Competitive and non-competitive inhibition of L-3, 4-dihydroxyphenylalanine uptake in Opossum kidney cells.
The present study aimed to determine the kinetics of L-3,4-dihydroxyphenylalanine (L-DOPA) uptake in Opossum kidney (OK) cells and to define the type of inhibition produced by L-5-hydroxytryptophan (L-5-HTP), cyanine 863 and 3,3'-diethyloxacarbocyanine (3,3'-DOC). Non-linear analysis of the saturation curves revealed for L-DOPA a Km (in microM) of 129 (114, 145) and a Vmax (in nmol/mg protein per 6 min) of 30.0 +/- 0.4 IC50 values for L-5-HTP (1454 microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were almost 4-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used (330). IC50 values for cyanine 863 and 3,3'-DOC (638 and 353 microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were similar to those obtained when non-saturating (25 microM) concentrations of L-DOPA were used (654 and 339 microM). Vmax values (in nmol/mg protein per 6 min) for L-DOPA uptake were identical in the absence (36.4 +/- 0.7) and the presence of L-5-HTP (39.2 +/- 1.3), but Km values (microM) were significantly greater (P < 0.05) when L-DOPA uptake was studied in the presence of L-5-HTP (121 (100, 142) versus 318 (237, 399)). In contrast, the effect of cyanine 863 and 3,3'-DOC was to cause a significant reduction in Vmax values without significant changes in Km values. It is concluded that L-5-HTP exerts a competitive type of inhibition of L-DOPA uptake in cultured OK cells, whereas both cyanine 863, an organic cation transport inhibitor and 3,3'-DOC behave as non-competitive inhibitors.
Gomes
P
P
Faculty of Medicine, Institute of Pharmacology and Therapeutics, Porto, Portugal.
Serrão
M P
MP
Vieira-Coelho
M A
MA
Soares-da-Silva
P
P
D000818:Animals; D001667:Binding, Competitive; D002232:Carbocyanines / Q000494:pharmacology*; D002478:Cells, Cultured; D004396:Coloring Agents / Q000494:pharmacology; D007668:Kidney / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D007980:Levodopa / Q000378:metabolism*; D009893:Opossums; D011806:Quinolinium Compounds / Q000494:pharmacology*; D012701:Serotonin / Q000494:pharmacology*
D002232:Carbocyanines; D004396:Coloring Agents; D011806:Quinolinium Compounds; D012701:Serotonin; C081554:3,3'-diethyloxacarbocyanine; D007980:Levodopa; C016855:cyanine 863