Cobalamin-related redox metabolic pathways in neuronal cells.
Endocytosis brings TC-bound Cbl species to lysosomes where axial ligands are removed by MMACHC and MeCbl or AdoCbl are subsequently formed by SAM and ATP-dependent pathways, respectively. MeCbl is a required cofactor for methionine synthase, whose activity supports a large number of methylation reactions, including DNA methylation, as well as dopamine-stimulated phospholipid methylation, carried out by the D4 dopamine receptor (D4R). AdoCbl supports MMACoA mutase in mitochondria. Cysteine, which is rate-limiting for GSH synthesis, can be provided either by cellular uptake via the cysteine/glutamate transporter EAAT3 (excitatory amino acid transporter 3) or by transsulfuration of HCY via cystathionine. The latter pathway is restricted in human brain, increasing the importance of growth factor-dependent cysteine uptake by EAAT3.