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In this article, we reviewed several aspects of ammonia inhibition. Ammonia accumulation in cell cultures can be a serious problem for cell growth, productivity, and product quality.

Cell culture

Cation exchange membrane

Strip/fixation side

Gas phase

Figure 11. Ammonia removal from cell culture using cation exchange membranes. Source: Adapted from Thommes et al. (164).

Figure 11. Ammonia removal from cell culture using cation exchange membranes. Source: Adapted from Thommes et al. (164).

Ammonia is generated mainly by glutamine degradation and glutamine metabolism. Cell physiology can be affected by ammonia concentrations between 2 and 10 mM. Different cell lines display varying tolerance to ammonia inhibition. Cell growth rate and final cell density in batch and fed-batch cultures decrease at high ammonia levels. Under normal conditions, ammonia does not directlyinflu-ence the death rate or the decrease in viability of the cultures.

Ammonia is involved in several metabolic pathways and can influence the metabolic rates. Specific metabolic rates for glucose, glutamine, ammonia, lactate, amino acids are accelerated at higher ammonia concentrations. The ammonia yield from glutamine decreases, and more alanine is produced at higher ammonia levels. Although ammonia does not seem to alter the specific oxygen consumption rate, the ATP production rate increases at elevated ammonia concentrations. Cells become more active meta-bolically and generate more ATP for maintenance when their growth is suppressed by ammonia.

Specific rates for product secretion are not influenced by ammonia concentration. Cultures at higher ammonia levels result in lower product concentrations because ofthe low cell concentrations achieved. Ammonia can influence product quality, glycosylation, and sialyation; this was demonstrated for a number of cell lines and products.

Several mechanisms have been hypothesized and examined experimentally in an attempt to understand ammonia inhibition. Alteration of intracompartmental pH and membrane potential, futile cycles, metabolic inhibition, and alteration of critical ribonucleotides are identified as potential mechanisms for ammonia inhibition. Although the dynamics of ammonia transport to and from the cells were studied and intracellular pHs were measured, the data on metabolic rates could not be explained by internal pH hypothesis. Energy dissipation caused by futile cycles and disturbance of membrane potentials are viable explanations for ammonia inhibition. Ammonia was shown to elevate levels in several ribonucleotide pools. Am monia increases the size of UDP-GNAc pool and results in growth inhibition.

Several techniques were investigated to minimize ammonia accumulation in cell culture. Generation of ammonia resulting from glutamine degradation and metabolism can be reduced by controlling the environment of the cells. By maintaining glutamine at low levels, the ammonia generation can be minimized. Glutamine can also be replaced by several other amino acids; however, the cells have to be adapted or genetically altered for this strategy to be successful. Cells can also be adapted to high ammonia concentrations and can thus tolerate higher ammonia levels.

Methods for ammonia removal in cell culture were investigated to improve culture performance. These techniques involve use of adsorbents, gas exchange and ion exchange membranes, and electrodialysis. These systems are fairly complicated, and more work has to be done to improve the effectiveness and reliability for commercial production.

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Your Metabolism - What You Need To Know

Your Metabolism - What You Need To Know

If you have heard about metabolism, chances are it is in relation to weight loss. Metabolism is bigger than weight loss, though, as you will learn later on. It is about a healthier, better you. If you want to fire up your metabolism and do not have any idea how to do it, you have come to the right place. If you have tried to speed up your metabolism before but do not see visible results, you have also come to the right place.

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