However, it is also possible that the detected signal is not related to mARC1 but stems from an unspecific reaction of the antibody. The protein levels of the protein assumed to be mARC1 are increased with fasting but decreased after 24 h of fasting, indicating a fast breakdown of the protein with prolonged fasting time. Thus protein turnover is different for the three components of the N-reductive system. This phenomenon is as well described for another three component system composed of cytochrome b5 and its reductase together with stearyl-CoA desaturase. The latter undergoes more rapid turnover than the other two members of the system for regulatory reasons. The same LY2835219 mechanism could also be assumable for mARC-containing enzyme system. Reasons or mechanisms for the initial increase of the assumed mARC1 protein due to fasting followed by its decrease with prolonged fasting time are not yet understood, as well as the interplay of mARC1 and mARC2 during fasting. Surprisingly, the abundances of mARC2 and mARC1 mRNA and protein levels in mice show opposite tendencies due to 24 h of fasting. This may indicate a posttranscriptional or posttranslational regulation that is so far unknown. Discrepancies and even opposite expression patterns for mRNA and protein levels have already been described for other proteins. These findings indicate that further studies on mARCs must not solely be based on RNA data but also on protein levels to be reliable. Nevertheless, mARC-proteins abundance and complex activity levels after 24 h of fasting match regarding tendencies. With respect to the 18 h fasting period we assume that both reduced levels of CYB5B and CYB5R are responsible for decreased N-reductive activity, regardless of the stable levels for mARC2 and increased levels for the assumed mARC1. CYB5B has been proven to be essential for N-reductive activity in vitro enzyme assay and cell culture,, as well as CYB5R in vitro, but only faint amounts of this protein are necessary. The latter might be the reason why others could not prove an involvement of CYB5R in N-reduction in cell culture. In case of the 24 h fasting period, all three components of the enzyme system show decreased abundance in fasted mice compared to control and are thus likely to cause a decline in N-reductive activity. Concerning the two isoforms of the mARC protein, mARC1 and mARC2, our group already hypothesized that the isoform mainly involved in the N-reductive activity varies from species to species. For pig and rat, mARC2, but not mARC1 was found in the outer mitochondrial membrane, the site where the mARC protein is mainly localized. In contrast, human OMM was shown to contain mARC1. In consistence with this, we demonstrated that changes in mARC2 in murine cell line Hepa 1.6 and changes in mARC1 in human cell line HepG2 result in altered N-reductive activity, whereas the corresponding second isoform was below detection limit in both cell lines. Nevertheless, in murine liver tissues, both isoforms seem to be involved in altered N-reductive activities. No differences in metabolite concentrations in liver homogenates and plasma after benzamidoxime administration could be found between fasted and non-fasted mice.