The 58 kDa mouse selenoprotein is a BCNU‐sensitive thioredoxin reductase

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Abstract
The flavoprotein thioredoxin reductase [EC 1.6.4.5] (NADPH+H++thioredoxin‐S2→NADP++thioredoxin‐(SH)2) was isolated from mouse Ehrlich ascites tumour (EAT) cells. Like the counterpart from human placenta but unlike the known thioredoxin reductases from non‐vertebrate organisms, the mouse enzyme was found to contain 1 equivalent of selenium per subunit of 58 kDa. The K M values were 4.5 μM for NADPH, 480 μM for DTNB and 36 μM for Escherichia coli thioredoxin, the turnover number with DTNB being ≈40 s−1. As mouse is a standard animal model in cancer and malaria research, thioredoxin reductase and glutathione reductase [EC 1.6.4.2] from EAT cells were compared with each other. While both enzymes in their 2‐electron reduced form are targets of the cytostatic drug carmustine (BCNU), no immunologic cross‐reactivity between the two mouse disulfide reductases was observed.