Decolorization of cationic dyes under alkaline conditions by Iodidimonas sp. Q-1 multicopper oxidase.

Previously, we found that a multicopper oxidase (IOX) produced by Iodidimonas sp. Q-1, an iodide (I-)-oxidizing marine bacterium, exhibited significant decolorization activity toward various anionic dyes. In this study, the potential capacity of IOX for decolorization of cationic dyes such as malachite green (MG), crystal violet (CV), and methylene blue (MB) was determined. Decolorization of the dyes by IOX exhibited significant pH dependence, and effective decolorization was observed under alkaline conditions. At an optimum pH of 9.5, IOX decolorized more than 90% of MG, CV, and MB within 30 min, 2 h, and 6 h, respectively. The addition of iodide was indispensable for decolorization, suggesting that this halide ion serves as a redox mediator. Decolorization products of MG showed less toxicity than MG against Escherichia coli cells. These results suggest that this IOX-iodide system can be used for the decolorization and detoxification of cationic dyes under alkaline conditions.

Identification and characterization of proteinase B as an unstable factor for neutral lactase in the enzyme preparation from Kluyveromyces lactis.

The stability of the commercial lactase enzyme is important for the dairy industry. A destabilizing factor for neutral lactase in the enzyme preparation from Kluyveromyces lactis was investigated. We found that lactase had lower thermal stability when fragmented bands of lactase were confirmed on SDS-PAGE. After the destabilizing factor of lactase was purified, that was identified by BLAST search as a hypothetical protein in K. lactis similar to proteinase B (PRB) of Saccharomyces cerevisiae. The molecular mass of protease was estimated to be approximately 30 kDa with SDS-PAGE. The purified protease exhibited activity toward lactase and FITC-casein but not toward bovine serum albumin or milk casein. The optimal pH and temperature of the protease were 8.0 and 40 °C, respectively. The protease activity was strongly inhibited by Fe2+, Cu2+, and a serine protease inhibitor, but activated by Ca2+. Based on these properties, the protease was identified as PRB. Lactase fragmentation was accelerated by the addition of purified PRB to the lactase preparation and was suppressed by protease inhibitors. Thus, this is the first report to identify and characterize PRB as the unstable factor of neutral lactase in the K. lactis preparation.

Decolorization of recalcitrant dyes by a multicopper oxidase produced by Iodidimonas sp. Q-1 with iodide as a novel inorganic natural redox mediator.

A multicopper oxidase (IOX) produced by Iodidimonas sp. Q-1 has high catalytic efficiency for iodide (I-) oxidation to form molecular iodine (I2). In this study, the potential capacity of IOX for decolorization of recalcitrant dyes was determined. Although IOX did not decolorize any dyes in the absence of redox mediator, significant decolorization of Orange G, Indigo Carmine, Amido Black, and Remazol Brilliant Blue R (RBBR) was observed in the presence of iodide. Addition of 0.1 mM iodide was sufficient to decolorize a total of 3 mM Indigo Carmine, suggesting that iodide functions as a mediator. Such mediator-like function of iodide was not observed in commercially available fungal laccases. The IOX-iodide decolorization system showed much alkaline pH optima of 5.5-6.5 and stronger salt tolerance than fungal laccases did. In addition, actual wastewater discharged from a dyeing factory could be decolorized more than 50% by the system. Since iodide is naturally occurring, non-toxic, and cheaper than common synthetic mediators, the IOX-iodide system is potentially more advantageous than fungal laccase-mediator systems for decolorization of recalcitrant dyes.