Which of the following would likely increase the activity of glutamine synthetase?

Glutamine synthetase plays a crucial role in ammonia assimilation by catalyzing the conversion of glutamate and ammonia into glutamine. High levels of ammonia in the cellular environment can stimulate this enzyme's activity because the presence of excess ammonia creates a greater demand for the detoxification and incorporation of ammonia into organic molecules, primarily glutamine.

When ammonia concentrations are elevated, it promotes the reaction facilitated by glutamine synthetase, thereby enhancing its activity to help maintain nitrogen balance and prevent toxic accumulation of free ammonia in the body. This mechanism is particularly significant in tissues like the brain and liver, where the management of nitrogen and ammonia levels is vital for metabolic health.

Other substances like pyruvate, glutamate, or urea do not directly promote the activity of glutamine synthetase in the way that high ammonia levels do. Pyruvate primarily plays roles in energy metabolism rather than directly influencing glutamine synthesis. High levels of glutamate could potentially lead to feedback inhibition of the enzyme, as it is a product of the same metabolic route. Urea, being a formed product of nitrogen metabolism, would not stimulate the activity of glutamine synthetase either; instead, it suggests that nitrogen has already been processed and excreted.