Kinetics of l-Valine Uptake in Suspension-Cultured Cells and Protoplast-Derived Cells of Tobacco: Comparison of Wild-Type and the Val-2 Mutant.

A kinetic analysis was made of l-valine uptake in protoplast-derived cells (mesophyll protoplasts cultured for 6 days) and in suspension-cultured cells of tobacco (Nicotiana tabacum L., cv Xanthi). Cells from wild-type and Val(r)-2 mutant plants were compared. A low-K(m) component was found in protoplast-derived cells (K(m) = 45 +/- 5 micromolar) as well as in suspension-cultured cells (K(m) = 84 +/- 21 micromolar). In the mutant cells the V(max) of this component was 12- to 14-fold less than in wild-type cells. A second component (K(m) = 2.4 +/- 0.7 millimolar) was found in suspension-cultured cells but not in protoplast-derived cells; its V(max) was the same in wild-type and mutant cells. A third component was apparently unsaturable (linear component). It was present in protoplast-derived cells but not in suspension-cultured cells, and had the same magnitude in wild-type and mutant cells. The results are discussed with reference to the uptake of l-valine in leaf tissue, in which the three kinetic components have been found simultaneously. The reduced V(max) of the low-K(m) component in the Val(r)-2 mutant, and the differential expression of the other two components in suspension-cultured cells and protoplast-derived cells indicate that the kinetically distinguishable components represent physically distinct transport systems.