Placental Transport of Free Amino Acids

Abstract

Measurement of the rate of accumulation of nitrogen in the foetus during gestation is difficult and has not been made in many species. The difficulties have been clearly described by Dr. Elsie Widdowson (I) who provided the first figures for the human infant, which contains about 15 g protein at 20 weeks gestation and 500 g. at term. These values give an indication of the net transfer of amino nitrogen from the mother to the foetus across the placental membrane throughout gestation (2, 3). Our knowledge of some of the mechanisms of this transport is already twenty years old. Christensen first suggested, and his colleagues subsequently proved, that the qualitative aspects of amino acid transfer across the placental membrane were very similar to the transfer of amino acids across the other cells in the body (4, 5). Active transport was demonstrated by the selective transfer of the natural L-isomer of histidine (6, 7); competitive inhibition between members of the same transport group and saturation of the transport mechanisms were also observed. Finally, transport occurs against a concentration gradient; the foetal plasma levels of amino acids are usually about twice that found in the maternal blood stream but may be as much as five times the maternal concentration in the guinea Pig, (4, 8).