Ornithine accumulation and metabolism in rat lens

The non-protein amino acid ornithine is readily accumulated into rat lenses cultured in TC-199 bicarbonate medium. This accumulation, measured by the lens water/lens medium ratios of radiolabeled ornithine, appears to be the result of an apparent energy-dependent basic amino acid transport system. Moreover, competition studies suggest that increased levels of ornithine can depress the concomitant lenticular accumulation of arginine and lysine. Examination of the trichloroacetic acid protein precipitates of these cultured lenses indicates that the radiolabel from ornithine can also be incorporated into lens proteins as proline. This results from the conversion by ornithine aminotransferase (EC 2.6.1.13) of radiolabeled ornithine to delta1-pyrroline-5-carboxylic acid (P5C), which is subsequently converted to proline by P5C reductase (EC 1.5.1.2). This incorporation of radiolabeled into cultured lens proteins is reduced upon addition of either 1mM proline or P5C to the culture medium or inhibited by the addition of the protein synthesis inhibitor, puromycin.