P-31 AND C-13 NUCLEAR-MAGNETIC-RESONANCE STUDIES OF GLUCOSE AND XYLOSE METABOLISM IN CELL-SUSPENSIONS AND AGAROSE-IMMOBILIZED CULTURES OF PICHIA-STIPITIS AND SACCHAROMYCES-CEREVISIAE

The metabolism of glucose and xylose as a function of oxygenation in P ichia stipitis and Saccharomyces cerevisiae cell suspensions was studi ed by P-31 and C-13 nuclear magnetic resonance spectroscopy. The rate of both glucose and xylose metabolism was slightly higher and the prod uction of ethanol was slightly lower in aerobic than in anoxic cell su spensions of P. stipitis. As well, the cytoplasmic pH of oxygenated ce lls was more alkaline than that of nonoxygenated cells. In contrast, i n S. cerevisiae, the intracellular pH and the rate of glucose metaboli sm and ethanol production were the same under aerobic and anoxic condi tions. Agarose-immobilized Pichia stipitis was able to metabolize xylo se or glucose for 24 to 60 h at rates and with theoretical yields of e thanol similar to those obtained with anoxic cell suspensions, Cell gr owth within the beads, however, was severely compromised. The intracel lular pH [pH((int))] of the entrapped cells fell to more acidic pH val ues in the course of the perfusions relative to corresponding cell sus pensions, Of importance was tile observation that no enhancement in th e rate of carbohydrate metabolism occurred in response to changes in t he PH(int) value. In contrast to P. stipitis, agarose-immobilized Sacc haromyces cerevisiae showed a dramatic twofold increase in its ability to metabolize glucose in the immobilized state relative to cell suspe nsions. This strain was also able to grow within the beads, although t he doubling time for the entrapped cells was longer, by a factor of 2, than the value obtained for log-phase batch cultures. Initially, the pH((int)) of the immobilized cells was more alkaline than was observed with the corresponding S. cerevisiae cell suspensions; however, over time, the intracellular pH became increasingly acidic. As with immobil ized P. stipitis, however, the pH((int)) did not play a key role in co ntrolling the rate of glucose metabolism.