Effect of pH on vitamin B12 binding by transcobalamins

An investigation has been made of the effect of varying pH at constant ionic strength on vitamin B₁₂ binding by human serum and by two transcbalamin fractions separated from serum by gel filtration. It was found that the methodology used had a considerable influence on the results obtained. Genuine effects of pH were largely confined to reduced vitamin B₁₂ binding at very acid and very alkaline pH. However, due to an adsorption artefact involving transcobalamin II, certain methods appeared to demonstrate a marked decrease in vitamin B₁₂ binding between pH 4.5 and pH 10.3, especially in the range of pH 5.3–7.5.