By which process ammonia is converted into urea in liver?

The conversion of ammonia to urea primarily occurs in the liver through a series of biochemical reactions known as the urea cycle, also called the ornithine cycle. Here's a simplified breakdown of the process:

Formation of Carbamoyl Phosphate: The urea cycle begins in the mitochondria of liver cells. Ammonia (NH₃) combines with carbon dioxide (CO₂) to form carbamoyl phosphate, catalyzed by the enzyme carbamoyl phosphate synthetase I (CPS-I). This reaction consumes one molecule of ATP.

Formation of Citrulline: The carbamoyl phosphate produced in the first step combines with ornithine to form citrulline, facilitated by the enzyme ornithine transcarbamylase (OTC). This step occurs in the mitochondrial matrix.

Transport of Citrulline: Citrulline is transported out of the mitochondria into the cytosol of the liver cell.

Formation of Argininosuccinate: In the cytosol, citrulline combines with aspartate to form argininosuccinate, catalyzed by argininosuccinate synthetase.

Formation of Arginine: Argininosuccinate is then converted into arginine by argininosuccinate lyase.

Formation of Urea: Arginine undergoes hydrolysis by arginase to produce urea and regenerate ornithine, completing the cycle. The urea produced is released into the bloodstream and eventually excreted by the kidneys.

The urea cycle helps to detoxify ammonia, a waste product of protein metabolism, by converting it into urea, which is less toxic and more easily excreted from the body.