5,10-Methenyltetrahydrofolate cyclohydrolase, rat liver and chemically catalysed formation of 5-formyltetrahydroflate

The 5,10-methenyltetrahydrofolate (5,10-CH=H(4)folate) synthetase catalyses the physiologically irreversible formation of 5,10-CH=H(4)folate from 5-formyltetrahydrofolate (5-HCO-H(4)folate) and ATP. It is not clear how (or if) 5-HCO-H(4)folate is formed in vivo. Using a spectrophotometric assay for 5-HCO-H(4)folate, human recombinant 5,10-CH=H(4)folate cyclohydrolase, which catalyses the hydrolysis of 5,10-CH=H(4)folate to 10-HCO-H(4)folate, was previously shown to catalyse inefficiently the formation of 5-HCO-H(4)folate at pH 7.3 [Pelletier and MacKenzie (1996) Bioorg. Chem. 24, 220-228]. In the present study, we report that (i) the human cyclohydrolase enzyme catalyses the conversion of 10-HCO-/5, 10-CH=H(4)folate into 5-HCO-H(4)folate (it is also chemically formed) at pH 4.0-7.0; (ii) rat liver has a very low capacity to catalyse the formation of 5-HCO-H(4)folate when compared with the traditional activity of 5,10-CH=H-4 folate cyclohydrolase and the activity of the 5, 10-CH=H, folate synthetase; and (iii) a substantial amount of 5-HCO-H(4)folate reported to be present in rat liver is chemically formed during analytical procedures. We conclude that (i) the cyclohydrolase represents some of the capacity of rat liver to catalyse the formation of 5-HCO-H(4)folate; (ii) the amount of 5-HCO-H(4)folate reported to be present in rat liver is overestimated (liver 5-HCO-H(4)folate content may be negligible); and (iii) there is little evidence that 5-HCO-H(4)folate inhibits one-carbon metabolism in mammals.