Enzymatic and physiological study of d-xylose metabolism by Candida shehatae

Summary Candida shehatae carbon metabolic pathways were correlated with fermentative activity under different growth conditions. Reduced nicotine adenine dinucleotide (NADPH) is the coenzyme preferred for xylose reductase by C. shehatae under in vitro anoxic cell culture conditions. To prevent a redox imbalance derived from intracellular accumulation of NADH in the second enzymatic step of xylose metabolism, the operation of phosphoketolase via in addition the classic pentose phosphate pathway essential for NADH dissimilation is suggested. Variation in cultivation conditions showed a different NADH/NADPH ratio coupled to xylose reductase activity. The existence of two xylose reductases is discussed. Like ethanol, xylitol accumulates only under oxygen-limited or anaerobic conditions. Xylitol accumulaiton under unaerobic conditions was higher when using respiring cells than respirofermentative cells. This fact suggests that cells pregrown under oxygen limitation are better adapted to starting alcoholic fermentation than cells previously grown under aerobic conditions.Offprint requests to: M. T. Amaral-Collaço