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

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.