Cellular folate vitamer distribution during and after correction of vitamin B12 deficiency: a case for the methylfolate trap.

Haematological sequellae of vitamin B12 deficiency are attributed to disturbed DNA synthesis, but vitamin B12 itself plays no role in DNA biosynthesis. A proposed explanation for this is the methylfolate trap hypothesis. This hypothesis states that B12 deficiency impairs overall folate metabolism because 5-methyltetrahydrofolate (5MTHF) becomes metabolically trapped. This trap results from the fact that 5MTHF can neither be metabolised via the methionine synthase pathway, nor can it be reconverted to its precursor, methylenetetrahydrofolate. Other manifestations of the methylfolate trap include cellular folate loss because of shorter 5MTHF polyglutamate chains and global hypomethylation. The methylfolate trap has never been demonstrated in humans. We describe a patient with B12 deficiency who was homozygous for the common methylenetetrahydrofolate reductase (MTHFR) C677T mutation. We analysed red blood cell (RBC) folate vitamers and global DNA methylation by liquid chromatography (LC) in combination with tandem mass spectrometry, and 5MTHF polyglutamate length by LC-electrochemical detection. Compared to post-B12 supplementation values, homocysteine was higher (52.9 micromol/l vs. 16.8 micromol/l), RBC folate was lower (268.92 nmol/l vs. 501.2 nmol/l), the 5MTHF fraction of RBC folate was much higher (94.5% vs. 67.4%), polyglutamate chain length was shorter (more tetra- and pentaglutamates), and global DNA methylation was 22% lower. This is the first time that virtually all features of the methylfolate trap hypothesis have been demonstrated in a human with vitamin B12 deficiency.

[6S]5-methyltetrahydrofolate or folic acid supplementation and absorption and initial elimination of folate in young and middle-aged adults.

OBJECTIVES: To assess the effects of supplementation with the diastereoisomer of 5-methyltetrahydrofolate ([6S]5-methylTHF), as an alternative supplement for folic acid, on folate absorption and elimination, in two age groups. DESIGN: A randomized, double-blind intervention study. SUBJECTS: A total of 12 young (<30 y) and 12 middle-aged (> or =50 y) healthy volunteers were recruited. METHODS: Volunteers were randomized to receive daily supplementation with 400 mug folic acid or equimolar amounts of [6S]5-methylTHF during 5 weeks. Before and after supplementation, absorption and initial elimination were calculated following oral [(2)H(2)]folic acid test doses using isotope kinetics in plasma. RESULTS: Folic acid absorption was lower in the middle-aged as compared to the young adults, both before (P = 0.03) and after (P = 0.05) supplementation. In the young adults, absorption decreased by 22% after [6S]5-methylTHF and increased by 21% after folic acid (P = 0.02). In the other age group, no such changes were found. The folate rate constant of elimination increased after folic acid supplementation in the young (+50%; P = 0.05) but not in the middle-aged (+18%; P = 0.5) adults. CONCLUSIONS: Young adults show increased folate turnover after folic acid supplementation relative to the effect of [6S]5-methylTHF supplementation. Similar differences are not observed in middle-aged adults, in whom folic acid absorption was found to be lower as compared to the young adults. SPONSORSHIP: Financial support was received from the European Union 5th Framework Programme (Grant QLRT-1999-00576).