title: The involvement of mitochondrial amidoxime reducing components 1 and 2 and mitochondrial cytochrome b5 in N-reductive metabolism in human cells.
abstract: The mitochondrial amidoxime reducing component mARC is a recently discovered molybdenum enzyme in mammals. mARC is not active as a standalone protein, but together with the electron transport proteins NADH-cytochrome b5 reductase (CYB5R) and cytochrome b5 (CYB5), it catalyzes the reduction of N-hydroxylated compounds such as amidoximes. The mARC-containing enzyme system is therefore considered to be responsible for the activation of amidoxime prodrugs. All hitherto analyzed mammalian genomes code for two mARC genes (also referred to as MOSC1 and MOSC2), which share high sequence similarities. By RNAi experiments in two different human cell lines, we demonstrate for the first time that both mARC proteins are capable of reducing N-hydroxylated substrates in cell metabolism. The extent of involvement is highly dependent on the expression level of the particular mARC protein. Furthermore, the mitochondrial isoform of CYB5 (CYB5B) is clearly identified as an essential component of the mARC-containing N-reductase system in human cells. The participation of the microsomal isoform (CYB5A) in N-reduction could be excluded by siRNA-mediated down-regulation in HEK-293 cells and knock-out in mice. Using heme-free apo-CYB5, the contribution of mitochondrial CYB5 to N-reductive catalysis was proven to strictly depend on heme. Finally, we created recombinant CYB5B variants corresponding to four nonsynonymous single nucleotide polymorphisms (SNPs). Investigated mutations of the heme protein seemed to have no significant impact on N-reductive activity of the reconstituted enzyme system.
authors.affiliation: Department of Pharmaceutical and Medicinal Chemistry, Pharmaceutical Institute, Christian-Albrechts-University of Kiel, 24118 Kiel, Germany.
authors.forename: Birte
authors.initials: B
authors.lastname: Plitzko
authors.forename: Gudrun
authors.initials: G
authors.lastname: Ott
authors.forename: Debora
authors.initials: D
authors.lastname: Reichmann
authors.forename: Colin J
authors.initials: CJ
authors.lastname: Henderson
authors.forename: C Roland
authors.initials: CR
authors.lastname: Wolf
authors.forename: Ralf
authors.initials: R
authors.lastname: Mendel
authors.forename: Florian
authors.initials: F
authors.lastname: Bittner
authors.forename: Bernd
authors.initials: B
authors.lastname: Clement
authors.forename: Antje
authors.initials: A
authors.lastname: Havemeyer
mesh_terms: D000818:Animals; D015153:Blotting, Western; D045744:Cell Line, Tumor; D042966:Cytochrome-B(5) Reductase / Q000235:genetics / Q000378:metabolism; D015786:Cytochromes b5 / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism*; D008982:Molybdenum / Q000378:metabolism; D009154:Mutation; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D010091:Oximes / Q000378:metabolism*; D020641:Polymorphism, Single Nucleotide; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D034622:RNA Interference
chemical_list: D008565:Membrane Proteins; D024101:Mitochondrial Proteins; D010091:Oximes; D020033:Protein Isoforms; D008982:Molybdenum; D015786:Cytochromes b5; C070491:Amidox; C570001:MTARC2 protein, human; D010088:Oxidoreductases; C534493:amidoxime reducing component, human; D042966:Cytochrome-B(5) Reductase
keywords: CYB5B; Drug Metabolism; Heme; MOSC; Mitochondria; Molybdenum; N-reduction; RNA Interference (RNAi); mARC