1. [3H]Folic acid infused intravenously into patients with biliary fistulae appeared promptly in bile, coincident with 131I-labelled Rose Bengal injected simultaneously. The radioactivity was distributed among several fractions of biological folates present in bile and was associated with folic acid and with an unidentified folate which chromatographed on DEAE-Sephadex close to 10-formyltetrahydrofolyl monoglutamate.

2. Based on affinity for DEAE-Sephadex and support of growth of Lactobacillus casei, Streptococcus faecalis and Pediococcus cerevisiae, we have tentatively identified some of the folates of human bile as 10-formyltetrahydrofolate, 10-formylfolate, 5-formyltetrahydrofolate, 5-methyltetrahydrofolate and tetrahydrofolate or 5,10-methylene tetrahydrofolate. After infusion of folic acid, the formyltetrahydrofolates increased more rapidly than did 5-methyltetrahydrofolate.

3. The unidentified radioactive folate contained both the pteridine and p-aminobenzoate portions of folate. It appeared not to support growth of the test microorganisms and not to be bound to a protein or a chain of γ-glutamates. It was present in the bile of a dog injected with [3H]folic acid but was absent from extracts of liver. This material may be a transport form of folate or a special modification imposed on folic acid during transport across the liver.

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