Methemoglobin formation by hydroxylamine metabolites of sulfamethoxazole and dapsone: implications for differences in adverse drug reactions.
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Differences in the incidence of adverse drug reactions to trimethoprim-sulfamethoxazole and dapsone may result from differences in the formation, disposition, toxicity, and/or detoxification of their hydroxylamine metabolites. In this study, we examine whether differences in the biochemical processing of sulfamethoxazole hydroxylamine (SMX-NOH) and dapsone hydroxylamine (DDS-NOH) by erythrocytes [red blood cells (RBCs)] contribute to this differential incidence. The methemoglobin (MetHgb)-forming capacity of both metabolites was compared after a 60-min incubation with washed RBCs from four healthy human volunteers. DDS-NOH was significantly more potent (P =.004) but equally efficacious with SMX-NOH in its ability to form MetHgb. The elimination of potential differences in disposition by lysing RBCs did not change the MetHgb-forming potency of either hydroxylamine. At pharmacologically relevant concentrations, greater reduction to the parent amine occurred with DDS-NOH. Maintenance of MetHgb-forming potency was dependent on recycling with glutathione, but no difference in cycling efficiency was observed between DDS-NOH and SMX-NOH. In contrast, the pharmacodynamics of hydroxylamine-induced MetHgb formation were not changed by pretreatment with the glucose 6-phosphate dehydrogenase inhibitor epiandrosterone or by compounds that alter normal antioxidant enzyme activity. Methylene blue, which stimulates NADPH-dependent MetHgb reductase activity, decreased MetHgb levels but did not alter the differential potency of these hydroxylamines. DDS-NOH was also significantly more potent when incubated with purified human hemoglobin A0. Collectively, these data suggest that the inherently greater reactivity of DDS-NOH with hemoglobin, the greater conversion of DDS-NOH to its parent amine, and potential differences in disposition of hydroxylamine metabolites may contribute to the preferential development of dapsone-induced hemotoxicity and sulfamethoxazole-induced hypersensitivity reactions. (+info)
Influence of nitric oxide modulators on cholinergically stimulated hormone release from mouse islets.
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1. We have investigated, with a combined in vitro and in vivo approach, the influence on insulin and glucagon release stimulated by the cholinergic, muscarinic agonist carbachol of different NO modulators, i.e. the nitric oxide synthase (NOS) inhibitors NG-nitro-L-arginine methyl ester (L-NAME), NG-monomethyl-L-arginine (L-NMMA) and 7-nitroindazole as well as the intracellular NO donor hydroxylamine. 2. At basal glucose (7 mM) carbachol dose-dependently stimulated insulin release from isolated islets with a half-maximal response at approximately 1 microM of the agonist. In the presence of 5 mM L-NAME (a concentration that did not influence basal insulin release) the insulin response was markedly increased along the whole dose-response curve and the threshold for carbachol stimulation was significantly lowered. 3. Carbachol-stimulated islets displayed an increased insulin release and a suppressed glucagon release in the presence of L-NAME, L-NMMA or 7-nitroindazole. Significant suppression of glucagon release (except for L-NAME) was achieved at lower concentrations (approximately 0.1-0.5 mM) of the NOS inhibitors than the potentiation of insulin release (1.0-5.0 mM). The intracellular NO donor hydroxylamine dose-dependently inhibited carbachol-induced insulin release but stimulated glucagon release only at a low concentration (3 microM). 4. In islets depolarized with 30 mM K+ in the presence of the KATP channel opener diazoxide, NOS inhibition by 5 mM L-NAME still markedly potentiated carbachol-induced insulin release (although less so than in normal islets) and suppressed glucagon release. 5. In vivo pretreatment of mice with L-NAME was followed by a markedly increased insulin release and a reduced glucagon release in response to an i.v. injection of carbachol. 6. The data suggest that NO is a negative modulator of insulin release but a positive modulator of glucagon release induced by cholinergic muscarinic stimulation. These effects were also evident in K+ depolarized islets and thus NO might exert a major influence on islet hormone secretion independently of membrane depolarization events. (+info)
Chemical modification of NADP-isocitrate dehydrogenase from Cephalosporium acremonium evidence of essential histidine and lysine groups at the active site.
