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5-Methyltetrahydrofolate transport in basolateral membrane vesicles from human liver
1Biochemistry Research Laboratory, Department of Veterans' Affairs Medical Center, Nashville, TN 37212-2637.
The American Journal of Clinical Nutrition
|July 1, 1993
Summary
Human liver basolateral membrane (BLM) vesicles utilize carrier-mediated cotransport for 5-methyltetrahydrofolate uptake, primarily driven by hydrogen ions across a pH gradient. This process exhibits saturable kinetics and is inhibited by related folate analogs.
Area of Science:
- Biochemistry
- Cell Biology
- Pharmacology
Background:
- 5-methyltetrahydrofolate is the primary circulating form of folate.
- Understanding its transport is crucial for nutrient absorption and drug interactions.
Purpose of the Study:
- To investigate the mechanism of 5-methyltetrahydrofolate transport across the human liver basolateral membrane (BLM).
Main Methods:
- Vesicle transport assays using isolated human liver BLM.
- Kinetic analysis (Km, Vmax) under varying pH conditions.
- Inhibition studies with structural analogs.
Main Results:
- Uptake occurred primarily via intravesicular transport, with minor membrane binding.
- Transport was significantly enhanced by an outward-to-inward pH gradient (pHout=5.0, pHin=7.5).
- pH gradient-driven transport exhibited transient overshoot and saturable kinetics (Km=0.55 µmol/L, Vmax=1.98 nmol/g protein/s).
- Transport was inhibited by 5-formyltetrahydrofolate, folic acid, and methotrexate.
- Transport was electroneutral.
Conclusions:
- 5-methyltetrahydrofolate transport in human BLM vesicles is mediated by a carrier system.
- The mechanism involves cotransport with hydrogen ions (H+).
- This carrier-mediated process is essential for folate homeostasis in the liver.