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.

Background: N-Reduction is catalyzed by a molybdenum-dependent three-component enzyme system. Results: Essential components include mitochondrial but not microsomal cytochrome b 5 , and the mitochondrial amidoxime reducing components 1/2. Conclusion: CYB5 heme is required for activity, and contribution of a particular mARC isoform to N-reduction is dependent on its expression level. Significance: These findings contribute to the understanding of N-reductive pathway in detoxication and drug metabolism.

Emotion discrimination deficits represent a well-established finding in schizophrenia. Although imaging studies addressed the cerebral dysfunctions underlying emotion perception in adult patients, the question of trait vs state characteristics is still unresolved. The investigation of juvenile patients offers the advantage of studying schizophrenia at an age where influences of illness course and long-term medication are minimized. This may enable a more detailed characterization of emotion discrimination impairments and their cerebral correlates with respect to their appearance and exact nature. A total of 12 juvenile patients with early onset schizophrenia and matched healthy juveniles participated in this study. fMRI data were acquired during an emotion discrimination task consisting of standardized photographs of faces displaying happy, sad, angry, fearful, or neutral facial expression. Similar to findings in adult patients, juvenile patients exhibited reduced performance specificity whereas sensitivity was unaffected. Independent of the valence, their processing of emotional faces was associated with hypoactivations in both fusiform gyri and in the left inferior occipital gyrus. In addition, hyperactivations in patients were found in the right cuneus common to happy, angry, and fearful faces. Further, most distinct changes were present in juvenile patients when processing sad faces. These results point to a dysfunction in cerebral circuits relevant for emotion processing already prominent in adolescent schizophrenia patients. Regions affected by a decrease in activation are related to visual and face processing, similar to deficits reported in adult patients. These changes are accompanied by hyperactivations in areas related to emotion regulation and attribution, possibly reflecting compensatory mechanisms.

Human molybdenum-containing enzyme mitochondrial amidoxime reducing component (mARC), cytochrome b 5 type B, and NADH cytochrome b 5 reductase form an N-reductive enzyme system that is capable of reducing N-hydroxylated compounds. Genetic variations are known, but their functional relevance is unclear. Our study aimed to investigate the incidence of nonsynonymous single nucleotide polymorphisms (SNPs) in the mARC genes in healthy Caucasian volunteers, to determine saturation of the protein variants with molybdenum cofactor (Moco), and to characterize the kinetic behavior of the protein variants by in vitro biotransformation studies. Genotype frequencies of six SNPs in the mARC genes (c.493A>G, c.560T>A, c.736T>A, and c.739G>C in MARC1; c.730G>A and c.735T>G in MARC2) were determined by pyrosequencing in a cohort of 340 healthy Caucasians. Protein variants were expressed in Escherichia coli. Saturation with Moco was determined by measurement of molybdenum by inductively coupled mass spectrometry. Steady state assays were performed with benzamidoxime. The six variants were of low frequency in this Caucasian population. Only one homozygous variant (c.493A; MARC1) was detected. All protein variants were able to bind Moco. Steady state assays showed statistically significant decreases of catalytic efficiency values for the mARC-2 wild type compared with the mARC-1 wild type (P < 0.05) and for two mARC-2 variants compared with the mARC-2 wild type (G244S, P < 0.05; C245W, P < 0.05). After simultaneous substitution of more than two amino acids in the mARC-1 protein, N-reductive activity was decreased 5-fold. One homozygous variant of MARC1 was detected in our sample. The encoded protein variant (A165T) showed no different kinetic parameters in the N-reduction of benzamidoxime.