Abstract. Sera from patients with viral hepatitis were examined for serum vitamin B12 concentration, unsaturated B12 binding capacity, total B12 binding capacity of whole serum and of separate transcobalamin‐I and transcobalamin‐II fractions. It was found that the high serum B12 concentrations observed were associated with an increase in the B12 binding of the transcobalamins. The B12 bound to transcobalamin‐I was found to be two to three times higher than that in normal subjects. The most striking difference was the finding of excessive amounts of B12 bound to transcobalamin‐II, as much as 30 to 40 times the normal. The unsaturated B12 binding capacity of hepatitis sera is usually very low. In addition to the B12 bound to transcobalamins, a considerable part of the elevated serum B12 in hepatitis was in a dialyzable form and was not bound to either transcobalamin‐I or transcobalamin‐II. This dialyzable B12 is apparently bound to a small peptide (M.W less than 10000) which permits its passage through dialysis membrane but makes the B12 molecule unavailable to the microorganism in the bioassay, unless it is released from the peptide complex by boiling.
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