1-Pyrroline-5-carboxylic acid is an enamine or an imino acid that forms upon the spontaneous dehydration of L-glutamate gamma-semialdehyde in aqueous solutions. The stereoisomer (S)-1-pyrroline-5-carboxylate is an intermediate in glutamate metabolism, arginine degradation, and proline biosynthesis and degradation. It can also be converted into or be formed from three amino acids: L-glutamate, L-ornithine, and L-proline. In particular, it is synthesized via the oxidation of proline by pyrroline-5-carboxylate reductase 1 (PYCR1) (EC 1.5.1.2) or by proline dehydrogenase (PRODH) (EC 1.5.99.8). It is hydrolyzed to L-glutamate by delta-1-pyrroline-5-carboxylate dehydrogenase (ALDH4A1) (EC 1.5.1.12). It is also one of the few metabolites that can act as a precursor to other metabolites of both the urea cycle and the tricarboxylic acid (TCA) cycle. Under certain conditions, pyrroline-5-carboxylate can act as a neurotoxin and a metabotoxin. A neurotoxin causes damage to nerve cells and nerve tissues. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of pyrroline-5-carboxylate are associated with at least five inborn errors of metabolism, including hyperprolinemia type I, hyperprolinemia type II, iminoglycinuria, prolinemia type II, and pyruvate carboxylase deficiency. Hyperprolinemia type II results in high levels of pyrroline-5-carboxylate. People with hyperprolinemia type II have signs and symptoms that vary in severity, but they are more likely than type I to have seizures or intellectual disability. Pyrroline-5-carboxylate is highly reactive and excess quantities have been shown to cause cell death and apoptosis.