Glutamine Synthetase catalyzes the conversion of ammonia and glutamate to glutamine. It is found in astrocytes as an octamer of identical 42 kDa subunits. The function of Glutamine Synthetase is the detoxification of brain ammonia. It also has an important role in the metabolic regulation of neurotransmitter glutamate. Because of the multiple functions and importance of Glutamine Synthetase in cellular metabolism, both catalytic activities and synthesis are highly regulated. The activity of Glutamine Synthetase is controlled by adenylylation. Its activity is decreased in the cerebral cortex of brains affected by Alzheimer's disease, particularly in the vicinity of senile plaques. It is also decreased under conditions of glucose deprivation. The level of expression of Glutamine Synthetase is increased during ischemia in vivo or hypoxia in culture.