Aspects of radiofolate absorption, metabolism and plasma binding

After 14C-methyl folate (14C-MeTHF) was taken by mouth, progressive incorporation of this istope into the dialysis-resistant plasma folate fraction occurred. At 6 hours 68,9% of the total plasma radioactive folate was dialysis-resistant. We have previously shown that 14C-folic acid (14C-PGA) taken by mouth is not similarly bound at 6 hours. Chromatography of plasma on DEA A50 after 14C-PGA absorption, showed that PGA in plasma (peak 1) was gradually converted to MeTHF (peak 2) and the absence of bound radiofolate 6 hours after 14C-PGA ingestion probably reflects this conversion phase. No radiofolate appeared in red cells up to 11 days after isotope ingestion. Initial divergence between plasma biofolate and radiofolate indicated that 'cold' storage folate was being displaced by abosrbed radiofolate. Urinary radiofolate resolved into 3 fractions (peaks 2, 3 and4) on DEAE A50 chromatography. One of these (peak 2) corresponded to MeTHF, but PGA (peak 1) was absent. Plasma showed peaks 1, 2 and 3, but at 3 hours no equivalent of urinary peak 4 was evident. Further studies are indicated to characterise fractions 3 and 4.