-heart mitochondria have indicated that aging is associated with a progressive loss in the capacity to balance the formation of toxic oxygen metabolites. Despite a clear induction, activities of protecting enzymes seem to be "swamped" by an age-dependent raise of steady state levels of dangerous oxygen intermediates. The resulting structural damage and functional impairment of mitochondria are discussed as the primary cause of cellular aging.

Aerobiosis↗
PubMed1981

[Physiological and pathophysiological significance of superoxide-radicals and the regulatory role of the enzyme superoxide dismutase (author's transl)].

The monovalent reduction of molecular oxygen, resulting in the formation of superoxide radicals (O(2)) is regarded as to be an ongoing physiological process involved in the respiration and other biological processes of aerobic cells. These reactive oxygen species have been reported to function as cofactors in many biosynthetic reaction steps. Thus, deviations from cellular steady state concentrations may lead to a multiplicity of clinical symptoms or may to a great deal determine the characteristic of a distinct malady. Decrease of cellular O(2)-concentration is discussed in connection with Trisomie 21 and various mental disorders. The role of O(2) in the biochemistry of inflammation, autoimmune diseases, various toxicological cases and the biological aging process is described. Hypothetical considerations concerning the involvement of O(2) in the pathogenetic mechanisms of Morbus Wilson, haemochromatosis, Parkinson syndrome, cataractogenesis and in carcinogenesis are presented. The physiological control of cellular O(2)-concentration is performed by formation rates of the various cellular O(2)-sources and the overall elimination rates of O(2)-consuming reaction steps. Superoxide dismutase (SOD) is of special interest within this cycle because it detoxifies O(2) radicals with velocity rates which are significantly faster than any other pathway involved in O(2) elimination. Thus attempts for a therapeutic interference on tissue levels of O(2)-radicals are mainly based on inhibition or activation of cellular SOD-activities depending on a supposed decrease or increase in cellular steady state concentrations of O(2). The availability of a drug version of SOD and of various synthetic SOD-active compounds allowing a therapeutic decrease of O(2)-tissue levels. Inhibition of cellular SOD is also possible, however, many still unknown toxic side effects should be expected because of unspecific action of the inhibitor available.

Biochemical Phenomena↗
PubMed1980

The metabolic fate of mitochondrial hydrogen peroxide.

1. Mitochondrial H2O2 formation is not in equilibrium with defence mechanisms that counteract an accumulation of H2O2 in rat-heart cells. 2. A model for the accumulation kinetics is proposed which is consistent with the data presented. 3. Four different pathways of H2O2 metabolism are described in rat-heart mitochondria. The major site for metabolic branching of H2O2 via different routes was found to be the mitochondrial catalase. 4. Glutathione (GSH) peroxidase accounts for only 15% of intramitochondrial H2O2 metabolism, while catalase-mediated destruction is four times more rapid. 5. Catalase activity is limited by its structural compartmentation in the matrix, while GSH peroxidase activity was found to be dependent on the availability of free GSH. 6. Catalase was shown to protect rat-heart mitochondria from upsetting redox states of GSH and pyridine nucleotides following H2O2 decomposition by GSH peroxidase. 7. Computer simulations of experimental data suggest the existence of a third sink for mitochondrial H2O2, possibly due to mitochondrial formation of OH . radicals; another fraction of the H2O2 matrix pool may cross the mitochondrial membrane and accumulate in the cytosol.

Animals↗
PubMed1979

Responses of mitochondrial superoxide dismutase, catalase and glutathione peroxidase activities to aging.

The previous observation (Eur. J. Biochem., 82 (1978) 563--567) that age-dependent accumulation of lipid peroxides follows as a consequence of increased radical formation in mitochondria has prompted an examination of the response of a set of protective enzymes to the above situation. Levels of mitochondrial catalase activity as well as selenium-dependent glutathione peroxidase activity were found to be increased with age, while superoxide dismutase activity remained unchanged. No selenium-independent glutathione peroxidase activity could be detected either in preparations from young 3-month-old controls or in preparations from 2-year-old rats. Both the relatively high and unchanged levels of reduced glutathione and kinetic considerations suggest that glutathione peroxidase is preferentially involved in lipid peroxide metabolism, while catalase predominantly metabolizes mitochondrial H2O2.

Aging↗