Fixation of CO by growing under CO in a chemostat was investigated. Under anaerobic conditions, CO provided 6.5 ± 1% of the total carbon of yeast grown on glucose and 1.6% of the total carbon of yeast grown on glucose plus excess aspartate. Under aerobic conditions, 2.6% of the yeast carbon was derived from exogenous CO during growth on glucose or glycerol, and 3.3% during growth on pyruvate or ethanol. The

The distribution of the fixed carbon among chemical components of the yeast, including some individual amino acids, was determined. Equilibration of CO across the cell membrane was probably not quite complete. Under anaerobic conditions, the similar molar radioactivities of aspartate and glutamate indicated that oxaloacetate was not metabolically compartmented. The unequal labelling of aspartate and glutamate during aerobic growth was consistent with operation of the glyoxylate bypass and/or compartmentation of oxaloacetate. Increased CO fixation and labelling of carbohydrate during growth on pyruvate or ethanol are ascribed to the activity of phospho pyruvate carboxykinase.


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