Total versus functional vitamin B12 are affected by levels of vitamin B12 binding proteins

Levels of vitamin B12 binding proteins alter the amount of total and functional vitamin B12. The majority of vitamin B12 is complexed with haptocorrin (HC, light blue boxes) and is not functional. In contrast, vitamin B12 bound to transcobalamin II (TC, yellow boxes) is functional; it can be transported into cells as the holotranscobalamin complex.

(A) Normal levels of vitamin B12 binding proteins HC and TC II, with normal total and functional vitamin B12.

(B) Reduction in HC results in low total vitamin B12, but TC II levels and levels of functional vitamin B12 are normal.