REGULATION OF PROLINE BIOSYNTHESIS - THE INHIBITION OF PYRROLINE-5-CARBOXYLATE SYNTHASE ACTIVITY BY ORNITHINE

Mammalian cells have the capacity for proline biosynthesis from ornithine or glutamic acid. Using a radioisotopic assay, the regulation by ornithine of .DELTA.1-pyrroline-5-carboxylate synthase, the enzyme that catalyzes the 1st step of proline biosynthesis from glutamic acid, was studied. In homogenates from Chinese hamster ovary cells, ornithine was a potent inhibitor of pyrroline-5-carboxylate synthase activity (50% inhibition at 0.37 mM). The effect was reversible and did not occur with amino acids other than ornithine. Preliminary findings suggest that the inhibition does not result from altered requirements for the cofactors NADPH and ATP. Significant inhibition was observed in 4 different Chinese hamster cell lines [ovary cell lines CHO-K1 (CCL 61), GR2, R25 and lung fibroblast cell line DON (CCL 16)]. Ornithine also to inhibited the conversion of 3H-glutamic acid to 3H-proline in intact human skin fibroblasts. Cells from patients with a rare ocular disease, gyrate atrophy of the choroid and retina, were used for these studies since they lack interfering ornithine aminotransferase activity. Ornithine may be a physiologic regulator of the rate of proline formation from glutamic acid. This information allowed construction of an hypothetical model for the overall regulation of proline biosynthesis and also suggested a pathophysiologic mechanism for the disease gyrate atrophy.