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NADP-isocitrate dehydrogenase from Cephalosporium acremonium CW-19 has been inactivated by diethyl pyrocarbonate following a first-order process giving a second-order rate constant of 3.0 m-1. s-1 at pH 6.5 and 25 degrees C. The pH-inactivation rate data indicated the participation of a group with a pK value of 6.9. Quantifying the increase in absorbance at 240 nm showed that six histidine residues per subunit were modified during total inactivation, only one of which was essential for catalysis, and substrate protection analysis would seem to indicate its location at the substrate binding site. The enzyme was not inactivated by 5, 5'-dithiobis(2-nitrobenzoate), N-ethylmaleimide or iodoacetate, which would point to the absence of an essential reactive cysteine residue at the active site. Pyridoxal 5'-phosphate reversibly inactivated the enzyme at pH 7.7 and 5 degrees C, with enzyme activity declining to an equilibrium value within 15 min. The remaining activity depended on the modifier concentration up to about 2 mm. The kinetic analysis of inactivation and reactivation rate data is consistent with a reversible two-step inactivation mechanism with formation of a noncovalent enzyme-pyridoxal 5'-phosphate complex prior to Schiff base formation with a probable lysyl residue of the enzyme. The analysis of substrate protection shows the essential residue(s) to be at the active site of the enzyme and probably to be involved in catalysis. (+info)
Peroxygenase metabolism of N-acetylbenzidine by prostaglandin H synthase. Formation of an N-hydroxylamine.
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Synthesis of prostaglandin H2 by prostaglandin H synthase (PHS) results in a two-electron oxidation of the enzyme. An active reduced enzyme is regenerated by reducing cofactors, which become oxidized. This report examines the mechanism by which PHS from ram seminal vesicle microsomes catalyzes the oxidation of the reducing cofactor N-acetylbenzidine (ABZ). During the conversion of 0.06 mM ABZ to its final end product, 4'-nitro-4-acetylaminobiphenyl, a new metabolite was observed when 1 mM ascorbic acid was present. Similar results were observed whether 0.2 mM arachidonic acid or 0.5 mM H2O2 was used as the substrate. This metabolite co-eluted with synthetic N'-hydroxy-N-acetylbenzidine (N'HA), but not with N-hydroxy-N-acetylbenzidine. The new metabolite was identified as N'HA by electrospray ionization/MS/MS. N'HA represented as much as 10% of the total radioactivity recovered by high pressure liquid chromatography. When N'HA was substituted for ABZ, PHS metabolized N'HA to 4'-nitro-4-acetylaminobiphenyl. Inhibitor studies demonstrated that metabolism was due to PHS, not cytochrome P-450. The lack of effect of 5,5-dimethyl-1-pyrroline N-oxide, mannitol, and superoxide dismutase suggests the lack of involvement of one-electron transfer reactions and suggests that hydroxyl radicals and superoxide are not sources of oxygen or oxidants. Oxygen uptake studies did not demonstrate a requirement for molecular oxygen. When [18O]H2O2 was used as the substrate, 18O enrichment was observed for 4'-nitro-4-acetylaminobiphenyl, but not for N'HA. A 97% enrichment was observed for one atom of 18O, and a 17 +/- 7% enrichment was observed for two 18O atoms. The rapid exchange of 18O-N'HA with water was suggested to explain the lack of enrichment of N'HA and the low enrichment of two 18O atoms into 4'-nitro-4-acetylaminobiphenyl. Results demonstrate a peroxygenase oxidation of ABZ and N'HA by PHS and suggest a stepwise oxidation of ABZ to N'-hydroxy, 4'-nitroso, and 4'-nitro products. (+info)
Palmitoylation of the three isoforms of human endothelin-converting enzyme-1.
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Endothelin-converting enzyme-1 (ECE-1) is a membrane-bound metalloprotease that catalyses the conversion of inactive big endothelins into active endothelins. Here we have examined whether the three isoforms of human ECE-1 (ECE-1a, ECE-1b and ECE-1c) are modified by the covalent attachment of the fatty acid palmitate and have evaluated a potential functional role of this modification. To do this, wild-type and mutant enzymes were expressed and analysed by metabolic labelling with [3H]palmitate, immunoprecipitation and SDS/PAGE. All three ECE-1 isoforms were found to be palmitoylated via hydroxylamine-sensitive thioester bonds. In addition, the isoforms showed similar levels of acylation. Cys46 in ECE-1a, Cys58 in ECE-1b and Cys42 in ECE-1c were identified as sites of palmitoylation and each of these cysteines accounted for all the palmitoylation that occured in the corresponding isoform. Immunofluorescence analysis demonstrated further that palmitoylated and non-palmitoylated ECE-1 isoforms had the same subcellular localizations. Moreover, complete solubility of the three isoforms in Triton X-100 revealed that palmitoylation does not target ECE-1 to cholesterol and sphingolipid-rich membrane domains or caveolae. The enzymic activities of ECE-1a, ECE-1b and ECE-1c were also not significantly affected by the absence of palmitoylation. (+info)
TAF25p, a non-histone-like subunit of TFIID and SAGA complexes, is essential for total mRNA gene transcription in vivo.
