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สารฮิ วมิ กช่วยเพิ่ มความสามารถการลดสี ... 208 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 Without the addition of any redox mediators (FAD nor AQDS), the specific decolorization rates of orange, violet and black dye of whole cells were significantly lower than the specific decolorization rates of the same dyes determined with cell extracts (Table 2). This suggested that either the cell membrane limited the uptake of highly polar sulfonated azo dyes or the lack of some cofactors (e.g., free flavins) limited the reduction of azo dyes by whole cells. The addition of FAD (0.05 mM) to whole cell suspensions and the cell membrane fraction of Paenibacillus sp. strain A5 did not significantly increase the azo reductase activities of whole cells and slightly increased the azo reducase activities present in membrane fraction. In contrast, it was observed that the addition of the same concentration of FAD in the cytoplasmic fraction of strain A5 resulting in dramatically increased in the specific reduction rates of all azo dyes used. On the other hand, the addition of the same concentration of AQDS, a quinoid redox mediator, was significantly enhanced azo reductase activities of whole cells and greatly stimulated the azo reductase activities present in membrane fraction of strain A5. In cyto plasmic fraction, however, this externally added quinoid mediator had rather lower stimulating effect on azo dyes reduction than the addition of FAD (Table 2). Thus the membrane-bound and cytoplasmic azo reductases of strain A5 are prob- ably different enzyme systems which the latter may have insignificant importance in the reduction of sulfonated azo dyes in vivo. Quinone reductase and flavin reductase activity in membrane and cytoplasmic fraction of strain A5 In cytoplasmic membranes of almost all prokaryotes, it has been shown that the reduction of soluble quinones is catalyzed by the membrane-bound respiratory NADH:quinone reductase (4, 5, 32, 58). Furthermore, it was previously suggested that in Sphingomonas xenophaga BN6 the (membrane-bound) NADH:quinone oxidoreductase of respiratory chain is responsible for the reduction of anthraquinone-2-sulfonate (and thus the azo reductase activity) (29). In the other hand, in earlier studies with facultatively anaerobic bacteria, it was repeatedly suggested that the reduced flavins generated