CONTROL OF THE SHIFT FROM HOMOLACTIC ACID TO MIXED-ACID FERMENTATION IN LACTOCOCCUS-LACTIS - PREDOMINANT ROLE OF THE NADH NAD(+) RATIO/

During batch growth of Lactococcus lactis subsp. lactis NCDO 2118 on v arious sugars, the shift from homolactic to mixed-acid metabolism was directly dependent on the sugar consumption rate. This orientation of pyruvate metabolism was related to the flux-controlling activity of gl yceraldehyde-3-phosphate dehydrogenase under conditions of high glycol ytic flux on glucose due to the NADH/NAD(+) ratio. The flux limitation at the level of glyceraldehyde-3-phosphate dehydrogenase led to an in crease in the pool concentrations of both glyceraldehyde-3-phosphate a nd dihydroxyacetone-phosphate and inhibition of pyruvate formate lyase activity. Under such conditions, metabolism was homolactic. Lactose a nd to a lesser extent galactose supported less rapid growth, with a di minished flux through glycolysis, and a lower NADH/NAD(+) ratio. Under such conditions, the major pathway bottleneck was most probably at th e level of sugar transport rather than glyceraldehyde-3-phosphate dehy drogenase. Consequently, the pool concentrations of phosphorylated gly colytic intermediates upstream of glyceraldehyde-3-phosphate dehydroge nase decreased. However, the intracellular concentration of fructose-1 ,6-bisphosphate remained sufficiently high to ensure full activation o f lactate dehydrogenase and had no in vivo role in controlling pyruvat e metabolism, contrary to the generally accepted opinion. Regulation o f pyruvate formate lyase activity by triose phosphates was relaxed, an d mixed-acid fermentation occurred (no significant production of lacta te on lactose) due mostly to the strong inhibition of lactate dehydrog enase by the in vivo NADH/NAD(+) ratio.