Aldehyde dehydrogenases (ALDHs) mediate NADP+-dependent oxidation of aldehydes into acids during the detoxification of alcohol-derived acetaldehyde, lipid peroxidation, and metabolism of corticosteroids, biogenic amines and neurotransmitters. ALDH18A1 (aldehyde dehydrogenase 18 family member A1), also known as GSAS (glutamate ?-semialdehyde synthetase), P5CS (?1- pyrroline-5-carboxylate synthetase) or PYCS, is the major enzyme involved in proline, arginine and ornithine biosynthesis. Localizing to the inner mitochondrial
membrane, ALDH18A1 catalyzes the reduction of glutamate to ?1- pyrroline-5-carboxylate. Due to alternative splicing events, two isoforms exist for ALDH18A1, differing by only two amino acids. The longer isoform is widely expressed while the shorter isoform predominantly functions in the gut. A mutation in the gene encoding ALDH18A1 results in P5CS deficiency, a disease characterized by progressive joint laxity, neurodegeneration, bilateral subcapsular cataracts and skin hyperelasticity.