Growth characteristics and metabolic flux analysis of Candida milleri

The growth characteristics of the sourdough yeast Candida milleri was studi ed in a carbon-limited aerobic chemostat culture on defined medium. The eff ect of glucose, xylose, and glucose-xylose mixture on metabolite production and on key enzyme activities was evaluated. Xylose as a sole carbon source was not metabolized by C. milleri. Glucose as a sole carbon source produce d only biomass and carbon dioxide. When a glucose-xylose mixture (125:125 C -mM) was used as a carbon source, a small amount of xylose was consumed and a low concentration of xylitol was produced (7.20 C-mM). Enzymatic assays indicated that C. milleri does not possess xylitol dehydrogenase activity a nd its xylose reductase is exclusively NADPH-dependent. In glucose medium b oth NAD(+)- and NADP(+)-dependent aldehyde dehydrogenase activities were fo und, whereas in a glucose-xylose medium only NADP(+)-dependent aldehyde deh ydrogenase activity was detected. The developed metabolic flux analysis cor responded well with the experimentally measured values of metabolite produc tion, oxygen consumption (OUR), and carbon dioxide production (CER). Turnov er number in generation and consumption of ATP, mitochondrial and cytosolic NADH, and cytosolic NADPH could be calculated and redox balance was achiev ed. Constraints were imposed on the flux estimates such that the directiona lity of irreversible reactions is not violated, and cofactor dependence of the measured enzyme activities were taken into account in constructing the metabolic flux network. (C) 2000 John Wiley & Sons, Inc.