Hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate at pH 2.5 to 4.5

At pH 4.0 to 4.5, 5, 10-methenyltetrahydrofolate is hydrolyzed to only 5-formyltetrahydrofolate if reducing agents are present or iron-redox cycling is suppressed. At pH 4.0, the equilibrium position for this hydrolysis is approximately equal concentrations of both folates. If no reducing agents are used or iron-redox cycling is promoted, considerable amounts of 10-formyldihydrofolate are also formed. It is likely that 10-formyldihydrofolate has been misidentified as 5,10-hydroxymethylenetetrahydrofolate, which was reported to accumulate during the hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate [Stover, P. and Schirch, V. (1992) Biochemistry 31, 2148-2155 and 2155-2164; (1990) J. Biol. Chem. 265, 14227-14233]. Since 5,10-hydroxymethylenetetrahydrofolate is reported to be the viable in vivo substrate for serine hydroxymethyltransferase-catalyzed formation of 5-formyltetrahydrofolate, and 5,10-hydroxymethylenetetrahydrofolate probably does not accumulate, the above folate metabolism is now doubtful. It is hypothesized that mildly acidic subcellular organelles provide an environment for the hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate in vivo, and there is no requirement for enzyme catalysis. Finally, 10-formyltetrahydrofolate is susceptible to iron-catalyzed oxidation to 10-formyldihydrofolate at pH 4 to 4.5.