The main site of urea synthesis in mammals is:
The intracellular ammonia is immediately trapped by glutamic acid to form glutamine, especially in brain cells. The glutamine is then transported to liver, where the reaction is reversed by the enzyme glutaminase . The ammonia thus generated is immediately detoxified into urea.
Reference: Vasudevan 7th  ed, pg 203
Liver
Skin
Intestine
Kidney