1. Binding of tritiated folic acid to rat liver cell cytoplasmic proteins has been studied in vitro and in vivo by Sephadex G75 column chromatography.
2. Studies in vitro show that folic acid is bound mainly in the ‘Y’ and ‘Z’ protein fractions, which have previously been described as binding other organic anions, e.g. bilirubin, sulphobromophthalein.
3. At 10 min after injection of tritiated folic acid in vivo, radioactive folates in liver cytoplasm were largely bound to X and Z protein fractions, with a smaller fraction bound to the Y fraction. At 24 and 48 h, radioactive folates were bound largely to the X and Y protein fractions.
4. It is suggested that the Y and Z proteins or closely related proteins may be involved in transfer of folates from plasma to liver, the major store of folate in the body.