Urate transport in Homarus americanus hepatopancreas: studies on membrane vesicles and R cells.
[2-14C]urate uptake was studied in hepatopancreatic basolateral membrane vesicles and in R cell suspensions of the American lobster by Millipore filtration techniques. Unspecific binding of urate to the vesicular membrane was 25.5 +/- 3.0% of equilibrium. Vesicular uptake showed a diffusional and a saturable component (Km) 0.37 +/- 0.04 mM and maximal velocity (Vmax) 16.5 +/- 1.2 pmol urate.mg protein-1.s-1). [2-14C]urate uptake was significantly trans-stimulated by urate. Purine analogues, probenecid, p-aminohippuric acid, pyrazinoic, and oxonic acid cis-inhibited urate transport. Urate uptake was not affected by Na+ or K+ transmembrane gradients but stimulated by 1 mM 2-oxoglutarate at the cis-side in Na(+)-containing media. Cellular urate uptake was inhibited by pyrazinoic acid. Uptake was saturable (Km 0.53 +/- 0.11 mM and Vmax 3.7 +/- 0.4 pmol urate.mg protein-1.s-1) and Na(+)-independent. However, 2-oxoglutarate stimulated uptake in Na(+)-containing media. These results suggest that urate uptake across the basolateral membrane occurs via a specific, Na(+)-independent transport system that may operate in the exchange mode accepting 2-oxoglutarate as countertransported substrate. In vivo, urate uptake thereby would be a tertiary active system driven by a 2-oxoglutarate gradient established across the cell membrane by the operation of a Na(+)-2-oxoglutarate cotransport system.
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AT
Universität Düsseldorf, Institut für Tierphysiologie, Germany.
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Kinne-Saffran
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Grieshaber
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D000818:Animals; D000838:Anions / Q000494:pharmacology; D001692:Biological Transport; D002412:Cations / Q000494:pharmacology; D007474:Ion Exchange; D007656:Ketoglutaric Acids / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D008121:Nephropidae / Q000378:metabolism*; D009995:Osmosis; D010179:Pancreas / Q000166:cytology / Q000378:metabolism*; D013379:Substrate Specificity; D013997:Time Factors; D014527:Uric Acid / Q000378:metabolism*
D000838:Anions; D002412:Cations; D007656:Ketoglutaric Acids; D014527:Uric Acid