MECHANISM OF PH-MODULATED TRANSPORT OF 5-METHYLTETRAHYDROFOLATE IN RAT JEJUNAL EVERTED SACS

Absorption and transport of 5-methyltetrahydrofolate, the dietary form of the vitamin after intestinal hydrolysis, was studied in rat everte d small intestinal segments. The folate transport from the mucosal to the serosal compartment was pH dependent and optimal at experimental p H4.0, and the transport was only 40-50% of that level at the physiolog ical pH of the gut, i.e., 6.5-7.0. The methyltetrahydrofolate transpor t to the serosal site was not linked with any cotransport with H+, but the optimum transport was dependent on the mucosal acidic pH of 4 irr espective of the serosal pH. Methyltetrahydrofolates were, however, bo und to their mucosal cell receptor optimally at pH 6.5. Addition of AT P at the mucosal site enhanced mucosal to serosal folate transport at pH 6.5 of the gut. A role for an intestinal mucosal Mg+-ATPase in the hydrolysis of ATP and formation of an acidic microclimate were observe d. Sodium azide, an inhibitor of Mg+-ATPase, hindered methyltetrahydro folate transport at pH 6.5 but not at pH 4.0. The overall results sugg est that 5-methyltetrahydrofolate is absorbed in the intestine by a pa ssive process in an acid microclimate at pH 4.0 generated by intestina l mucosal brush-border Mg+-ATPase.