The dehydrogenation (oxidation) of proline to L-1-pyrroline-5-carboxylate coupled to the conversion of FAD to FADH2 is the first step in proline breakdown (Bender et al. 2005). Proline dehydrogenase (PRODH), the enzyme that catalyzes this reaction, is found in liver, kidney, and brain cells, where it is tightly bound to the inner mitochondrial membrane.
proline dehydrogenase activity of PRODH(1-?) [mitochondrial inner membrane]