AMINO-ACID-METABOLISM DURING FETAL DEVELOPMENT

Fetal and placental amino acid metabolism has been studied using multi ple tracer methodology. Comparisons between species have been difficul t because of technical differences that make such comparisons inapprop riate. However, comparisons of the metabolism of different amino acids within the same species, the fetal lamb, are now possible because of the abundance of studies carried out in this species. The role of plac ental metabolism in contributing to the determination of the supply of amino acids to the fetus has been better defined. Some amino acids su ch as glutamate are catabolized significantly within the placenta, oth ers are transported with little placental catabolism. The organ specif icity noted in adult metabolism for specific amino acids is now well e stablished in fetal life as well. Fetal hepatic metabolism is characte rized by a large uptake of most amino acids and by a net release of gl utamate and serine. Amino acid uptake by the hindlimb differs signific antly from postnatal life not only in the magnitude of the amino acid uptake but also by the fact that amino acids normally released from th e hindlimb in postnatal life such as alanine and glutamine are taken u p by these tissues in fetal life. The impact of changes in the fetal b iologic state introduced by hypoxia or by undernutrition are just begi nning to be investigated in terms of the flux of amino acids to protei n synthesis versus catabolism and oxidation.