Nitrite Reductase and Nitric-oxide Synthase Activity of the Mitochondrial Molybdopterin Enzymes mARC1 and mARC2

Background: Nitrite reduction pathways are critical for biological NO production under hypoxia. Results: The mitochondrial enzyme mARC reduces nitrite to NO using cytochrome b(5) as electron donor. Conclusion: mARC forms an electron transfer chain with NADH, cytochrome b(5), and cytochrome b(5) reductase to reduce nitrite to NO. Significance: mARC proteins may constitute a new pathway for hypoxic NO production in vivo. Mitochondrial amidoxime reducing component (mARC) proteins are molybdopterin-containing enzymes of unclear physiological function. Both human isoforms mARC-1 and mARC-2 are able to catalyze the reduction of nitrite when they are in the reduced form. Moreover, our results indicate that mARC can generate nitric oxide (NO) from nitrite when forming an electron transfer chain with NADH, cytochrome b(5), and NADH-dependent cytochrome b(5) reductase. The rate of NO formation increases almost 3-fold when pH was lowered from 7.5 to 6.5. To determine if nitrite reduction is catalyzed by molybdenum in the active site of mARC-1, we mutated the putative active site cysteine residue (Cys-273), known to coordinate molybdenum binding. NO formation was abolished by the C273A mutation in mARC-1. Supplementation of transformed Escherichia coli with tungsten facilitated the replacement of molybdenum in recombinant mARC-1 and abolished NO formation. Therefore, we conclude that human mARC-1 and mARC-2 are capable of catalyzing reduction of nitrite to NO through reaction with its molybdenum cofactor. Finally, expression of mARC-1 in HEK cells using a lentivirus vector was used to confirm cellular nitrite reduction to NO. A comparison of NO formation profiles between mARC and xanthine oxidase reveals similar K-cat and V-max values but more sustained NO formation from mARC, possibly because it is not vulnerable to autoinhibition via molybdenum desulfuration. The reduction of nitrite by mARC in the mitochondria may represent a new signaling pathway for NADH-dependent hypoxic NO production.