Vitamin B12 (B12) deficiency may cause severe clinical complications, partly attributable to the “methylfolate trap”. When B12-dependent methionine synthase activity is impaired, 5-methyltetrahydrofolate (5-MTHF) becomes trapped because it cannot be used in the methionine synthase reaction or reconverted to 5,10-methylenetetrahydrofolate. This reduces the availability of folate cofactors, including tetrahydrofolate (THF), 5,10-methylene-THF, and 10-formyl-THF required for histidine catabolism, pyrimidine metabolism, and de novo purine synthesis (DNPS). We investigated whether this mechanism is reflected in urinary DNPS intermediates. Using LC–MS/MS, we quantified the histidine catabolite formiminoglutamate (FIGLU) and urinary DNPS intermediates in 28 patients with confirmed B12 deficiency and age-stratified controls. Serum methylmalonic acid (MMA) served as a proxy for B12 deficiency severity. Compared with controls, urinary FIGLU and DNPS intermediates—glycinamide ribonucleoside (GAr) and 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr)—were significantly increased across all age groups (p < 0.05), while succinyl-5-aminoimidazole-4-carboxamide ribonucleoside (SAICAr) was also increased, except in adults. These changes were more evident in severe B12 deficiency—defined as MMA ≥ 10× the age-specific upper limit (UL): THF depletion was reflected by marked FIGLU elevation (up to several dozen multiples of UL), while reduced folate cofactor availability was demonstrated by increased GAr, SAICAr, and AICAr (up to 3.4-, 2.7-, and 5.0-fold UL, respectively). In linear correlation analyses, urinary FIGLU correlated with serum MMA, and GAr, SAICAr, and AICAr correlated with FIGLU, a marker of THF deficiency (p < 0.05). These findings are consistent with methylfolate trapping, linking B12 deficiency to reduced availability of folate cofactors for histidine catabolism and DNPS.
Krijt et al. (Wed,) studied this question.