In vivo plasma and urine folate binding after ingestation of 3H-folic acid and 14C-methyl-folate.

After simultaneous ingestion of equivalent amounts of [3H]folic acid (3H-PteGlu) and [14C]N5-methyl-tetrahydrofolic acid (14C-CH3-H4PteGlu) we were able to demonstrate progressive macromolecular binding of radiofolate in plasma, which appeared to be near maximal at 6 h. Bound radiofolate was predominantly of 14C-CH3H4PteGlu origin, and only at 24 h could 3H incorporation be demonstrated. The binder eluted with albumin from Sephadex DEAE-A50 columns. In urine a smaller bound radiofolate fraction, with approximately equal amounts of 3H and 14C, appeared after 5.5 h. Plasma chromatography showed radio-PteGlu (peak 1) to be rapidly converted to CH3-H4PteGlu (peak 2), with subsequent appearance of two further radiofolate peaks (peaks 3 and 4) the nature of which is as yet unclear. Urine showed similarly placed fractions but their magnitude differed, and urinary peak 3 in particular was much more prominent than its plasma counterpart.