Metabolism of glutamine and glutamate in human lenses

Glutamate is important to lenses as a central intermediate in amino acid metabolism, as well as for synthesis of proteins and glutathione. In rat and calf lenses, the principal source of lenticular glutamate is glutamine, which enters the lens and is deamidated to form glutamate. In contrast, monkey lenses use external glutamate more readily than glutamine. Amino acid metabolism was studied in human lenses by incubating them with amino-labeled [15N]glutamine or [15N]glutamate. The lenticular free amino acids were then isolated and analysed by gas chromatography-mass spectrometry to determine 15N-labeled products. The results were compared with those of similar experiments with lenses from other species. Label was measured in aspartate, alanine, serine and proline, as well as lenticular glutamine and glutamate. Glutamine enters lenses more readily than glutamate in all the species examined. Nevertheless, aspartate, alanine and serine were more rapidly labeled by incubating human lenses in [15N]glutamate than in [15N]glutamine. This observation is similar to reports of experiments with monkey lenses, which unlike rat lenses, preferentially utilize glutamate rather than glutamine. In contrast, human lens proline was more rapidly labeled by incubating lenses with [15N]glutamine than with [15N]glutamate. In human lenses, the relatively slow utilization of glutamine for the transamination reactions which form aspartate, alanine and serine appears to result from slow deamidation. The relative preference for glutamine over glutamate as a precursor for proline synthesis in human lenses may be related to the mitochondrial location of the enzymes involved.