Glutamine Synthetase

Abstract

Glutamine synthetase (EC 6.3.1.2) was localized in the mitochondrial fractions of liver from three uricotelic species. These were chicken (Gallus domesticus), pigeon (Columba livia), and snake (Thamnophis elegans). In rat liver, glutamine synthetase is an extramitochondrial enzyme being mainly associated with microsomes during cellular fractionation. Agents causing the dissociation of the mammalian enzyme from liver microsomes did not release the activity from chicken liver mitochondria. However, the activity was released by agents known to release other mitochondrial enzymes suggesting that glutamine synthetase is an integral part of mitochondrial structure in uricotelic species. The amount of activity in uricotelic liver was two to six times greater than that in rat liver. Both the higher level of activity and the mitochondrial localization of glutamine synthetase in uricotelic liver suggest that the enzyme functions to detoxify ammonia generated intramitochondrially via amino acid catabolism by converting it to glutamine. Glutamine could then exit the mitochondrion for conversion to purine in the cytosol. This system is thus very similar to the system in ureotelic liver where intramitochondrially generated ammonia is converted to citrulline which exits the mitochondrion to be converted to urea in the cytosol.