A radioimmunoassay for 5-methyltetrahydrohomofolate

A radioimmunoassay for 5-methyltetrahydrohomofolate has been developed by using antibody induced in rabbits by 5-methyltetrahydrohomofolate-bovine serum albumin conjugates. The labeled drug was prepared by condensing it with [3H]histamine or [125I]histamine. The assay employing either isotope was simple and reproducible and had identical sensitivities. The specificity of the antibody was characterized by comparing the effectiveness of various related compounds in displacing labeled 5-methyltetrahydrohomofolate from the binding site of the antisera. At concentrations up to 1000 microgram/ml, homofolate acid, tetrahydrohomofolic acid, folic acid and methotrexate showed no competition for the binding. 5-methyltetrahydrofolic acid and 5-formyltetrahydrofolic acid cross-reacted with the antisera; the concentrations producing 50% binding inhibition were 2.8 and 24 microgram, respectively, as compared to 0.01 microgram for 5-methyltetrahydrohomofolate. The assay can be used for measuring the drug in plasma and tissues. This study supports its usability for clinical pharmacologic studies.