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Clostridium acetobutylicum exhibited diauxie growth in the presence of mixtures of glucose and xylose. Both glucose-and xylose-grown cells had a glucose uptake activity. On the other hand, growth on xylose was associated with the induction of a xylose permease activity, which was repressed by glucose in xylose-induced cells. The rate of sugar uptake with increasing sugar concentrations showed saturation kinetics with an apparent Km of 1.25 x 10-5 M for glucose and 5 x 10-3 M for xylose. Concomitant with the production of solvents, the activities of the glucose and xylose transport systems decreased. Among the main products of fermentation, butanol was shown to be a potent inhibitor of the growth of the organism and of the rate of sugar uptake as well as of sugar incorporation into cell materials. These inhibitory effects of butanol were more pronounced in xylose-grown cells than in glucose-grown cells. Butanol completely inhibited growth at a concentration of 14 g/liter for cultures growing on glucose and 8 g/liter for cultures growing on xylose. Concentrations of 7 and 10.5 g/liter of butanol caused a 50% inhibition of the xylose and glucose incorporations into cell materials. These inhibitory levels of butanol were found in typical glucose or xylose fermentation.

The metabolism of CIostridium butyricum DSM 5431 was studied in chemostat culture under carbon limitation using either glucose or glycerol. On glycerol, the enzymes glycerol dehydrogenase, diol dehydratase and 1,3-propanediol (1,3-PD) dehydrogenase constitute the branch point that partitions the carbon flux between the competing pathways, i.e. formation of either 1,3-PD or acetate and butyrate. The increasing levels of these enzyme activities with increasing dilution rates (D) explained the constant proportion of glycerol conversion into 1,3-PD. The production of acetate or butyrate constitutes another important branch point and when D increased (i) large amounts of intracellular acetyl-CoA accumulated, (ii) the carbon flux switched from butyric acid to acetic acid, (iii) the specific activity of thiolase was not affected, suggesting this enzyme may be the bottleneck for carbon flux to butyrate biosynthesis providing an explanation for the accumulation of large amounts of intracellular acetyl-CoA, and (iv) high levels of NADH were found in the cell. Oxidation of NADH by 1,3-PD dehydrogenase was linked to the production of 3-hydroxypropionaldehyde (3-HPA) by glycerol dehydratase. The fact that high intracellular concentrations of NADH were found means that diol dehydratase activity is the rate-limiting step in 1,3-PD formation, avoiding the accumulation of 3-HPA which is a very toxic compound. The specific rate of glucose catabolism (9gluc,,se = 11.1 mmol h-' g-') was around four times lower than the specific rate of glycerol catabolism (qglycero, = 57-4 mmol h-' g-'). On glucose-grown cells, reducing equivalents which are released in the glycolytic pathway were reoxidized by the butyric pathway and the low specific formation rate of butyric acid led to an increase in the intracellular level of acetyl-CoA and NADH. Carbon flow was higher on glycerol due to the reoxidation of NADH by both butyric and PD pathways. INTRODUCTIONOver recent years there has been an increase in research and development efforts involving clostridial fermentation, namely the fermentation of glucose by butyric acid bacteria (Woods, 1993). Clostridia are able to ferment the carbon source to a variety of end-products by means of branched fermentation pathways. Generally, the different branches of fermentation are not equivalent with respect to the generation of energy or reducing equivalents, andAbbreviations: DHA, dihydroxyacetone; fd, ferredoxin; 3-HPA, 3-hydroxypropionaldehyde; PD, propanediol.the amount of reduced versus neutral and oxidized products has the potential for a great deal of natural variation. It was shown (Jungermann et al., 1973; Petitdemange e t d. , 1976) that the reduced electron carrier ferredoxin (fd) plays a pivotal role in electron distribution in the cell in that it can either transfer electrons via hydrogenase to generate hydrogen or transfer electrons to the pyridine nucleotides via the appropriate fd oxidoreductase. The activities of three enzymes, NADH-fd oxidoreductase, NADPH-fd oxidoreductase and hydrogen...

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