On the mechanisms of maternofetal transfer of human albumin and gamma-G globulin in the mouse.

Human serum albumin and human gammaG globulin were labeled with (131)I, and the labeled proteins were then mixed with different amounts of the respective unlabeled protein. These mixtures were injected intravenously into pregnant mice near term, and the amounts of protein-bound radioactivity present in the fetuses and in maternal serum 24 hr later were determined. The concentration of human albumin found in the fetus was proportional to the maternal serum concentration of this protein over the maternal range studied, from 0.03 to 935 mg/100 ml. On the other hand, the fetal concentration of human gammaG first increased rapidly as the maternal concentration increased to approximately 200 mg/100 ml and then decreased as the maternal concentration continued to increase above this level; however, as the maternal human gammaG level increased above approximately 1100 mg/100 ml, the fetal concentration again increased and became proportional to the maternal concentration. The data suggest that maternofetal transfer of human gammaG in the mouse may be mediated by two processess; one of these, as with the transfer of human albumin, appears to be first order in relation to the maternal serum concentration, and the other appears to be consistent with a carrier or enzymatic process that is directly or indirectly inhibited at high maternal serum levels.