Phosphorus-31 and carbon-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 Pichia stipitis and Saccharomyces cerevisiae cell suspensions was studied by 31P and 13C nuclear magnetic resonance spectroscopy. The rate of both glucose and xylose metabolism was slightly higher and the production of ethanol was slightly lower in aerobic than in anoxic cell suspensions of P. stipitis. As well, the cytoplasmic pH of oxygenated cells was more alkaline than that of nonoxygenated cells. In contrast, in S. cerevisiae, the intracellular pH and the rate of glucose metabolism and ethanol production were the same under aerobic and anoxic conditions. Agarose-immobilized Pichia stipitis was able to metabolize xylose or glucose for 24 to 60 h at rates and with theoretical yields of ethanol similar to those obtained with anoxic cell suspensions. Cell growth within the beads, however, was severely compromised. The intracellular pH [pH(int)] of the entrapped cells fell to more acidic pH values in the course of the perfusions relative to corresponding cell suspensions. Of importance was the observation that no enhancement in the rate of carbohydrate metabolism occurred in response to changes in the pH(int) value. In contrast to P. stipitis, agarose-immobilized Saccharomyces cerevisiae showed a dramatic twofold increase in its ability to metabolize glucose in the immobilized state relative to cell suspensions. This strain was also able to grow within the beads, although the 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 immobilized P. stipitis, however, the pH(int) did not play a key role in controlling the rate of glucose metabolism.