Amino acid synthesis from ornithine: enzymes and quantitative comparison in brain slices and detached retinas from rats and chicks

The regional distribution of proline biosynthetic enzymes, ornithine-delta-aminotransferase and delta 1-pyrroline-5-carboxylate reductase, in the rat brain, and basic conditions for proline synthesis from ornithine in rat and chicken brain slices and chicken retinas were investigated. The cerebral regions relating to memory formation and imprinting (cortex, hypothalamus and hippocampus) exhibited a high activity of ornithine-delta-aminotransferase, while delta 1-pyrroline-5-carboxylate reductase activity was ubiquitously high. Amino acids were determined fluorometrically after separation and reaction with o-phthaldialdehyde by high-performance liquid chromatography. Proline formation was both ornithine- and 2-oxoglutarate-dependent, and the proline level was suppressed by a high-potassium medium in the brain slices but not in the detached retinas. Its main precursor in vitro seemed to be ornithine but not arginine. The retinas from formoguanamine (2,4-diamino-S-triazine)-treated chicks showed a 10-fold higher level of proline and a marked decrease in gamma-aminobutyric acid, presumably due to an impairment of the blood-retina barrier. The different response in proline level to the high-potassium medium in the brain slices and detached neuroretinas suggests that cellular distribution of the enzymes relating to ornithine and proline metabolism is different in the brain and the neuroretina.