The metabolism of amino acids in the bovine lens. Their oxidation as a source of energy

Abstract

1. The metabolism by the bovine lens of nine (14)C-labelled l-amino acids was studied. These were: alanine, aspartate, glutamate, leucine, lysine, proline, serine, tyrosine and tryptophan. 2. All were taken up by the tissue and incorporated into protein. 3. Aspartate and glutamate, although poorly taken up, were readily metabolized to CO(2). Radioactivity from glutamate was also found in glutathione, glutamine, proline and ophthalmic acid. Aspartate was converted into glutamate, glutathione, proline, alanine and lactate. 4. Alanine was largely converted into lactate, which was released into the medium, but incorporation of radioactivity into CO(2), glutamate, glutathione, aspartate and lipids also occurred. 5. Radioactivity from leucine was detected in CO(2), lipids, glutamate, glutathione, proline and glutamine. 6. Lysine was only slightly broken down by the bovine lens; radioactivity was observed in CO(2), glutamate, glutathione, proline and two unidentified compounds. 7. Proline was metabolized to glutamate from which CO(2), glutathione and glutamine were formed. Hydroxyproline in the capsule collagen was labelled. 8. Radioactivity from serine was found in CO(2), lipids, glutathione, glycine, cystine, ATP, lactate and three unidentified compounds, one of which was probably taurine. 9. Neither tyrosine nor tryptophan were metabolized by the bovine lens. 10. The ability of the lens to metabolize amino acids was also shown by measurement of NH(3) production: more NH(3) was formed when glucose was absent from the incubation medium. 11. These experiments suggest that oxidation of amino acids is a source of energy for the lens.

The effect of single and repeated UVB radiation on rabbit lens

“…The UVR-induced elevated sorbitol levels, as well as the increase in the amount of the reactive oxygen species in the lens, might thus be responsible for the observed changes in alanine concentration. Moreover, prior examinations of the metabolism of amino acids in normal bovine lenses showed that alanine is largely deaminated in the lens fibres and that the pyruvate formed is reduced to lactate [34]. A significant decrease in lactate content (14% for UVB1, 26% for UVB2) found in the present study would be thus in accordance with a concomitant alanine reduction.…”

Glutaminolysis in Animal Cells

“…This suggested the presence of an important aerobic substrate other than glucose for lens in vitro (Trayhurn and Van Heyningen, 1971). The substrate was subsequently identified as amino acids (Trayhurn and Van Heyningen, 1973a). Of the amino acids tested, glutamate was the most extensively oxidized.…”

Alpha-crystallin mutations alter lens metabolites in mouse models of human cataracts

“…These data may indicate elevated degraded protein levels and increased amino acid transport across the cell membrane in mutant lenses. Early work on lens metabolism showed that amino acid oxidation in the lens can be an energy source [32]. In our recent RNA-seq studies with lenses from postnatal Cryab-R120G-het mice, we found 5-and 27-fold increases in Slc6a13 and Slc1a1 gene expression, respectively [15].…”