title: Cell inward transport of L-DOPA and 3-O-methyl-L-DOPA in rat renal tubules.
abstract: 1. The present study has determined the kinetics of the uptake of L-3,4-dihydroxyphenylalanine (L-DOPA) and 3-O-methyl-L-DOPA (3-OMDOPA) in rat renal tubules and examined the effect of 3-OMDOPA on the inward transport of L-DOPA and on its conversion into dopamine in kidney homogenates. 2. The accumulation of both L-DOPA and 3-OMDOPA in renal tubules was found to occur through non-saturable and saturable mechanisms. The kinetics of the saturable component of L-DOPA and 3-OMDOPA uptake in renal tubules were as follows: L-DOPA, Vmax = 11.1 nmol mg-1 protein h-1 and Km = 216 microM (n = 6); 3-OMDOPA, Vmax = 8.1 nmol mg-1 protein h-1 and Km = 231 microM (n = 5). The diffusion constant of the non-saturable component for the accumulation of L-DOPA and 3-OMDOPA was 0.0010 and 0.0014 mumol-1, respectively. 3. 3-OMDOPA (100 to 2000 microM) was found to produce a concentration-dependent decrease (29% to 81% reduction) of the saturable component of the tubular uptake of L-DOPA; the Ki value of 3-OMDOPA for inhibition of L-DOPA uptake was found to be 181 microM (n = 5). The accumulation of L-DOPA obtained in experiments conducted at 4 degrees C was not affected by 3-OMDOPA. 4. In experiments conducted in kidney homogenates only L-DOPA (10 to 5000 microM) was found to be decarboxylated. The Vmax and Km values for aromatic L-amino acid decarboxylase determined in the absence of 3-OMDOPA (Vmax = 14.1 nmol mg-1 protein h-1; Km =62 MicroM) were not significantly different from those observed when the decarboxylation of L-DOPA was carried out in the presence of 1000 MicroM 3-OMDOPA (Vmax = 15.7 nmol mg-1 protein h-1; Km = 68 MicroM).5. It is concluded that the tubular uptake of both L-DOPA and 3-OMDOPA occur through nonsaturable and saturable mechanisms; only the saturable tubular uptake of L-DOPA was found to be inhibited by 3-OMDOPA. It is further shown that 3-OMDOPA neither undergoes decarboxylation into 3-MT nor affects the decarboxylation of L-DOPA.
authors.affiliation: Institute of Pharmacology and Therapeutics, Faculty of Medicine, Porto, Portugal.
authors.forename: P
authors.initials: P
authors.lastname: Soares-da-Silva
authors.forename: M H
authors.initials: MH
authors.lastname: Fernandes
authors.forename: P C
authors.initials: PC
authors.lastname: Pinto-do-O
mesh_terms: D000818:Animals; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D066298:In Vitro Techniques; D007684:Kidney Tubules / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D007980:Levodopa / Q000378:metabolism* / Q000493:pharmacokinetics; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics
chemical_list: D014443:Tyrosine; D007980:Levodopa; D001142:Aromatic-L-Amino-Acid Decarboxylases; C008404:3-methoxytyrosine