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tion rate of ATP can be estimated using the lactate production and oxygen consumption rates (87,92,93):

the contribution of glycolysis increased with elevated ammonia levels (6,52).

9ATP = ?Lac + 2(pl°) • qO2 = ?Lac + 6 • qO2 (3)

where qATP, qLac, and qO2 are the production rates of ATP, lactate, and oxygen, respectively, and P/O is the phosphorylation ratio. The term qLac corresponds to the contribution of glycolysis, and the term 6 • qO2 corresponds to the contribution of oxidative phosphorylation.

Miller et al. (52) observed a decrease in oxygen uptake at high ammonia concentrations in a continuous reactor operation. On the other hand, Kimura et al. (94) and Oz-turk et al. (6) reported no significant effect of ammonia on oxygen consumption rates for a human leukemia line and a hybridoma line, respectively. The data of Ozturk et al. (6) on the effect of ammonia on oxygen consumption rates are presented in Figure 4A.

Miller et al. (52) and Ozturk et al. (6) reported an increase in the ATP production rate at high ammonia concentrations. The dependence of the ATP production rate on ammonia is presented in Figure 4a. Although cells generated more ATP at higher ammonia levels, this increased energy production was not, however, used for growth, because the growth rate was inhibited (6). It appears that the cells produce more energy under stressful conditions and use it for maintenance. The relative contribution of oxidative phosphorylation was observed to decrease, and

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