Abstract
The mitochondrial amidoxime reducing component (mARC) enzymes are molybdenum (Mo) -dependent oxidoreductases that function as part of a three-enzyme metabolon with cytochrome b5 (CYB5) and cytochrome b5 reductase (CYB5R). Previous studies in our lab have demonstrated that nitrite reduction to nitric oxide by the mARC-1 metabolon is regulated by oxygen via superoxide. However, the effect of oxygen on mARC-2, the mARC paralog expressed in human vascular tissue, has not been reported. Therefore, this study was conducted to determine the effect of oxygen on nitrite reduction to nitric oxide by mARC-2 and determine if superoxide is involved. We used purified recombinant mARC-2, CYB5, and CYBR enzymes and kinetic nitric oxide measurements to measure the effect of anoxia, hypoxia, and normoxia on nitrite reduction. We found that the NADH-dependent CYB5/CYB5R/mARC-2 metabolon is more sensitive to oxygen than the mARC-1 metabolon. However, the role of superoxide is less pronounced. This is in contrast with previous studies on the mARC-1 metabolon. Our data suggests that mARC-2 oxygen inhibition proceeds via a superoxide-independent mechanism, possibly direct and irreversible oxidation of the Mo active site.
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