N-alpha-Acetyl-L-citrulline, also known as N-acetylcitrulline, is an N-acetylated metabolite of citrulline that is part of the arginine biosynthetic pathway. Arginine biosynthesis is notable for its complexity and variability at the genetic level, and by its connection with several other pathways, such as pyrimidine and polyamine biosynthesis, and certain degradative pathways. The initial steps of the arginine biosynthetic pathways proceed via N-acetylated intermediates. The presumed reason for this is that the acetylation prevents the spontaneous cyclization of glutamate derivatives, which leads to proline biosynthesis. N-acetyl-L-ornithine can be transcarbamylated directly by the enzyme acetylornithine transcarbamylase, resulting in N-acetyl-L-citrulline. The enzyme acetylornithine deacetylase can accept N-acetyl-L-citrulline as a substrate and can deacetylate it into citrulline. N-alpha-Acetyl-L-citrulline is found in cases of deficiency of the urea cycle enzyme argininosuccinate synthase (EC 6.3.4.5) that leads to increased concentrations of citrulline and N-acetylcitrulline in the urine.