EFFECT OF OXYGEN-TRANSFER RATE ON LEVELS OF KEY ENZYMES OF XYLOSE METABOLISM IN DEBARYOMYCES-HANSENII

The titers of key enzymes of xylose metabolism were measured and correlated with the kinetics of xylitol production by Debaryomyces hansenii under different oxygen transfer rates (OTR) in a batch reactor. An OTR change from 2.72 to 4.22 mmol O-2 l(-1) min(-1) resulted in a decrease in NADPH-dependent xylose reductase (XR) and NAD(+)-dependent xylitol dehydrogenase (XDH) activities. For higher values of OTR (12.93 mmol O-2 l(-1) min(-1)), the XDH titer increased twofold whereas the XR titer did not show a significant change. At the lowest OTR (2.72 mmol O-2 l(-1) min(-1)), xylitol (and ethanol) production rates showed the highest values. However, xylitol specific productivity was twice as high as ethanol specific productivity. The titer of the NADPH-forming enzyme, glucose-6-phosphate dehydrogenase (GPDH), increased from 333 to 412 mU mg(-1) when the OTR was increased. However, 6-phosphogluconate dehydrogenase (PGDH) activity remained unchanged and at a lower level, which indicates that this enzyme is responsible for the carbon flux control of the oxidative branch of the pentose phosphate pathway. The activity of the alcohol-forming enzyme was repressed at the higher amount of oxygen, decreasing its activity more than 50%. The changes in ADH suggested that two different metabolic regions under oxygen-limited conditions can be hypothesized for xylose metabolism by D. hansenii. For low OTR values (up to 4.22 mmol O-2 l(-1) min(-1)) a fermentative-type activity is displayed At higher OTR values (above 4.22 mmol O-2 l(-1) min(-1)), no significant fermentative activity is reported.