trans-4-Hydroxy-L-proline-d9 is deuterium labelledtrans-4-Hydroxy-L-proline (H952376).A natural constituent of animal structural proteins such as collagen and elastin. Several microorganisms producing proline trans-4- and cis-3-hydroxylase were discovered and these enzymes were applied to the industrial production of trans-4- and cis-3-hydroxy-L-proline.