Saccharopine is an intermediate in the degradation of lysine, formed by the condensation of lysine and alpha-ketoglutarate. The saccharopine pathway is the main route for lysine degradation in mammals, and its first two reactions are catalyzed by enzymatic activities known as lysine-oxoglutarate reductase (LOR) and saccharopine dehydrogenase (SDH), which reside on a single bifunctional polypeptide (LOR/SDH) (EC 1.5.1.8). The reactions involved with saccharopine dehydrogenases have very strict substrate specificity for L-lysine, 2-oxoglutarate, and NADPH. LOR/SDH has been detected in a number of mammalian tissues, mainly in the liver and kidney, contributing not only to the general nitrogen balance in the organism but also to the controlled conversion of lysine into ketone bodies. A tetrameric form has also been observed in human liver and placenta. LOR activity has also been detected in brain mitochondria during embryonic development, and this opens up the question of whether or not lysine degradation has any functional significance during brain development. As a result, there is now a new focus on the nutritional requirements for lysine in gestation and infancy. Finally, LOR and/or SDH deficiencies seem to be involved in a human autosomal genetic disorder known as familial hyperlysinemia, which is characterized by serious defects in the functioning of the nervous system and characterized by a deficiency in lysine-ketoglutarate reductase, saccharopine dehydrogenase, and saccharopine oxidoreductase activities. Saccharopinuria (high amounts of saccharopine in the urine) and saccharopinemia (an excess of saccharopine in the blood) are conditions present in some inherited disorders of lysine degradation. If present in sufficiently high levels, saccharopine can act as an acidogen and a metabotoxin. 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. Saccharopine is an organic acid. 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). Many affected children with organic acidemias experience intellectual disability or delayed development.