The adaptation of BHK cells to a non-ammoniagenic glutamate-based culture medium.

Although glutamine is a major carbon source for mammalian cells in culture, its chemical decomposition or cellular metabolism leads to an undesirable excess of ammonia. This limits the shelf-life of glutamine-supplemented media and may reduce the cell yield under certain conditions. We have attempted to develop a less ammoniagenic medium for the growth of BHK-21 cells by a mole-to-mole substitution of glutamine by glutamate. This results in a medium that is thermally stable but unable to support an equivalent growth yield. However, supplementation of the glutamate-based medium with asparagine (3 mM) and a minimal level of glutamine (0.5 mM) restored the original growth capacity of the cultures. Substitution of the low level of glutamine with the glutamine dipeptides, ala-gln (1 mM), or gly-gln (3 mM) resulted in an equivalent cell yield and in a thermally stable medium. The ammonia accumulation in cultures with glutamate-based medium was reduced significantly (>60%). Factors mediating growth and adaptation in medium substituted with glutamate were also investigated. The maximum growth capacity of the BHK-21 cells in glutamate-based medium (without glutamine) was achieved after a period of adaptation of 5 culture passages from growth in glutamine-based cultures. Adaptation was not influenced by increases in glutamate uptake which was constitutively high in BHK cells. Adaptation was associated with changes in the activities of enzymes involved in glutamate or glutamine metabolism. The activities of glutamine synthetase (GS) and alanine aminotransferase (ALT) increased significantly and the activity of phosphate-activated glutaminase (PAG) decreased significantly. The activity of glutamate dehydrogenase (GDH) showed no significant change after adaptation to glutamate. These changes resulted in an altered metabolic profile which included a reduced ammonia production but an increased alanine production. Alanine production is suspected of being an alternative route for removal of excess nitrogen.