Tetrahydrofolic acid as the principal congener of plasma folates in pigs.

Concentrations of pig plasma folates were determined by a high-performance liquid chromatography with an electrochemical detector (HPLC-ECD). Tetrahydrofolic acid (THF) and 5-methyltetrahydrofolic acid (5-MF) were found to be the plasma folates. The concentration of THF (10.3 +/- 4.3 ng/ml) was significantly higher than that of 5-MF (3.5 +/- 1.0 ng/ml). The sum of the THF and 5-MF concentrations determined by the HPLC-ECD analysis in Göttingen miniature pigs was comparable to the total folate concentration of the same plasma sample determined by a radioligand assay. No or only trace amounts of THF were found in the plasma of the rats, mice, rabbits, dogs, cows, horses, or humans. Although 5-MF is generally recognized as the only principal plasma folate in many species, THF may be the principal congener with 5-MF as a secondary congener in the pig.