METABOLISM OF CELLOBIOSE BY CLOSTRIDIUM-CELLULOLYTICUM GROWING IN CONTINUOUS-CULTURE - EVIDENCE FOR DECREASED NADH REOXIDATION AS A FACTOR LIMITING GROWTH

Previous results indicated that molar growth yields are reduced when C lostridium cellulolyticum is cultured in media containing cellobiose c oncentrations greater than 1 g l(-1). Continuous cultures were examine d to determine the physiological basis of these poor growth yields. Ac etate was the main product of C. cellulolyticum metabolism, whereas th e production of reduced compounds such as ethanol or lactate was low. Such patterns of product formation were accompanied by a 12-fold incre ase in intracellular NADH concentration when the cellobiose flow was i ncreased. Catabolic enzymic activities were measured in vitro. Glycera ldehyde-3-phosphate dehydrogenase (GAPDH), acetate kinase and phosphor oclastic activities were found at similar levels as in cells metaboliz ing higher substrate concentrations. In contrast, lactate dehydrogenas e activity was low and correlated with the rate of lactate production. Furthermore, an inhibition of GAPDH activity by high NADH/NAD(+) rati os was established. These results suggested that a decreased NADH reox idation could be responsible for limiting C. cellulolyticum growth. La ctate and ethanol production were not sufficient to balance out the NA DH produced in the GAPDH step of glycolysis. One consequence of poor N ADH reoxidation would be an increase in intracellular concentration of NADH, which in turn could inhibit GAPDH activity.