Amino Acid Transport in Plasma‐Membrane Vesicles from Rat Liver

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Abstract
Plasma‐membrane vesicles, isolated from rat liver, catalyze active transport of L‐alanine. The transient accumulation of L‐alanine requires the presence of a Na+ concentration gradient (outside> inside). The alanine‐Na+ symport is an electrogenic process, since it is stimulated under conditions that allow compensatory ion movements: both co‐transported anions as well as counter‐transported cations influence the rate of alanine‐Na+ symport. However, no uptake is observed in the presence of a membrane potential, when no Na+ concentration gradient is present. Sodium‐gradient‐stimulated alanine uptake is dependent on temperature and pH, stereospecific, and is affected by the presence of other amino acids. The activity of the L‐alanine transport system is influenced both by the Na+ and by the l‐alanine concentration. In the presence of 100 mM Na+, an apparent Km for alanine of 2 mM is observed; lowering the Na+ concentration results in an increase in the apparent Km, and a decrease in the apparent V. The apparent Km for Na+ is 34 mM in the presence of 0.2 mM l‐alanine. Increasing the L‐alanine concentration also results in a lower apparent Km for Na+ and a higher V.

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