The irreversible conversion of adenylosuccinate to adenosine 5'-monophosphate and fumarate is catalyzed by adenylosuccinate lyase. The active form of this enzyme is a cytosolic tetramer (Stone et al. 1993). The enzyme also catalyzes the conversion of 5'-phosphoribosyl-5-aminoimidazole-4-N-succinocarboxamide (SAICAR) to 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide (AICAR) and fumarate. Humans lacking the enzyme accumulate dephosphorylated forms of both substrates, indicating that the enzyme mediates both reactions in vivo as well (Race et al. 2000).