Defect in Vitamin B 12 Release from Lysosomes: Newly Described Inborn Error of Vitamin B 12 Metabolism

Cultured diploid fibroblasts from a patient with a previously undescribed inborn error of cobalamin metabolism accumulate unmetabolized, nonprotein-bound vitamin B 12 in lysosomes. These cells are able to endocytose the transcobalamin II-B 12 complex and to release B 12 from transcobalamin II. The freed vitamin B 12 is not released from lysosomes into the cytoplasm of the cell. This suggests that there is a specific lysosomal transport mechanism for vitamin B 12 in the human.