Figure 9 .

Cobalamin uptake and utilization in the cell. The TC protein carries absorbed Cbl to the TCblR on the cell surface. The TC-Cbl complex is internalized, TC is degraded in the lysosome, and free Cbl is released in the cytoplasm. Some of the Cbl is converted to methyl Cbl and participates as a cofactor for the enzyme. Additional Cbl is transported into the mitochondria and converted to 5′-deoxyadenosyl Cbl (AdoCbl) for utilization in the MMCoA mutase dependent conversion of MMCoA to succinyl CoA. Production of succinyl-CoA to feed the TCA cycle and optimum mitochondrial function appear to be critical determinants for the production of glycolipids and sphingomyelin.