The encoded protein catalyzes the reduction of glutamate to delta1-pyrroline-5-carboxylate, a critical step in the de novo biosynthesis of proline, ornithine and arginine.

Mutations in this gene lead to hyperammonemia, hypoornithinemia, hypocitrullinemia, hypoargininemia and hypoprolinemia and may be associated with neurodegeneration, cataracts and connective tissue diseases.

[6] As reported by Bruno Reversade and colleagues, ALDH18A1 deficiency or dominant-negative mutations in P5CS in humans causes a progeroid disease known as De Barsy Syndrome.

The gamma-glutamic semi-aldehyde is in tautomeric equilibrium with P5C and it is the obligatory intermediate in the interconversions of proline, ornithine, and glutamate.

Because all three of these amino acids are a part of very significant processes, the presence of P5CS becomes an important regulator which makes sure that none of these three become deficient.

[16] Therefore, a lack of P5CS, due to mutations in the ALDH18A1 gene, often leads to neurodegeneration, joint laxity, skin hyperelasticity, bilateral sub capsular cataracts, and a plethora of other complications associated with impaired proline and ornithine synthesis.