Vitamin B12 binding proteins in bovine serum.

Examination of bovine serum by the diethylaminoethyl cellulose small column method revealed three proteins binding vitamin B12. The elution pattern suggested that they are similar to the three transcobalamins recognized in human serum. Distribution of unbound binding capacity among serum binders was assessed in serum from normal, ketotic, and B12- and Factor B-supplemented cows in early lactation. No major differences were observed among groups; however, cow serum displayed a pattern different from human serum. Mean total binding capacity of bovine serum for B12 as well as mean unbound binding capacity were lower than the corresponding means for human serum.