Clinical meaning
Vitamin B12 (cobalamin) serves as an essential cofactor for two critical enzymatic reactions. First, methylmalonyl-CoA mutase requires adenosylcobalamin to convert methylmalonyl-CoA to succinyl-CoA in the mitochondria; when B12 is deficient, methylmalonic acid (MMA) accumulates because this conversion is impaired. Second, methionine synthase requires methylcobalamin to convert homocysteine to methionine while simultaneously regenerating tetrahydrofolate (THF) from methyltetrahydrofolate; B12 deficiency traps folate in the methylTHF form (the methyl-folate trap), impairing thymidylate and purine synthesis needed for DNA replication, causing megaloblastic changes. Serum B12 levels have limitations as a standalone diagnostic test: the normal range (200-900 pg/mL) includes a gray zone (200-400 pg/mL) where functional deficiency may exist despite borderline-normal levels because serum B12 measures total cobalamin bound to both haptocorrin (80%, biologically inactive) and transcobalamin II (20%, the active delivery protein). Methylmalonic acid (MMA) is the most specific functional marker for B12 deficiency because it is elevated only in B12 deficiency (not folate deficiency). Homocysteine is elevated in both B12 and folate deficiency, making it sensitive but not specific for B12 alone. In pernicious anemia, intrinsic factor antibodies (blocking and binding types)...