What is the primary source of NH4+ in the urea cycle?

The primary source of NH4+ (ammonium ion) in the urea cycle is glutamate. In the process of ammonia detoxification, the urea cycle requires nitrogen in the form of NH4+ to convert it into urea, which is then excreted from the body. Glutamate plays a crucial role in this process.

Glutamate is formed in various metabolic pathways and can release NH4+ through the action of the enzyme glutamate dehydrogenase, which catalyzes the oxidative deamination of glutamate. This reaction produces α-ketoglutarate and free NH4+. This NH4+ then contributes directly to the formation of urea within the urea cycle.

While other molecules, like aspartate, do contribute nitrogen to the urea cycle, its primary source for NH4+ is glutamate, making it fundamental to the cycle's nitrogen supply. Alanine and pyruvate do not directly provide ammonia for the urea cycle, further solidifying glutamate's essential role.