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Metal adsorption of the acidic polysaccharide produced from lVlethylobacterium organophilum was investigated. The polysaccharide non-specifically adsorbed 21% for copper and 18% for lead of the polysacclmride used after 30 minutes of reaction time at pH 7. The metal adsorption may be caused by the interaction between metal cations and negative charges of acidic residues of the polysaceharide such as uronic, pyruvic and acetic acid. I N T R O D U C T I O N Microbial polysaecharides show a wide variety of chemical compositions and their physical and biochemicalproperties are now understood (Bikales, 1973). Polysaccharides are commercially used to produce gels, to thicken and stabilize foods, medicines and industrial products. The production of microbial exopolysaccharides provides a valid alternative, either through the development of products with properties almost identical to currently used polysaccharides, or materials with better rheological characteristics which can be used for new applications.Microbial polysaccharides also assist in its removal and accumulation of metal cations from aqueous solutions (Norberger and Rydin, 1984). Microorganisms including bacteria, yeast and algae have also been used as adsorbing agents for heavy metals. Interaction between metal ions and microbial cells(or exopolysaccharides) can occur through the adsorption of metal ions to cell surface or to extracellular microbial polysaecharides and the adsorbed metal ions were accumulated within the cells or removed as metallic complexes with the polysaecharides (Norberger and Rydin, 1984). Polysaccharides especially can be recommended as good adsorbing agents for heavy metals because, for examples, bacterial extracellular polysaccharides in activated sludge maintain an extensive complexing capacity for heavy metals (Norberger and Enfors, 1982) and acidic polysaccharides can function as polyelectrolytes and thus form the basis for metal adsorption (Kurita et al., 1979).The main purpose of this study was to investigate metal adsorption of the polysaccharide produced from Afethylobacterium organophilum (Choi et al., 1991 ). 1161

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The yeast Candida tropicalis produces xylitol, a natural, low-calorie sweetener whose metabolism does not require insulin, by catalytic activity of NADPH-dependent xylose reductase. The oxidative pentose phosphate pathway (PPP) is a major basis for NADPH biosynthesis in C. tropicalis. In order to increase xylitol production rate, xylitol dehydrogenase gene (XYL2)disrupted C. tropicalis strain BSXDH-3 was engineered to co-express zwf and gnd genes which, respectively encodes glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6-PGDH), under the control of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) promoter. NADPH-dependent xylitol production was higher in the engineered strain, termed "PP", than in BSXDH-3. In fermentation experiments using glycerol as a co-substrate with xylose, strain PP showed volumetric xylitol productivity of 1.25 g l(-1) h(-1), 21% higher than the rate (1.04 g l(-1) h(-1)) in BSXDH-3. This is the first report of increased metabolic flux toward PPP in C. tropicalis for NADPH regeneration and enhanced xylitol production.

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