Energy Coupling in H+-Amino Acid Cotransport

Experiments were undertaken in order to test the mechanism of energy coupling for amino acid uptake proposed in the cotransport hypothesis. According to the hypothesis an electrochemical potential difference in H(+) is established by active H(+) extrusion. That potential difference then drives the cotransport of H(+) and amino acids into the cells. Application of amino acids to oat (Avena sativa var. Victory) coleoptiles induced transient depolarizations of the cell membrane electrical potentials considered to reflect the joint uptake of H(+) and amino acids followed by an enhanced H(+) extrusion. In the presence of KCN, cysteine induced strong depolarizations, but the rate of repolarization depended linearly upon the cyanide-adjusted ATP level of the tissue. At an ATP level 44% of normal, the membrane potential was 74% of normal, but the repolarization after cysteine-induced depolarization was practically nil. Sudden transitions from room temperature to temperatures below 15 degrees C induced sharp depolarizations of the membrane which then repolarized within 3 min; the ATP content of the tissues was unaffected. Cysteine and alanine induced strong depolarizations at temperatures between 5 and 25 degrees C, and the Q(10) for the rate of depolarization was 1.5 for cysteine and 1.6 for alanine. The Q(10) for the rate of repolarization was 3.0 for cysteine and 2.0 for alanine. These experiments support the prevailing view that the depolarizations are caused by the passive joint influx of H(+) and amino acids and that the repolarizations depend upon the ATP-dependent extrusion of H(+).