L-3,4-dihydroxyphenylalanine and l-5-hydroxytryptophan share the same transporter in Opossum kidney cells

Abstract

Opossum kidney (OK) cells, which have the ability to synthesise dopamine and 5-HT, have been used as an in vitro model for the study of renal actions of dopamine and 5-hydroxytryptamine (5-HT). The present study reports on the uptake of their immediate precursors l-3,4-dihydroxyphenylalanine ( l-DOPA) and l-5-hydroxytryptophan ( l-5-HTP). IC 50 values for l-5-HTP (1569 μM) obtained in the presence of a nearly saturating (250 μM) concentration of l-DOPA were 6-fold those obtained when using non-saturating (0.25 and 25 μM) concentrations of l-DOPA (251 and 266). V max values (in nmol mg protein −1 6 min −1) for l-DOPA uptake are identical in the absence (13.6) and the presence of 250 μM l-5-HTP (13.3), but K m values ( μM) are significantly greater ( P<0.05) when l-DOPA uptake was studied in the presence of l-5-HTP (90 vs 179). IC 50 values for l-DOPA (679 μM) obtained in the presence of a near saturating (250 μM) concentration of l-5-HTP were almost 3-fold those obtained when non-saturating (0.25 and 25 μM) concentrations of l-5-HTP were used (254 and 220). V max values (in nmol mg protein −1 6 min −1) for l-5-HTP uptake are identical in the absence (11.2) and the presence of 250 μM l-DOPA (11.7), but K m values ( μM) are significantly greater ( P<0.05) when l-5-HTP uptake was studied in the presence of l-DOPA (103 vs 220). It is concluded that l-DOPA and l-5-HTP share the same transporter(s) and each compound exerts a competitive type of inhibition upon the other.