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H. L. KERN, D. BRASSIL; Effect of Sodium Ions on the Transport of Neutral Amino Acids by Calf Lens. Invest. Ophthalmol. Vis. Sci. 1968;7(4):452-461.
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Replacement of Na+ in the incubation medium depressed uptake of glycine and L-hydroxyproline and increased net loss of glycine from the calf lens. Passive, nonmediated diffusion was not altered markedly when Na+ was removed from the medium. A mediated component of influx was demonstrable for Na+, and the capacity of this system was comparable with that concerned with transport of sarcosine. The findings suggest that external Na+ may activate the system capable of accumulating amino acids by combining with a carrier associated with both systems of transport. Sodium ions and amino acids are considered to combine at different sites on the mutual carrier. Dependence on Na+ was noted for uptake of representative neutral amino acids at the A-site and the X-site (a special site for amino acids of low molecular weight). In contrast, uptake of L-leucine at the L-site did not depend significantly on the level of external Na+. These experimental results indicate that there are both Na+-dependent and Na+-independent systems concerned with the transport of neutral amino acids in the lens.
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