ALDH4A1 and hyperprolinemia type 2: P5C dehydrogenase (P5CD) degrades proline via Δ‐1‐pyrroline‐5‐carboxylate (P5C) into glutamate.31 Deficiency of P5CD results in hyperprolinemia with concomitant increase of urinary P5C (hyperprolinemia type II; HPII).32, 33 These high P5C levels deactivate PLP (vitamin B6), a co‐factor for many enzymes.34, 35 An effect of P5CD deficiency on glutamate levels has not been reported.