Kinetics of l-Valine Uptake in Suspension-Cultured Cells and Protoplast-Derived Cells of Tobacco: Comparison of Wild-Type and the $\text{Val}^{\text{r}}$-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 $\text{Val}^{\text{r}}$-2 mutant plants were compared. A low-Km component was found in protoplast-derived cells (Km = 45 ± 5 micromolar) as well as in suspension-cultured cells (Km = 84 ± 21 micromolar). In the mutant cells the Vmax of this component was 12- to 14-fold less than in wild-type cells. A second component (Km = 2.4 ± 0.7 millimolar) was found in suspension-cultured cells but not in protoplast-derived cells; its Vmax 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 Vmax of the low-Km component in the $\text{Val}^{\text{r}}$-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.