Substrate shift experiments in chemostat cultures with either methanol or glucose as carbon source were performed with the yeast Candida boidinii variant 60. At low dilution rates of 0.064 h "1 the culture may be easily shifted from methanol to glucose medium and back again to methanol. From these experiments it can be seen that glucose does not give rise to any catabolite inhibition of alcohol oxidase. Alcohol oxidase and formaldehyde dehydrogenase seem to be regulated by a repression-derepression mechanism, as small basal activities of both these enzymes can still be measured during growth on glucose. On the other hand, formate dehydrogenase activity is completely absent in the presence of glucose. This kind of regulation seems to favor the smooth switch from growth on glucose to methanol metabolism.
With methanol or glucose, growth yields (Ys) of 0.3 and 0.35, respectively may be obtained, and oxygen consumption (Qo2) is much higher in methanol cultures than in glucose-grown cells. Accordingly, the RQ values during growth on methanol decrease to about 0.5. Based on the yield coefficient of 0.3, it is possible to calculate that 38% of the methanol consumed must he incorporated into biomass, whereas 62% of the methanol is oxidized to CO 2. The corresponding RQ of 0.56 could not be experimentally ascertained.
The activities of three mitochondrial enzymes were found to be higher in methanol-grown cells than in cells from glucose cultures. The low activites of enzymes for the phosphogluconate route in methanol-grown cells indicates that a cyclic oxidation of formaldehyde via hexose phosphate to CO 2 cannot be of great importance for methanol metabolism.
Microbial growth on methanol requires special enzyme systems, including new pathways for energy-yielding reactions (