SODIUM-ION DEPENDENT ACTIVE-TRANSPORT OF LEUCINE IN METHANOSPHAERA-STADTMANAE

A Na+-dependent active transport system for leucine has been observed in Methanosphaera stadtmanae. The K(m) for leucine as determined was 2 0 muM with a V(max) of 3.2 nmol . min-1 . mg protein-1. A minimum of 5 mM Na+ was required for optimal uptake rates. After correction for un specific binding and incorporation into trichloroacetic acid precipita ble materials, [C-14]leucine was accumulated inside the cell to concen trations > 100 times higher than in the medium. The uptake of leucine into active cells was inhibited by the protonophore 3,3',4'5-tetrachlo rosalicylanilide but stimulated by the synthetic Na+-H+ antiporter mon ensin. A 500 mM NaCl pulse was able to drive leucine into resting cell s. Of the amino acids tested, only valine and isoleucine competed effe ctively with leucine for transport.