Active Transport of L-Dopa in the Intestine

Abstract

IN a discussion of the problems associated with the metabolism and absorption of L-3,4-dihydroxyphenylalanine (L-dopa) in patients with Parkinson's disease¹, Bianchine et al. suggested that the small intestine is the major site of absorption, as is the case with many other amino-acids. The existence of a carrier-mediated transport process was implied by the suggestion that the absorption of L-dopa is subject to competitive inhibition by other amino-acids. But a study² of the absorption of ¹⁴C-L-dopa in vitro by sacs of everted rat intestine did not suggest an active transport mechanism. On the contrary, there was no significant difference in mucosal uptake when the tissue was gassed with oxygen or rendered anoxic with nitrogen. These data appeared to be consistent with a passive diffusion mechanism for the entry of L-dopa into the mucosa of both stomach and small intestine. On the other hand, there is evidence that L-dopa inhibits the absorption of L-phenylalanine and L-tyrosine in patients with Parkinson's disease³. We have now investigated whether there is an active component in the L-dopa transport mechanism in the small intestine of the rat in vitro.