Na+-independent exchange of amino acids in Ehrlich cells

Many hydrophilic amino acids are transported via exchange by the Na+-independent L system in the Ehrlich cells. However, this exchange is measurable only at relative high concentrations of the small hydrophilic amino acids. Whereas leucine saturated this transport system at 0.2 mM, with threonine, serine, or alanine the system was below saturation at 30 mM. All amino acids capable of exchanging showed similar EA values (congruent to 20 X 10(3) cal X mol-1, 1 cal = 4.1868 J) and maximum velocities. Depending on the temperature, the apparent Km values for different amino acids differed by one or two orders of magnitude. These data suggest that for the small hydrophilic amino acids, the relatively slow rate of transport is due to the unfavorable Km values.