Reductive Activation Of Cr(Vi) By Nitric Oxide Synthase

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• 作者：Ryan Porter ; Marie Já ; chymová ; Pavel Martá ; sek ; B. Kalyanaraman ; and Jeannette Vá ; squez-Vivar
• 刊名：Chemical Research in Toxicology
• 出版年：2005
• 出版时间：May 2005
• 年：2005
• 卷：18
• 期：5
• 页码：834 - 843
• 全文大小：291K
• 年卷期：v.18,no.5(May 2005)
• ISSN：1520-5010

文摘

Chromium(VI) is a recognized toxicant whose effects have been linked to its reduction tolower oxidation states. Although Cr(VI) is reduced by several systems, it is anticipated thatits reduction by nitric oxide synthase (NOS) could have significant effects in endothelial andbrain cells that express high constitutive levels of the enzyme. This possibility was examinedby electron paramagnetic resonance that showed the formation of a stable Cr(V) species fromNOS/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 (BH₄)-free NOSwith calcium/calmodulin diminished Cr(V) steady-state levels while increasing superoxideformation. Since SOD restored Cr(V) to control levels, this result was taken as evidence for areaction between Cr(V) and superoxide. Supplementation of NOS with BH₄ cofactor not onlyfailed to increase Cr(V) yields but generated superoxide and hydroxyl radical. Since theholoenzyme does not generate superoxide, this reaction indicated that Cr(V) mediates theoxidation of BH₄-bound to the enzyme. In the presence of L-arginine, however, Cr(VI) neitherenhances superoxide release nor inhibits NO formation from fully active NOS. This suggeststhat L-arginine protects BH₄ from Cr(V)-mediated oxidation. While Cr(V) was inactive towardNO, spin trapping experiments with 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide andoxygen 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 NOSoccurs in resting and fully active states. It is likely that the reaction between Cr(V) andsuperoxide influences the cytotoxic mechanisms of Cr(VI) in cells.