Vitamin B 12 Binding Proteins in the Human Embryo

No data has been published on the unsaturated [that is, without vitamin B₁₂ (B₁₂)] binding proteins in embryos of any species. This is in contrast to the many studies which have elucidated these proteins in adults 1 , 2 , 3 , 4 , 5 . In humans, three proteins are present which bind B_12. These are intrinsic factor (I.F.) with a molecular weight of 55,000 daltons produced in the gastric mucosa and functioning in transporting B₁₂ across the intestinal mucosa; transcobalamin II (TC-II) with a molecular weight of 35,000 transports B₁₂ into cells of peripheral tissues, and transcobalamin I (TC-I) which has a molecular weight of 110,000 and whose function is not understood 1 . The sites of synthesis of TC-I and TC-II are not known. TC-I and TC-II are found in serum. In addition, TC-I is found in many body fluids including gastric juice, saliva, and milk 1 . Evidence has been presented that a third serum binder transcobalamin III with distinctive charge properties exists 6 , 7 , 8 . It is not certain, however, that this is not one of the above glycoproteins with different sugars producing the different charge characteristics.

The extensive work of Kumento has shown that cord serum from term human infants contains TC-III and a protein with a molecular weight of 110,000 which differs in charge from TC-I, but which, as discussed above, may be a very similar molecule 9 . The present work was undertaken to establish the sequence of appearance of the above proteins during human development. As such, it presents the first information of embryonic vitamin B₁₂ binding proteins of any species. We establish that in the earliest samples studied embryonic stomachs contain proteins similar if not identical to I.F. and TC-I.