The Vitamin B12 Binding Capacity of Transcobalamin I and II of Normal Human Serum

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Abstract
The unsaturated serum vitamin B12 binding capacity (UB12BC) of transcobalamin O, I and II (TC O, I and II) of 50 normal subjects was determined by gel filtration on Sephadex G‐200. TC O bound from 0 to 76 pg. per ml. of serum, accounting for up to 5.2 per cent of the UB12BC, while TC I bound from 72 to 1057 pg. per ml., or up to 47.1 per cent of the UB12BC. TC II bound from 650 to 1891 pg. per ml., accounting for up to 90.3 per cent of the UB12BC.There was a significant positive correlation between the binding capacity of TC I and of TC II when each was compared with the total unsaturated B12 binding capacity. A positive correlation therefore existed between the binding capacity of TC I and that of TC II.TC O, I and II were the only binders of added B12 over a range from 75 to 20,760 pg. per ml. of serum. The relative amount of B12 bound to TC I and to TC II remained fairly constant in an individual serum regardless of the amount of B12 added.