Partial purification and characterization of sulfhydryl oxidase from Aspergillus niger

Sulfhydryl oxidase (SOX) was purified after extraction and the contaminating catalase activity was completely eliminated in the last chromatography step. A yield of 25% was obtained with a purification factor higher than 300. The isoelectric point was 3.7 and the molecular weight 110 kDa. SOX exhibited an optimal activity at pH 5.6 and its efficiency (V-mapp/K-mapp) increased from pH 4.5 to 6.5. At pH 5.6, the K-mapp values were 0.5, 2.5, 10.5, 110, and 450 mM for GSH, cysteine, g-glu-cys, dithiothreitol, and homocysteine, respectively, and the V values represented 2, 34, 24, and 44% of the V-mapp value found for GSH, respectively. Cys-gly was not oxidize by SOX. In the presence of GSH, SOX is able to catalyze the oxidation of cysteine and cys-gly at a significant rate.