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Table 1

Specific folic acid binding in R5 and HeLa cells

Folic acid binding was assessed by exposing cells to 5 nm [3H]folic acid in the presence or absence of 500 nm nonlabeled folic acid at 4°C for 15 min. Folic acid bound to the cell surface was released by pH 3.5 buffer. Specific folic acid bound is the difference in tritium released in the presence and absence of 500 nm unlabeled folic acid. Data are expressed in units of pmol/mg of protein based on the average of three independent experiments ± SE.

Cell line[3H]Folic acid bound (A)[3H]Folic acid bound in the presence of 500 nm non-labeled folic acid (B)Specific [3H]folic acid bound (AB)
HeLa in folic acid medium 0.38 ± 0.06 0.014 ± 0.002 0.37 
R5 in folic acid medium 0.15 ± 0.03 0.009 ± 0.001 0.14 
HeLa in 5-CHO-THFa medium 0.59 ± 0.09 0.02 ± 0.002 0.57 
R5 in 5-CHO-THF medium 0.37 ± 0.05 0.014 ± 0.001 0.36 
Cell line[3H]Folic acid bound (A)[3H]Folic acid bound in the presence of 500 nm non-labeled folic acid (B)Specific [3H]folic acid bound (AB)
HeLa in folic acid medium 0.38 ± 0.06 0.014 ± 0.002 0.37 
R5 in folic acid medium 0.15 ± 0.03 0.009 ± 0.001 0.14 
HeLa in 5-CHO-THFa medium 0.59 ± 0.09 0.02 ± 0.002 0.57 
R5 in 5-CHO-THF medium 0.37 ± 0.05 0.014 ± 0.001 0.36 
a

5-CHO-THF, 5-formyltetrahydrofolate.

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