Decarboxylation of orally administered l-dopa in the human digestive tract

Summary

Carboxyl labelled l-dopa was administered orally and intravenously to Parkinsonian patients and control subjects. Total radioactivity was measured in the expired air, urine and plasma. The activity in plasma was also measured before and after removal of carbon dioxide-dicarbonate. After intravenous administration about four times more radioactivity was recovered in the urine than after oral administration with a roughly corresponding decrease in the recovery from expired air, whereas a considerably smaller proportion of the total radioactivity in plasma was derived from carbon dioxide-bicarbonate. It is concluded that the major fraction of the orally administered l-dopa undergoes decarboxylation in the digestive tract before reaching the general circulation. No difference was observed between the Parkinsonian and the non-Parkinsonian group.