A new synaptosomal biosynthetic pathway of proline from ornithine and its negative feedback inhibition by proline

The biosynthesis of L-proline (Pro) was studied in sonicates of mouse brain synaptosomes (P2). Incubation with L-[14C]glutamic acid (Glu) resulted in formation of [14C]gamma-aminobutyric acid (GABA) and L-[14C]aspartic acid (Asp), but not [14C]Pro. On the other hand, L-ornithine (Orn) was found to be an effective precursor of Pro. Incubation of [3H]Orn with the sonicated mouse brain P2 fraction not only resulted in the extensive formation of [3H]Pro, but also yielded [3H]Glu and [3H]GABA. Almost 50% of [3H]Orn consumed was converted into [3H]Pro. In vitro addition of Pro (0.1-1 mM) inhibited the formation of [3H]Pro from [3H]Orn to the extent of 40-75%, while increasing the accumulations of [3H]Glu and [3H]GABA without affecting the consumption of [3H]Orn. These results clearly demonstrate that a potential site exists for negative feedback inhibition in Pro biosynthesis from Orn in the mammalian central nervous system.