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Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2023.

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Figure 1. . Intracellular metabolism of cobalamin.

Figure 1.

Intracellular metabolism of cobalamin. The intracellular cobalamin metabolism and related pathways – including the complementation groups and corresponding genes – are shown.

Endocytosis of cobalamin bound to its blood carrier transcobalamin (TC) occurs through the transcobalamin receptor (TCblR). Inside the lysosome, cobalamin is released from transcobalamin and then transported into the cytoplasm where it undergoes enzyme-mediated reduction (Cbl III to Cbl II) and then mitochondrial adenosylation to form adenosylcobalamin (AdoCbl) or cytosolic methylation to form methylcobalamin. The color-coded boxes around the cobalamin-processing enzymes indicate their role in causing: (1) isolated AdoCbl deficiency and associated increase in MMA (blue); (2) isolated MeCbl deficiency and hyperhomocysteinemia (green); (3) both AdoCbl and MeCbl deficiencies causing elevations in MMA and homocysteine (purple).

From: Disorders of Intracellular Cobalamin Metabolism

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