4-nitrocatechol and catechol

The theory of "proton-assisted process" can well explain the catalytic mechanism of homoprotocatechuate 2,3-dioxygenase (2,3-HPCD) with a monoanionic substrate (homoprotocatechuate, HPCA). Here a "non-proton-assisted process" is presented to interpret catalytic mechanism of 2,3-HPCD with a dianionic substrate (4-nitrocatechol, 4NC). The ONIOM calculation is performed to investigate the reaction pathway of a wild-type 2,3-HPCD with 4NC (H200H-4NC system). The catalytic reaction is comprised of four steps: (1) A dioxygen attacks the aromatic ring to produce an alkylperoxo species. (2) O-O bond cleavage and the formation of an epoxide species occur. (3) A seven-membered O-heterocyclic compound is generated by the extinction of the epoxy structure. (4) The seven-membered ring undergoes ring opening to form the final product (C2-C3 cleavage product). The effective free energy barrier of the catalytic reaction of the H200H-4NC system is 26.2 kcal mol

Topics: Biodegradation, Environmental; Catalysis; Catechols; Dioxygenases; Ferric Compounds; Oxygen; Protons

Although skin is the largest organ of the human body, cutaneous drug metabolism is often overlooked, and existing experimental models are insufficiently validated. This proof-of-concept study investigated phase II biotransformation of 11 test substrates in fresh full-thickness human skin explants, a model containing all skin cell types. Results show that skin explants have significant capacity for glucuronidation, sulfation, N-acetylation, catechol methylation, and glutathione conjugation. Novel skin metabolites were identified, including acyl glucuronides of indomethacin and diclofenac, glucuronides of 17β-estradiol, N-acetylprocainamide, and methoxy derivatives of 4-nitrocatechol and 2,3-dihydroxynaphthalene. Measured activities for 10 μM substrate incubations spanned a 1000-fold: from the highest 4.758 pmol·mg skin(-1)·h(-1) for p-toluidine N-acetylation to the lowest 0.006 pmol·mg skin(-1)·h(-1) for 17β-estradiol 17-glucuronidation. Interindividual variability was 1.4- to 13.0-fold, the highest being 4-methylumbelliferone and diclofenac glucuronidation. Reaction rates were generally linear up to 4 hours, although 24-hour incubations enabled detection of metabolites in trace amounts. All reactions were unaffected by the inclusion of cosubstrates, and freezing of the fresh skin led to loss of glucuronidation activity. The predicted whole-skin intrinsic metabolic clearances were significantly lower compared with corresponding whole-liver intrinsic clearances, suggesting a relatively limited contribution of the skin to the body's total systemic phase II enzyme-mediated metabolic clearance. Nevertheless, the fresh full-thickness skin explants represent a suitable model to study cutaneous phase II metabolism not only in drug elimination but also in toxicity, as formation of acyl glucuronides and sulfate conjugates could play a role in skin adverse reactions.

Topics: Acetylation; Adult; Aged; Biotransformation; Catechols; Diclofenac; Female; Glucuronides; Glutathione; Humans; Liver; Male; Metabolic Detoxication, Phase II; Methylation; Middle Aged; Naphthols; Skin; Sulfates

A complex between native, iron(II) soybean lipoxygenase 3 and 4-nitrocatechol, a known inhibitor of the enzyme, has been detected by isothermal titration calorimetry and characterized by X-ray crystallography. The compound moors in the central cavity of the protein close to the essential iron atom, but not in a bonding arrangement with it. The iron ligands experience a significant rearrangement upon formation of the complex relative to their positions in the native enzyme; a water molecule becomes bound to iron in the complex, and one histidine ligand moves away from the iron to become involved in a hydrogen bonding interaction with the catechol. These changes in position result in a trigonal pyramid coordination geometry for iron in the complex. Molecular modeling and force field calculations predict more than one stable complex between 4-nitrocatechol and the central cavity of lipoxygenase 3, but the interaction having the small molecule in the same orientation as the one found in the crystal structure was the most favorable. These observations reveal specific details of the interaction between lipoxygenase and a small molecule and raise the possibility that changes in the ligand environment of the iron atom could be a feature of the product activation reaction or the catalytic mechanism.

Topics: Binding Sites; Calorimetry; Catechols; Computer Simulation; Crystallography, X-Ray; Enzyme Activation; Glycine max; Lipoxygenase; Macromolecular Substances; Models, Molecular; Plant Proteins; Structure-Activity Relationship