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We demonstrate, utilizing a temperature conditional mutant allele of the gene encoding TAF25p, that this non-histone-like TBP-associated factor, which is shared between the TFIID and SAGA complexes, is required for bulk mRNA gene transcription by RNA polymerase II in vivo. Immunoblotting experiments indicate that at the restrictive temperature, inactivation of TAF25p function results in a reduction of the levels of numerous TFIID and SAGA subunits, indicating its loss of function, like the histone-like TAFs, causes degradation of the constituents of these two multisubunit complexes. These data suggest that TAF25p plays a key structural role in maintaining TFIID and SAGA complex integrity. This is the first demonstration that a non-histone-like TAF is required for continuous, high level RNA polymerase II-mediated mRNA gene transcription in living yeast cells. (+info)
Repetitive activation of corticostriatal fibers produces long-term depression (LTD) of excitatory synaptic potentials recorded from striatal spiny neurons. This form of synaptic plasticity might be considered the possible neural basis of some forms of motor learning and memory. In the present study, intracellular recordings were performed from rat corticostriatal slice preparations to study the role of glutamate and other critical factors underlying striatal LTD. In current-clamp, but not in voltage-clamp experiments, brief focal applications of glutamate, as well as high-frequency stimulation (HFS) of corticostriatal fibers, induced LTD. This pharmacological LTD and the HFS-induced LTD were mutually occlusive, suggesting that both forms of synaptic plasticity share common induction mechanisms. Isolated activation of either non-NMDA-ionotropic glutamate receptors (iGluRs) or metabotropic glutamate receptors (mGluRs), respectively by AMPA and t-ACPD failed to produce significant long-term changes of corticostriatal synaptic transmission. Conversely, LTD was obtained after the simultaneous application of AMPA plus t-ACPD. Moreover, also quisqualate, a compound that activates both iGluRs and group I mGluRs, was able to induce this form of pharmacological LTD. Electrical depolarization of the recorded neurons either alone or in the presence of t-ACPD and dopamine (DA) failed to mimic the effects of the activation of glutamate receptors in inducing LTD. However, electrical depolarization was able to induce LTD when preceded by coadministration of t-ACPD, DA, and a low dose of hydroxylamine, a compound generating nitric oxide (NO) in the tissue. None of these compounds alone produced LTD. Glutamate-induced LTD, as well as the HFS-induced LTD, was blocked by L-sulpiride, a D2 DA receptor antagonist, and by 7-nitroindazole monosodium salt, a NO synthase inhibitor. The present study indicates that four main factors are required to induce corticostriatal LTD: (1) membrane depolarization of the postsynaptic neuron; (2) activation of mGluRs; (3) activation of DA receptors; and (4) release of NO from striatal interneurons. (+info)
Ni(2+) transport and accumulation in Rhodospirillum rubrum.
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The cooCTJ gene products are coexpressed with CO-dehydrogenase (CODH) and facilitate in vivo nickel insertion into CODH. A Ni(2+) transport assay was used to monitor uptake and accumulation of (63)Ni(2+) into R. rubrum and to observe the effect of mutations in the cooC, cooT, and cooJ genes on (63)Ni(2+) transport and accumulation. Cells grown either in the presence or absence of CO transported Ni(2+) with a K(m) of 19 +/- 4 microM and a V(max) of 310 +/- 22 pmol of Ni/min/mg of total protein. Insertional mutations disrupting the reading frame of the cooCTJ genes, either individually or all three genes simultaneously, transported Ni(2+) the same as wild-type cells. The nickel specificity for transport was tested by conducting the transport assay in the presence of other divalent metal ions. At a 17-fold excess Mn(2+), Mg(2+), Ca(2+), and Zn(2+) showed no inhibition of (63)Ni(2+) transport but Co(2+), Cd(2+), and Cu(2+) inhibited transport 35, 58, and 66%, respectively. Nickel transport was inhibited by cold (50% at 4 degrees C), by protonophores (carbonyl cyanide m-chlorophenylhydrazone, 44%, and 2,4-dinitrophenol, 26%), by sodium azide (25%), and hydroxyl amine (33%). Inhibitors of ATP synthase (N, N'-dicyclohexylcarbodiimide and oligomycin) and incubation of cells in the dark stimulated Ni(2+) transport. (63)Ni accumulation after 2 h was four times greater in CO-induced cells than in cells not exposed to CO. The CO-stimulated (63)Ni(2+) accumulation coincided with the appearance of CODH activity in the culture, suggesting that the (63)Ni(2+) was accumulating in CODH. The cooC, cooT, and cooJ genes are required for the increased (63)Ni(2+) accumulation observed upon CO exposure because cells containing mutations disrupting any or all of these genes accumulated (63)Ni(2+) like cells unexposed to CO. (+info)