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 14C-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 CO2. 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 CO2, glutamate, glutathione, aspartate and lipids also occurred. 5. Radioactivity from leucine was detected in CO2, lipids, glutamate, glutathione, proline and glutamine. 6. Lysine was only slightly broken down by the bovine lens; radioactivity was observed in CO2, glutamate, glutathione, proline and two unidentified compounds. 7. Proline was metabolized to glutamate from which CO2, glutathione and glutamine were formed. Hydroxyproline in the capsule collagen was labelled. 8. Radioactivity from serine was found in CO2, 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 NH3 production: more NH3 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.