Studies of the role of a particulate folate-binding protein in the uptake of 5-methyltetrahydrofolate by cultured human KB cells.
The characteristics of the uptake by human epidermoid carcinoma (KB) cells of 5-methyltetrahydrofolate at extracellular concentrations in the physiologic range and the possible role of a membrane-associated folate binder in folate uptake by KB cells have been investigated. Uptake of 5-methyltetrahydrofolate was specific, saturable, and time-, temperature-, and concentration-dependent. Trypsin treatment released 50% of the 5-methyltetrahydrofolate accumulated by KB cells at 4 degrees C, but only 12% at 37 degrees C, indicating that most of the accumulated ligand was intracellular at 37 degrees C, thus demonstrating transport. Accumulated 5-methyltetrahydrofolate was bound to a membrane-associated protein which required detergent for its solubilization, and a significant amount of which was oriented to the cell exterior as demonstrated by its release by trypsin treatment of intact KB cells. The membrane-associated folate binder was immunoprecipitated by antiserum to purified human placental folate receptor, and this antiserum inhibited 5-methyltetrahydrofolate uptake by intact KB cells in a concentration-dependent manner. These data support the hypothesis that the membrane-associated folate-binding protein of human cells participates in the transport of folates under physiologic conditions.
D002294:Carcinoma, Squamous Cell / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000378:metabolism; D006801:Humans; D008970:Molecular Weight; D011956:Receptors, Cell Surface; D013696:Temperature; D013763:Tetrahydrofolates / Q000378:metabolism*; D013997:Time Factors; D014357:Trypsin / Q000378:metabolism
D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D011956:Receptors, Cell Surface; D013763:Tetrahydrofolates; D005492:Folic Acid; D014357:Trypsin; C005984:5-methyltetrahydrofolate