L-dopa decarboxylation in chronically treated patients.

We measured decarboxylation of oral L-dopa in patients chronically treated with L-dopa, and in untreated controls. Chronic L-dopa and carbidopa administration did not affect the extent of whole-body decarboxylation, and it is therefore unlikely that on-off fluctuations are related to chronic changes in the activity of L-aromatic amino acid decarboxylase. The observed duration of action and dose-response properties of carbidopa suggested that current empirically based dose schedules are optimal and supported the concept that decarboxylase inhibitors enhance the clinical effect of L-dopa largely by reducing the extent of first-pass metabolism rather than through an action on the decarboxylase enzyme in cerebral capillaries.