A flow-injection analysis (FIA) for superoxide dismutase (SOD) activity was developed based on the use of tetrazolium salt, WST-1 (4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate sodium salt) and an enzyme reactor packed with Sepharose 4B on which xanthine oxidase (XO) and catalase were co-immobilized. WST-1 is highly water-soluble, and no adhesion of the reduced form to the FIA line was observed during continuous operation for 3 months. As optimized conditions, a sample (9 vol) was mixed with a reagent solution (1 vol) containing 3 mM hypoxanthine and 2 mM WST-1, and the mixed solution (20 µl) was injected into a carrier stream of 50 mM carbonate buffer (pH 10.2) at a flow rate of 0.4 ml/min. Under the conditions, the concentration of the SOD preparation giving 50% inhibition (IC50) was 2.7 µg/ml and the sampling frequency was 30 samples/h. When the SOD activity in erythrocytes of rats was determined by the present FIA method, the values were linearly related to those obtained by the conventional nitroblue tetrazolium (NBT) assay (r =0.975; n =18). The enzyme reactor was stable for at least 200 repetitive injections.
A trinuclear cobalt(III) complex, [Co3(l-his)3(μ-OH)3(μ3-O)]ClO4·H2O (l-Hhis: l-histidine), containing an incomplete cubane Co3O4 core was isolated and the structure was compared with [Co3(edma)3(μ-OH)3(μ3-O)]+ (Hedma: NH2CH2CH2NHCH2COOH) isomers. Using the edma and l-his as N–N–O type ligands, [Mo3(L)3(μ-S)3(μ3-S)]+ complexes with a Mo3S4 incomplete cubane core were also prepared, and their structures were determined by the X-ray diffraction method. A structural similarity between [Mo3(L)3(μ-S)3(μ3-S)]+ and [Co3(L)3(μ-OH)3(μ3-O)]+ is discussed; it is suggested that three N–H···O inter-ligand (intra-molecular) attractive interaction has an important role in stabilizing the incomplete cubane structure.
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