ABSTRACT
Under high dose treatment with sulfamethoxazole (SMX)/trimethoprim (TMP), hypersensitivity reactions occur with a high incidence. The mechanism of this adverse drug reaction is not fully understood. Several steps in the toxification pathway of SMX were investigated. The aim of our study was to investigate the reduction of sulfamethoxazole hydroxylamine (SMX-HA) in this toxification pathway, which can possibly be catalyzed by the mARC-containing N-reductive enzyme system. Western blot analyses of subcellular fractions of porcine tissue were performed with antibodies against mARC-1, mARC-2, cytochrome b5 type B, and NADH cytochrome b5 reductase. Incubations of porcine and human subcellular tissue fractions and of the heterologously expressed human components of the N-reductive enzyme system were carried out with SMX-HA. mARC-1 and mARC-2 knockdown was performed in HEK-293 cells. Kinetic parameters of the heterologously expressed human protein variants V96L, A165T, M187 K, C246S, D247H, and M268I of mARC-1 and G244S and C245W of mARC-2 and N-reductive activity of 2SF, D14G, K16E, and T22A of cytochrome b5 type B were analyzed. Western blot analyses were consistent with the hypothesis that the mARC-containing N-reductive enzyme system might be involved in the reduction of SMX-HA. In agreement with these results, highest reduction rates were found in mitochondrial subcellular fractions of porcine tissue and in the outer membrane vesicle (OMV) of human liver tissue. Knockdown studies in HEK-293 cells demonstrated that mARC-1 and mARC-2 were capable of reducing SMX-HA in cell metabolism. Investigations with the heterologously expressed human mARC-2 protein showed a higher catalytic efficiency toward SMX-HA than mARC-1, but none of the investigated human protein variants showed statistically significant differences of its N-reductive activity and was therefore likely to participate in the pathogenesis of hypersensitivity reaction under treatment with SMX.
Subject(s)
Mitochondria/metabolism , Sulfamethoxazole/analogs & derivatives , Amino Acid Substitution , Animals , Biocatalysis , Chromatography, High Pressure Liquid , Cytochrome-B(5) Reductase/metabolism , Cytochromes b5/genetics , Cytochromes b5/metabolism , HEK293 Cells , Humans , Kinetics , Liver/metabolism , Mitochondrial Proteins/antagonists & inhibitors , Mitochondrial Proteins/genetics , Mitochondrial Proteins/metabolism , Oxidation-Reduction , Oxidoreductases/antagonists & inhibitors , Oxidoreductases/genetics , Oxidoreductases/metabolism , RNA Interference , RNA, Small Interfering/metabolism , Recombinant Proteins/biosynthesis , Recombinant Proteins/chemistry , Recombinant Proteins/genetics , Sulfamethoxazole/chemistry , Sulfamethoxazole/metabolism , SwineABSTRACT
The "mitochondrial Amidoxime Reducing Component" (mARC) is the newly discovered fourth molybdenum enzyme in mammals. All hitherto analyzed mammals express two mARC proteins, referred to as mARC1 and mARC2. Together with their electron transport proteins cytochrome b(5) and NADH cytochrome b(5) reductase, they form a three-component enzyme system and catalyze the reduction of N-hydroxylated prodrugs. Here, we demonstrate the reductive detoxification of toxic and mutagenic N-hydroxylated nucleobases and their corresponding nucleosides by the mammalian mARC-containing enzyme system. The N-reductive activity was found in all tested tissues with the highest detectable conversion rates in liver, kidney, thyroid, and pancreas. According to the presumed localization, the N-reductive activity is most pronounced in enriched mitochondrial fractions. In vitro assays with the respective recombinant three-component enzyme system show that both mARC isoforms are able to reduce N-hydroxylated base analogues, with mARC1 representing the more efficient isoform. On the basis of the high specific activities with N-hydroxylated base analogues relative to other N-hydroxylated substrates, our data suggest that mARC proteins might be involved in protecting cellular DNA from misincorporation of toxic N-hydroxylated base analogues during replication by converting them to the correct purine or pyrimidine bases, respectively.
