Effects of oxygen and acetoin on fermentation in yeasts

In contrast to the Pasteur effect, found in many yeasts, a negative Pasteur effect is observed in the fermentation of glucose by yeasts belonging to the genus Brettanomyces (Custers, 1940; Wik6n et al., 1961). The Pasteur effect is generally regarded as an inhibition of fermentation by respiration in consequence of a shortage of ADP. The negative Pasteur effect, i.e. the inhibition of fermentation under anaerobic conditions, is attributed to a shortage of NAD (oxidized form), which is required for the oxidation of phosphoglyceraldehyde; this inhibition can be abolished by 02, which oxidizes NADH2 via the respiratory chain, or by certain carbonyl compounds, able to oxidize NADH2 enzymatically (Scheffers, 1961). As soon as the initial shortage of NAD, which may be brought about by the activity of various redox systems in the cell after addition of glucose, is overcome by the action of 02 or a carbonyl compound, dehydrogenation of phosphoglyceraldehyde can take place and consequently acetaldehyde is produced, which reoxidizes the NADH2 formed in the dehydrogenation of phosphoglyceraldehyde. In Warburg experiments with Brettanomyces spp., addition of the carbonyl compound aeetoin (1 raM) stimulates the anaerobic fermentation of glucose to rates beyond those of the aerobic fermentation (in air). Also, in presence of low concentrations of Oz (ca. 0.I ~), the rate of fermentation may surpass the rate in air. Apparently, at certain low O2 tensions, the anaerobic inhibition of fermentation is overcome by oxidation of NADH2 via the respiratory chain, whereas the depressing effect of respiration oll fermentation, known as Pasteur effect, does not yet manifest itself to the full extent. It is concluded, that in Brettanomyces spp. the normal Pasteur effect is obscured by the negative Pasteur effect, but may become evident after the anaerobic inhibition of fermentation is abolished by addition of O~ or acetoin in low concentrations. On the other hand, acetoin increase~ the rate of anaerobic fermentation in a number of yeasts showing a normal Pasteur effect, e.g. in species belonging to the genera Endomycopsis, Pichia, Hansenula, and Torulopsis. Low concentrations of 02 may have a similar effect. It is concluded, that in these yeasts a partial inhibition of fermentation by anaerobic conditions is masked by the normal Pasteur effect. Finally, in many yeasts, e.g. in species belonging to the genera Saccharomyces, Hanseniaspora, Candida, and Kloeckera, an initial inhibition of anaerobic fermentation is found, which is overcome after a short period of time even without addition of 02 or acetoin. Apparently, in these cases the yeast itself is able to reoxidize NADHz via a side reaction. By addition of low concentrations of 02 or acetoin, this lag phase in anaerobic fermentation is eliminated. in summary, it is concluded that in one and the same yeast, fermentation may be inhibited, on the one hand, by higher Oz tensions leading to a shortage of ADP as a consequence of respiratory chain phosphorylation; on the other hand, by anaerobic conditions leading to a shortage of NAD. The ratio between both inhibitions determines, whether a normal or a negative Pasteur effect is observed.