Effect of substrate structure on coupling ratio for Na+-dependent transport of amino acids

Abstract

We have measured the coupling ratio, i.e. the ratio of the augmentation of Na+ entry due to the presence of a substrate amino acid, to the augmentation of the entry of that amino acid due to the presence of Na+, for a characteristic transport system studied in the pigeon red blood cell. The value of this ratio has been shown to be approximately independent of the interval of observation and of the concentrations of the two cosubstrates. It does, however, depend strongly on the structure of the amino acid, varying from about 0.2 for proline to about 4 for threonine and 5 for cysteine. The coupling ratio both for entry and for exodus also depends on the composition of the internal pool of substrate amino acids. The presence of a hydroxyl group on carbons 3 or 4 produces sharp increases in the coupling ratio. A sulfhydryl or carboxamide group on carbon 3 appears to have a similar effect. A model consistent with these features is presented, which indicates that the presence of a hydroxyl group probably acts by disproportionately accelerating the release, displacement or replacement of Na+ from the transport complex at the interior surface of the membrane.