Determination of redox potential levels critical for cell respiration and suitable for L-leucine production.

The effect of oxygen supply on L-leucine fermentation was investigated employing a leucine-producing mutant of Brevibacterium lactofermentum. Since it was not possible to measure oxygen tension below 0.01 atm by a Teflon-coated oxygen electrode, the degree of satisfaction of the cells' oxygen demand (cells' respiration rate/maximum oxygen demand of cells, rab/KrM) and the redox potential of the culture medium (E, mV) were used as indices to oxygen supply in cultures under low oxygen tension. When the oxygen demand of the cells was satisfied (rab/KrM = 1.0) and the E value was between --90 and --110mV, L-leucine formation was 26.5 mg/ml. When the oxygen demand of the cells was not satisfied (rab/KrM = 0.85) and the E value was between --200 and --220 mV, L-leucine accumulation was 29.7 mg/ml. When the oxygen supply was extremely limited (rab/KrM = 0.27) and the E value was --280 mV, L-leucine formation was 12.9 mg/ml. A new method which simultaneously measures the redox potential and dissolved oxygen was applied to the determination of the critical dissolved oxygen level for cell respiration (PL crit), whick was too small to be detected by conventional oxygen electrodes. The value of PL crit of the leucine producer was estimated as 0.0002 atm.