Folates in plasma and bile of man after feeding folic acid—3H and 5-formyltetrahydrofolate (folinic acid)

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Abstract
During the 1st hr after feeding folic acid—3H (3H-PteGlu) to fasting human volunteers, plasma S. faecalis and 3H activity were elevated to an equivalent degree, whereas after this, the 3H activity exceeded S. faecalis activity, which suggests gradual conversion of folic acid—3H to methyltetrahydrofolate-3H (5-CH3H4 PteGlu). The increase of L. casei activity exceeded the increase of S. faecalis and 3H activity, which is consistent with flushing of endogenous methyltetrahydrofolate from the tissues by the administered folic acid—3H. Feeding of 5-formyltetrahydrofolate (±5CHOH4PteGlu) produced a large increase of plasma L. casei activity and only a slight increase of S. faecalis and P. cerevisiae activity, which is consistent with very rapid conversion of folinic acid to methyltetrahydrofolate. Bile folate concentration determined microbiologically was 2.3-9.8 times plasma folate. 40-80% of the bile folate was S. faecalis-active and 20-35% P. cerevisiae-active. Chromatography of bile folates on TEAE-cellulose showed several folates including four tentatively identified as 10-formyltetrahydrofolate (10-CHO-H4PteGlu), 10-formylfolate (10-CHO-PteGlu), and/or 10-formyldihydrofolate (10-CHOH2PteGlu), methyltetrahydrofolate, and possibly a triglutamate folate. After folate ingestion bile folate concentration increased rapidly. The distribution of bile folates measured by microbiological assay was similar after either folic or folinic acid feeding. Most of the 3H label of folic acid—3H appeared in the biological folates of bile rather than in the folic acid fraction, which shows that the administered folic acid was rapidly transformed to other folates. Folate polyglutamate deconjugating enzyme activity was found to be much less than in serum. Polyglutamates of the type found in yeast were not found in bile. It is suggested that biliary folate may reflect the hepatic intracellular oligoglutamate folate pool rather than the folate as it appears in the hepatic portal blood.