Normal erythrocyte uptake of L-DOPA in Parkinson's, Huntington's, and related diseases.

We measured the kinetic constants for the unidirectional influx of L-DOPA into red blood cells of patients with Parkinson's disease (seven patients), Huntington's disease (seven patients), and other extrapyramidal diseases (11 patients), and in five controls. Influx consisted of two components with low affinity and high exchange capacity. In individual subjects, the L-DOPA concentration giving half-maximal influx (Km) varied between 0.04 and 2.19 mM, and the maximum velocity (Vmax) of the saturable transport component was between 20 and 578 mumol/l cell water/h, which is compatible with the neutral amino acids of low affinity for the transport system. The range of Kd (the first-order rate constant for the unsaturable component) was between 0.11 and 0.36 hour-1. There was no gross deficit of the L-DOPA uptake process in patients with Parkinson's disease, Huntington's disease, dystonia, or other extrapyramidal diseases.