NAGS and hepatocellular carcinoma: On the other hand, in hepatoma cells, the expression levels enzymes involved in ornithine biosynthesis are increased, namely arginase 1, ornithine aminotransferase (OAT), and N-acetylglutamate synthase (NAGS), as well as enzymes that convert glutamate to α-ketoglutarate (GLUD1 and GLUD2) and utilize ammonium.