สำนักราชบัณฑิตยสภา

213 สมศั กดิ์ ด� ำรงค์เลิ ศ และคณะ วารสารราชบัณฑิตยสถาน ปีที่ ๓๗ ฉบับที่ ๑ ม.ค.-มี.ค. ๒๕๕๕ azo dyes though the cell membrane, are presumably responsible for the unspecific reduction of various sulfonated azo dyes by Paenibacillus sp. strain A5. Several quinoide redox mediator compounds, for example; anthaquinone-2-sulfonate (AQS), 2-hydroxy-1,4-naphthoquinone (lawsone), 4-amino-1,2-naphthoquinone and AQDS, have been shown to enhance degradation of sulfonated azo dyes by acting as electron shuttles that facilitate reduction of the azo dye (26, 27, 29, 30, 42, 43). In this article we report that external added AQDS not only stimulate quinone- dependent azo reductase activity in detergent-soluble membrane fractions but also enhance an- aerobic reduction of intact strain A5 cells. In addition, it could be demonstrated in the cell-frac- tioning experiments that the NADH: quinone oxidoreductase activity was almost restrictively present in the membrane fraction (Table 3). These results suggest the existence of an NADH-de- pendent quinone reductase in Paenibacillus sp. strain A5 membranes that catalyze the reduction of endogenous quinones (e.g., menaquinone), may responsible for the reduction of exogenous quinones (e.g., AQDS) which then transfer reduction equivalents to sulfonated azo dyes outside the cells. The anaerobic decolorization occurred only in the presence of cells, indicting that the cells reduced the AQDS to the corresponding hydroquinone (AH 2 QDS). This reaction may be catalyzed anaerobically by NADH-quinone oxidoreductase (NDH) of the respiratory chain, which appeared to have a low substrate specificity of the quinone-binding site in several bacterial genera (18, 29, 42, 43, 50, 57). NDH play their most important role as a primary dehydrogenase, linked with the central metabolism, in the respiratory chain of all organisms having an aerobic or anaerobic electron-transport system. Several types of NDH occur in bacteria, the most common of which are referred to as NDH-1 and NDH-2 (16). NDH-1 is an energy-transducing enzyme meanwhile NDH-2 appears to have no role in energy transduction (16). Some gram-positive bacteria such as Bacillus subtilis have non- energy generating NDH II but not NDH I (3). In addition, it has recently suggested that oxygen-insensitive nitroreductase (NfsA and NfsB) are capable of effectively reducing not only nitro compounds but also quinones, which may not be natural substrates (60). The nitroreductase, and other enzymes in different families, which had sequence homologies to a certain group of NfsA