A novel sodium-dependent uptake system for l-leucine in rat brain synaptosomes.

A novel Na(+)-dependent system for the uptake of l-leucine by rat brain synaptosomes is described. At a constant osmolarity of 270 mOsm/L, it functions optimally at a Na(+) concentration of 10 mM. This system is determined by measuring the difference in leucine uptake in the presence and absence of Na(+) as well as by measuring the residual uptake of leucine in the presence of an excess amount of 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid in a Na(+) -containing medium. Mg(2+) is found to enhance leucine uptake in the presence and absence of Na(+). However, its presence in the incubation medium reduces net leucine uptake rates due to Na(+) and also alters K(m) and V(max) values for the Na(+) -dependent uptake system.