Health, Safety & Environment

Mitochondrial amidoxime-reducing component (mARC)     

Mitochondrial amidoxime-reducing component 2 (mARC2) has a significant role in N-reductive activity and energy metabolism

The mitochondrial amidoxime-reducing component (mARC) is a mammalian molybdenum-containing enzyme. All annotated mammalian genomes harbor two mARC genes, MARC1 and MARC2, which share a high degree of sequence similarity. Both molybdoenzymes reduce a variety of Nhydroxylated compounds. Besides their role in Nreductive drug metabolism, only little is known about their physiological functions. In the present study, we characterized an existing knockout (KO) mouse model lacking the functional MARC2 gene and fed a high-fat diet (HFD) and also performed in vivo and in vitro experiments to characterize reductase activity toward known mARC substrates. The MARC2KO significantly decreased reductase activity toward several Noxygenated substrates, and for typical mARC substrates, only a small residual reductive activity was still detectable in the MARC2-KO mice. Residual detected reductase activity in the MARC2-KO mice could be explained by mARC1 expression that was hardly unaffected by the KO, and we found no evidence for significant activities of other reductase enzymes. These results clearly indicate that mARC2 is mainly responsible for Nreductive biotransformation in mice. Striking phenotypical features of the MARC2-KO mice were a lower body weight, increased body temperature, decreased levels of total cholesterol, and increased glucose levels, supporting previous findings that mARC2 affects energy pathways. Of note, the MARC2-KO mice were resistant to HFD-induced obesity. We propose that the MARC2-KO mouse model could be a powerful tool for predicting mARC-mediated drug metabolism and further investigating mARC's roles in energy homeostasis.

S. Rixen, A. Havemeyer, A. Tyl-Bielicka, K. Pysniak, M. Gajewska, M. Kulecka, J. Ostrowski, M. Mikula, and B. Clement,Mitochondrial amidoxime-reducing component 2 (mARC2) has a significant role in N-reductive activity and energy metabolism, The Journal of biological chemistry, 2019.

Mitochondrial amidoxime reducing component

The "mitochondrial amidoxime reducing component" (mARC) is the most recently discovered molybdenum-containing enzyme in mammals. All mammalian genomes studied to date contain two mARC genes: MARC1 and MARC2. The proteins encoded by these genes are mARC-1 and mARC-2 and represent the simplest form of eukaryotic molybdenum enzymes, only binding the molybdenum cofactor. In the presence of NADH, mARC proteins exert N-reductive activity together with the two electron transport proteins cytochrome b5 type B and NADH cytochrome b5 reductase. This enzyme system is capable of reducing a great variety of N-hydroxylated substrates. It plays a decisive role in the activation of prodrugs containing an amidoxime structure, and in detoxification pathways, e.g., of N-hydroxylated purine and pyrimidine bases. It belongs to a group of drug metabolism enzymes, in particular as a counterpart of P450 formed N-oxygenated metabolites. Its physiological relevance, on the other hand, is largely unknown. The aim of this article is to summarize our current knowledge of these proteins with a special focus on the mammalian enzymes and their N-reductive activity.

Ott G(1), Havemeyer A, Clement B.The mammalian molybdenum enzymes of mARC. J Biol Inorg Chem. 2015 Mar;20(2):265-75. doi: 10.1007/s00775-014-1216-4. Epub 2014 Nov 26

Mitochondrial Amidoxime Reducing Component mARC

Detoxification of Trimethylamine N-Oxide by the Mitochondrial Amidoxime Reducing Component mARC

Although known for years, the toxic effects of trimethylamine N-oxide (TMAO), a physiological metabolite, were just recently discovered and are currently under investigation. It is known that elevated TMAO plasma levels correlate with an elevated risk for cardiovascular disease (CVD). Even though there is a general consensus about the existence of a causal relationship between TMAO and CVD, the underlying mechanisms are not fully understood. TMAO is an oxidation product of the hepatic flavin-containing monooxygenases (FMO), mainly of isoform 3, and it is conceivable that humans also have an enzyme reversing this toxification by reducing TMAO to its precursor trimethylamine (TMA). All prokaryotic enzymes that use TMAO as a substrate have molybdenum-containing cofactors in common. Such molybdenum-containing enzymes also exist in mammals, with the so-called mitochondrial amidoxime reducing component (mARC) representing the most recently discovered mammalian molybdenum enzyme. The enzyme has been found to exist in two isoforms, mARC1 and mARC2, both being capable of reducing a variety of N-oxygenated compounds, including nonphysiological N-oxides. To investigate whether the two isoforms of this enzyme are able to reduce and detoxify TMAO, we developed a suitable analytical method and tested TMAO reduction with a recombinant enzyme system. We found that one of the two recombinant human mARC proteins, namely, hmARC1, reduces TMAO to TMA. The N-reductive activity is relatively low and identified via the kinetic parameters with Km = (30.4 +/- 9.8) mM and Vmax = (100.5 +/- 12.2) nmol/(mg protein.min). Nevertheless, the ubiquitous tissue expression of hmARC1 allows a continuous reduction of TMAO whereas the counter-reaction, the production of TMAO through FMO3, can take place only in the liver where FMO3 is expressed. TMAO reduction in porcine liver subfractions showed the characteristic enrichment of N-reductive activity in the outer mitochondrial membrane. TMAO reduction was also found in human cell cultures. These findings indicate the role of hmARC1 in the metabolomic pathway of TMAO, which might contribute to the prevention of CVD. This also hints at a physiological function of the molybdenum enzyme, which remains mainly unknown to date.

J. Schneider, U. Girreser, A. Havemeyer, F. Bittner, and B. Clement,Detoxification of Trimethylamine N-Oxide by the Mitochondrial Amidoxime Reducing Component mARC, Chemical research in toxicology, 2018, 31, 447-453.