Na+ and pH dependence of 5-methyltetrahydrofolic acid and methotrexate transport in freshly isolated hepatocytes.
The dependence of the high-affinity transport systems for 5-methyltetrahydrofolic acid (5-CH3-H4PteGlu) and methotrexate on sodium ions and on pH was examined in freshly isolated rat hepatocytes. Previous studies indicated that transport of these folate derivatives was sodium-dependent. Experiments to determine the Km for sodium of 5-CH3-H4PteGlu transport showed no dependence on extracellular sodium. However, uptake was sodium-dependent when hepatocytes were preincubated for 30 min in sodium-free medium, a treatment which resulted in an increase in the transmembrane pH gradient (delta pH = pH out-pH in) and a decrease in the uptake of 5-CH3-H4PteGlu. Uptake of methotrexate displayed a linear dependence on extracellular sodium ions. Uptake of 5-CH3-H4PteGlu increased linearly as the transmembrane pH gradient decreased; i.e., as the medium became more acid with respect to the cytosol. Lineweaver-Burk and Scatchard plots of 5-CH3-H4PteGlu uptake indicated an apparent Km for H+ of about 24 nM, equivalent to a pH of 7.6. Hill-plots suggested a stoichiometry of 1:1 for the interaction of protons with the 5-CH3-H4PteGlu transport system. Both the Km and Vmax for 5-CH3-H4PteGlu transport were increased at pH 5.5 compared to pH 7.4, suggesting that extracellular protons increased the number of and/or the activity of the membrane carrier. In contrast, methotrexate transport was maximal at pH 7 where the transmembrane pH gradient was zero. These results suggest the possibility that 5-CH3-H4PteGlu may be cotransported along with H+ ions in hepatocytes, although they do not rule out a 'catalytic coupling' whereby protons interact with the carrier to stimulate substrate flux without concomitant H+ transport.
Horne
D W
DW
Biochemistry Research Laboratory, Veterans Administration Medical Center, Nashville, TN.
D000818:Animals; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008727:Methotrexate / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*; D013763:Tetrahydrofolates / Q000378:metabolism*
D013763:Tetrahydrofolates; D012964:Sodium; D005947:Glucose; C005984:5-methyltetrahydrofolate; D008727:Methotrexate