Intestinal transport of 5-methyltetrahydrofolate

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Abstract
Enzymatically prepared 5-methyltetrahydrofolate (5-CH3THF) was used to characterize transport by rat intestinal loops in vivo and everted jejunal sacs in vitro. Transport of 5-CH3THF is saturable (Km = 5.2 .mu.M) and highly pH dependent, with the rate of maximal transport occurring at pH 5.8. Transport is competitively inhibited by folic acid (Ki = 4.2 .mu.M) and methotrexate (Ki = 4.65 .mu.M). Metabolic poisons and anaerobiosis greatly reduce 5-CH3THF transport. 5-CH3THF transport in the rat intestine evidently occurs by the same structure-specific mechanism responsible for the transport of unreduced folic acid and other monoglutamyl folates.