Summary
Membrane vesicles obtained from the basal lateral membranes of the rat intestinal epithelium were used to study the pathways for neutral amino acid transport.
In the absence of sodium there was a stereospecific uptake ofl-alanine which exhibited saturation kinetics (K m 0.73mm andV max 5.3 nmol/mg min at 22°C). The activation energy for this process was 8.1 kcal/mole between 5 and 25°C. Preloading the vesicles with alanine increased the unidirectional influx of alanine into the vesicle. Competition experiments indicated that the affinity of the sodium-independent transport system was glutamine > threonine > alanine > phenylalanine > valine > methionine > glycine > histidine > proline, N-MeAIB. These are the characteristics of the classical “L” transport system.
External sodium increased the rate of the stereospecificl-alanine uptake. The Na-dependent flux had aK m of 0.04mm and aV max of 0.26 nmol/mg min at 22°, and an activation energy of 9.1 kcal/mole between 5 and 25°C. Competition experiments suggest the existence of three separate pathways for alanine transport in the presence of sodium. A major pathway is shared by all other amino acids tested (i.e., threonine, glutamine, methionine, phenylalanine, valine, proline and N-MeAIB). This resembles the classical “A” system. A second pathway is unavailable to either phenylalanine or N-MeAIB; this is reminiscent of the classical “ASC” system; and the third is a novel pathway which is shared by N-MeAIB but not phenylalanine.
The sodium-independent and the sodium-dependent transport ofl-alanine was blocked by PCMBS and significantly inhibited by DTP and NEM. It is concluded that the sodium-independent system (the “L”-like system) accounts for the efflux of neutral amino acids from the epithelium to the blood during the absorption of amino acids from the gut, and that the sodium-dependent transport processes may play an important role in the supply of amino acids to the epithelium in the absence of amino acids from the gut lumen.
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Mircheff, A.K., van Os, C.H. & Wright, E.M. Pathways for alanine transport in intestinal basal lateral membrane vesicles. J. Membrain Biol. 52, 83–92 (1980). https://doi.org/10.1007/BF01869009
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DOI: https://doi.org/10.1007/BF01869009