Oxochromium(V) species formed with 2,3-dehydro-2-deoxy-N-acetylneuraminic or N-acetylneuraminic (sialic) acids: an in vitro model system of oxochromium(V) species potentially stabilized in the respiratory tract upon inhalation of carcinogenic chromium(VI) compounds.
The human respiratory tract is rich in sialoglycoconjugates. These polyfunctional, oxygen-rich molecules stabilize potentially genotoxic oxoCr(V) species, which has relevance to the metabolic fate of carcinogenic Cr(VI) compounds when inhaled as dusts, mists, and fumes during occupational exposure. A series of oxoCr(V) species formed from solutions of Cr(VI) and the reductant, glutathione (GSH), in the presence of an excess of small molecule ligands that model sialoglycoconjugates, either 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (I) or N-acetylneuraminic acid (II), have been characterized as a function of pH value, using electron paramagnetic resonance (EPR) spectroscopy (Cr(V) = d(1)). Where [I] or [II] is in large excess of [Cr(V)-GSH], oxoCr(V)-I or -II species form (g(iso) = 1.97-1.98) via ligand-exchange reactions in which oxygen donor atoms from the carboxylic acid (I), tert-2-hydroxycarboxylato(2-) (II), and/or diolato(2-) groups (I, II) feature in the oxoCr(V) coordination sphere. At pH approximately 7.1 (where [Cr(V)-GSH]:[II] << 1), the oxoCr(V)-II complexes remain EPR active for 24 h (with t(1/2) approximately 380 min). The parent EPR spectrum from oxoCr(V)-I or -II solutions represents the sum of spectra from individual linkage isomers; using EPR spectral simulation procedures, coupled with empirical EPR spectroscopic data for previously characterized oxoCr(V)-hydroxycarboxylato and -diolato species, a signature EPR spectrum has been predicted for each oxoCr(V)-I or -II linkage isomer, with defining g(iso) and (1)H a(iso) values and signal multiplicities. At pH values > 7.0, the major EPR signal (g(iso) approximately 1.980) is a septet, indicating the presence of oxoCr(V)-I or oxoCr(V)-II species with coordination from two O(8),O(9)-diolato(2-) chelates, with six magnetically equivalent protons ((1)H a(iso) approximately 0.7 x 10(-)(4) cm(-)(1)). At pH values < 7.0, where carboxylato binding competes with diolato binding, the species (g(iso) approximately 1.979) most likely feature mixed binding modes from one diolato(2-) and one tert-2-hydroxycarboxylato(2-) chelate (oxoCr(V)-II) or one diolato(2-) chelate, one monodentate carboxylato(1-) group, and aqua and/or hydroxo oxygen donors (oxoCr(V)-I). In I and II, the relative concentration of the oxoCr(V) linkage isomer featuring donation via the O(7),O(8) diolato group is about 1 order of magnitude smaller than the concentration of the O(8),O(9) linkage isomer. The dominance of oxoCr(V)-I or -II linkage isomers with O(8),O(9) donation correlates with the dihedral angle in these isomers (O(9)-C(9)-C(8)-O(8) approximately 65 degrees ) being more favorably disposed toward metal binding, as compared to the O(7),O(8) analogue (O(7)-C(7)-C(8)-O(8) approximately 163 degrees ). These results have important implications with respect to the nature of the oxoCr(V) species that may potentially form in the sialoglycoconjugate-rich environment of the human respiratory tract upon inhalation of carcinogenic Cr(VI) compounds.
Codd
Rachel
R
Centre for Heavy Metals Research, School of Chemistry, University of Sydney, NSW 2006, Australia. r.codd@chem.usyd.edu.au
Lay
Peter A
PA
D002274:Carcinogens, Environmental / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry* / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D005978:Glutathione / Q000737:chemistry / Q000378:metabolism; D066298:In Vitro Techniques; D019158:N-Acetylneuraminic Acid / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism; D009942:Organometallic Compounds / Q000138:chemical synthesis*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism; D012137:Respiratory System / Q000737:chemistry / Q000378:metabolism
D002274:Carcinogens, Environmental; D009942:Organometallic Compounds; D002857:Chromium; C074702:chromium hexavalent ion; C036137:2-deoxy-2,3-dehydro-N-acetylneuraminic acid; D005978:Glutathione; D019158:N-Acetylneuraminic Acid; D010100:Oxygen