Effect of alkali ions on the active transport of neutral amino acids into Streptomyces hydrogenans.

The active transport of neutral amino acids into Streptomyces hydrogenans is inhibited by external Na+. There is no indication that in these cells amino acid accumulation is driven by an inward gradient of Na+. The extent of transport inhibition by Na+ depends on the nature of the amino acid. It decreases with increasing chain length of the amino acid molecules i.e. with increasing non-polar properties of the side chain. Kinetic studies show that Na+ competes with the amino acid for a binding site at the amino acid carrier. There is a close relation between the Ki values for Na+ and the number of C atoms of the amino acids. Other cations also inhibit neutral amino acid uptake competitively; the effectiveness decreases in the order Li+ greater than Na+ greater than K+ greater than Rb+ greater than Cs+. Anions do not have a significant effect on the uptake of neutral amino acids. After prolonged incubation of the cells with 150 mM Na+, in addition to the competitive inhibition of transport Na+ induces an increase in membrane permeability for amino acids.