Dihydroorotate dehydrogenase is an enzyme that plays a crucial role in the biosynthesis of pyrimidines, which are essential components of nucleic acids like DNA and RNA. It catalyzes the conversion of dihydroorotate to orotate, using NAD+ as a cofactor. This reaction is a key step in the de novo synthesis pathway of pyrimidine nucleotides. There are two main forms of dihydroorotate dehydrogenase: one located in the cytosol and another in the mitochondria. The mitochondrial form is particularly important for cellular metabolism and is a target for certain immunosuppressive drugs, such as leflunomide, which are used to treat autoimmune diseases.