CPSI (carbamoyl-phosphate synthetase I) is the first key enzyme in the urea cycle, responsible for catalyzing the reaction that synthesizes carbamoyl phosphate from ammonia and carbon dioxide. It is a large multifunctional enzyme primarily located in the mitochondrial matrix. CPSI plays a crucial role in regulating ammonia detoxification, urea synthesis, and nitrogen metabolism balance. In the liver, CPSI expression is specific, and mutations or dysregulation of its expression are associated with various genetic diseases, including carbamoyl-phosphate synthetase I deficiency, a rare metabolic disorder that leads to hyperammonemia and neurological damage. The activity of CPSI is regulated by various factors, including substrate concentrations, metabolites, and hormone levels.