Measurement of the Amounts of the Individual Vitamin B₁₂ Binding Proteins in Plasma : I. Studies of Normal Plasma

The findings of the present study, when combined with those of previous studies, support the following concept of binding of vitamin B₁₂ by serum or plasma in vitro. There are primary binders of B₁₂ which preferentially take B₁₂ when small amounts are added and combine firmly with it. They include two already identified substances, TC I and TC II, which are of the seromucoid fraction of plasma. At concentrations of added B₁₂ of the order of 1.0 ng./ml. of plasma, these proteins become saturated and secondary binders become important. The latter are relatively unimportant at lower concentrations of B₁₂; they are weak binders of B₁₂; they cannot be saturated by concentrations used to date, and they are not seromucoids.