Glutamine is not stable in aqueous solutions and is degraded to yield a five-member ring structure, pyrrolidone carboxylic acid, and ammonia (18-20):
Glutamine r Pyrrolidone carboxylic acid + NH/ (1)
This reaction is irreversible, and the extent of the reaction is affected by the chemical environment of the medium. Glutamine degradation follows first-order kinetics, the half-life of glutamine is reported to be between 6 and 20 days, depending on the conditions (16,21-23).
Glutamine degradation is pH (18,21,22,24) and temperature (20) dependent. Alkaline pH increases the degradation rate, and altering pH from 7.2 to 7.6 was reported to elevate the degradation rate by 300% (21,22). The presence of serum, especially if not heat inactivated, can elevate degradation of glutamine and subsequent ammonia accumulation caused by glutaminase and asparaginase activity (25). The media composition, especially the phosphate levels in the media, affects glutamine degradation significantly (22).
Glutamine degradation is an important problem for cell culture. The degradation not only generates inhibitory ammonia, but also decreases the level of glutamine available to the cells. The cell performance is affected negatively because of these two factors.
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