Transport of l-glutamic acid and l-glutamine and their incorporation into lenticular glutathione

Abstract

The dicarboxylic amino acids penetrate the cells of the lens (calf, rat) with difficulty. l-Aspartic acid was confined to the extracellular space of the bovine lens. In contrast, l-glutamine was transported as a typical neutral amino acid in both species. This amino acid was found to equilibrate readily with lenticular glutamate in the rat and the calf, and the latter amino acid was then incorporated into glutathione (γ-glutamylcysteinylglycine). A turnover rate of 2% per hr was calculated for the tripeptide in the calf lens. Since twice as much glutamine as glutamic acid was found in bovine aqueous humor, the amide appears to be the natural source of glutamine, glutamic acid, and the glutamyl residue of glutathione in the lens. l-Glutamic acid entered the cells of the rodent lens primarily by a special class of transporters (Glu −). Influx of the dicarboxylic amino acids was 20% or less than that of l-glutamine. Incorporation into glutathione was noted of both l-glutamate and l-glutamine in the rat, with glutamine giving about four times the specific activity of glutamate in this compound. A turnover rate of 6% per hr was obtained. Conversion of both glutamine and glutamate to proline was observed in these lenses.