In vitamin B12 deficiency, higher serum folate is associated with increased homocysteine and methylmalonic acid concentrations.

To the Editor: In the December 11, 2007, issue of PNAS, Selhub et al. (1) suggest that the association between elevated serum folate and methylmalonic acid (MMA) concentrations in vitamin B12 deficiency resulted from disruption of MMA metabolism by folate. An alternative explanation is that the increase in both serum folate and MMA concentrations resulted from the severity of the vitamin B12 deficiency. An association between MMA and vitamin B12 is expected (2)—an association between vitamin B12 deficiency and elevated folate concentrations is not. However, several studies (e.g., refs. 3 and 4) show elevated serum folate concentrations associated with vitamin B12 deficiency; folate is present in the serum as monoglutamates (predominantly 5-methyl-tetrahydrofolate). To maintain a serum-tissue concentration gradient (5), folate monoglutamates are elongated to polyglutamates within the tissue. Because 5-methyl-tetrahydrofolate is a poor substrate (3, 5) for folate polyglutamate synthase, it must be demethylated to tetrahydrofolate (in a vitamin B12-dependent reaction) before being polyglutamated. In severe vitamin B12 deficiency, as might be marked by elevated serum MMA, 5-methyl-tetrahydrofolate is inefficiently demethylated, and cannot be polyglutamated (4) nor retained by the cell (i.e., the ‘‘methyl-trap’’ hypothesis). Consequently, the serum-tissue concentration gradient collapses (4). Because serum folate is no longer efficiently removed by the tissue, its concentration increases (3, 4). Conversely, tissue folate concentrations (4), particularly the more biologically active polyglutamates (3), decrease. This reduction, coupled with the vitamin B12 deficiency, decreases homocysteine remethylation, which increases serum homocysteine concentrations. In conclusion, the severity of the vitamin B12 deficiency may explain the observed association among serum folate, homocysteine, and MMA concentrations.