Bound, free, and total L-dopa measurement in plasma of Parkinson's disease patients.

Peaks and troughs of levodopa in plasma contribute to pulsatile postsynaptic dopamine receptor stimulation in patients with Parkinson's disease. Measurement of levodopa plasma levels mostly only considers the total levodopa plasma concentration. Objectives were to determine bound, free, and total plasma levodopa and to investigate their correlations to each other. We employed reversed-phase high-performance liquid chromatography combined with electrochemical detection. Bound levodopa was computed as difference between total and free L-dopa values. Close correlations between free and total (R = 0.93, p < 0.0001), bound and total (R = 0.91, p < 0.0001) plasma levodopa appeared. A considerable variability of levodopa concentrations occurred. The ratio between bound and free levodopa did not differ in patients with a higher and lower oral daily levodopa dosing. Free, bound, and total levodopa plasma levels are closely related. Estimation of the total levodopa level only seems to be meaningful.