Medical College of Wisconsin
CTSICores SearchResearch InformaticsREDCap

Reductive activation of Cr(Vi) by nitric oxide synthase. Chem Res Toxicol 2005 May;18(5):834-43

Date

05/17/2005

Pubmed ID

15892577

DOI

10.1021/tx049778e

Scopus ID

2-s2.0-18944406786 (requires institutional sign-in at Scopus site)   17 Citations

Abstract

Chromium(VI) is a recognized toxicant whose effects have been linked to its reduction to lower oxidation states. Although Cr(VI) is reduced by several systems, it is anticipated that its reduction by nitric oxide synthase (NOS) could have significant effects in endothelial and brain cells that express high constitutive levels of the enzyme. This possibility was examined by electron paramagnetic resonance that showed the formation of a stable Cr(V) species from NOS/Cr(VI). The formation of Cr(V) was calcium/calmodulin-independent indicating that Cr(VI) to Cr(V) reduction occurs at the flavin-containing domain of NOS. Accordingly, Cr(VI) reduction by the reductase domain of NOS and the chimera protein cytochrome-P450-reductase+tail-nNOS also generated Cr(V). Activation of tetrahydrobiopterin (BH(4))-free NOS with calcium/calmodulin diminished Cr(V) steady-state levels while increasing superoxide formation. Since SOD restored Cr(V) to control levels, this result was taken as evidence for a reaction between Cr(V) and superoxide. Supplementation of NOS with BH(4) cofactor not only failed to increase Cr(V) yields but generated superoxide and hydroxyl radical. Since the holoenzyme does not generate superoxide, this reaction indicated that Cr(V) mediates the oxidation of BH(4)-bound to the enzyme. In the presence of L-arginine, however, Cr(VI) neither enhances superoxide release nor inhibits NO formation from fully active NOS. This suggests that L-arginine protects BH(4) from Cr(V)-mediated oxidation. While Cr(V) was inactive toward NO, spin trapping experiments with 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide and oxygen consumption measurements showed that Cr(V) reacts with superoxide by a one-electron-transfer mechanism to generate oxygen and Cr(IV). Thus, reduction of Cr(VI) to Cr(V) by NOS occurs in resting and fully active states. It is likely that the reaction between Cr(V) and superoxide influences the cytotoxic mechanisms of Cr(VI) in cells.

Author List

Porter R, Jáchymová M, Martásek P, Kalyanaraman B, Vásquez-Vivar J

Authors

Balaraman Kalyanaraman PhD Professor in the Biophysics department at Medical College of Wisconsin
Jeannette M. Vasquez-Vivar PhD Professor in the Biophysics department at Medical College of Wisconsin




MESH terms used to index this publication - Major topics in bold

Animals
Arginine
Brain
Calcium
Calmodulin
Chromates
Chromium
Cytochrome P-450 Enzyme System
Endothelial Cells
Flavins
Hydroxyl Radical
Nitric Oxide Synthase
Oxidation-Reduction
Oxygen
Superoxides
Time Factors