Intestinal Conversion of Folinic Acid to 5‐Methyltetrahydrofolate in Man

Abstract
Summary. In three adult subjects undergoing diagnostic umbilical vein catheteriza‐tion, the active stereoisomer of folinic acid (dl,‐5‐formyltetrahydrofolate) was metabolized during absorption and appeared in hepatic portal venous plasma as 5‐methyltetrahydrofolate. A small amount of 5‐formyltetrahydrofolate and probably of 10‐formylfolate was detected in hepatic portal venous plasma early during absorption as well.Peripheral venous plasma folate changes in these subjects and in two achlorhydric subjects fed folinic acid were similar, suggesting that isomerization of 5‐formyl‐ to 10‐formyltetrahydrofolate by gastric acid was not necessary for its subsequent metabolism by the intestinal cell.Similar increments in peripheral venous plasma folate followed feeding of comparable doses of folinic acid and folic acid, evidence that folinic acid may be effectively administered by mouth. When so administered, the effect in respect of all tissues except the intestine is essentially equivalent to the feeding of 5‐methyltetra‐hydrofolate itself.