[Carrier-mediated uptake of L-leucine at the points of growth separated from potato tubers].

The uptake of 14C-L-leucine by the points of growth separated from potato tubers was studied. The pH optimum of the process was about 7.0. The leucine transport over a wide concentration range was biphasic. For descriptive convenience the isotherm was divided into two segments, both of which obeyed the Michaelis-Menten kinetics. The Km value for the high affinity transport was within the range of 7.0 x 10(-6) mM, while that for the low affinity transport was within the range of 5 x 10(-4) mM. The maximum velocities were 3 x 10(-8) mM/g of dry weight/hour and 3.8 x 10(-7) mM/g of dry weight/hour, respectively. Cycloheximide and 2.4-dinitroftorphenol caused a considerable inhibition of net transport of L-leucine, while actinomycin D had no such effect. The uptake of L-leucine was unaffected by phenylalanine and isoleucine, but was strongly inhibited by D-leucine. The results obtained are indicative of a biphasic carrier-mediated uptake system with a high affinity and high substrate specificity for leucine.