SAICAR (or (S)-2-[5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxamido]succinate) is a substrate for the multifunctional protein ADE2. SAICAR is an intermediate in purine metabolism. SAICAR is converted from 5-amino-1-(5-phospho-D-ribosyl) imidazole-4-carboxylate via phosphoribosylaminoimidazole-succinocarboxamide synthase (EC: 6.3.2.6) or SAICAR synthase. This enzyme catalyses the seventh step out of ten in the biosynthesis of purine nucleotides. The appearance of succinylaminoimidazolecarboxamide riboside (SAICAriboside) and succinyladenosine (S-Ado) in cerebrospinal fluid, in urine, and, to a lesser extent, in plasma is characteristic of a heritable deficiency known as adenylosuccinate lyase deficiency. Adenylosuccinate lyase deficiency is responsible for a range of symptoms that involve psychomotor retardation, often accompanied by epileptic seizures, and autistic features. When present in sufficiently high levels, SAICAR can act as an oncometabolite, a metabotoxin, and an acidogen. An oncometabolite is a compound that promotes tumour growth and survival. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. As an oncometabolite, high levels of SAICAR stimulate pyruvate kinase isoform M2 and promote cancer cell survival in glucose-limited conditions such as aerobic glycolysis (PMID: 23086999). As an organic acid, SAICAR is associated with acidosis. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart, liver, and kidney abnormalities, seizures, coma, and possibly death. These are also the characteristic symptoms of untreated adenylosuccinate lyase deficiency. Many affected children with organic acidemias experience intellectual disability or delayed development.