Protein and Amino Acid Metabolism in the Fetal-Placental Unit

Abstract

After completing this article, readers should be able to: 1. Determine the rate of fetal nitrogen accretion and compare it with the fetal growth rate. 2. Characterize fetal urea production on a weight-specific basis. 3. Explain the effects of hypoglycemia on the fetus. 4. Explain the effects of intrauterine growth retardation on the fetal amino acid profile. Protein synthesis and accretion are the cornerstones of growth that provide the structural framework and enzymatic machinery necessary for fetal development. The maternal uterine circulation supplies the components that the fetus uses for protein accretion. All are derived from the maternal diet or maternal tissue stores. It is important to explore the regulatory processes within the maternal, placental, and fetal compartments that determine fetal amino acid retention. In addition to tissue synthesis, use of free amino acids as oxidative fuel is extensive in the well-nourished fetus and increases significantly during periods of maternal food deprivation. The changes in fetal amino acid utilization represent a significant mechanism by which the fetus adapts to energy deprivation. Before examining the available data, it is necessary to understand the models and methodology by which the data are derived. In general, four methods have been applied to fetal nitrogen metabolism: 1) carcass analysis, 2) net umbilical uptake determinations, 3) measurement of the rate of incorporation of a tracer amino acid into fetal protein, and 4) studies of the kinetics of amino acids in the fetal arterial free amino acid pool. Many studies have combined aspects of two or more of the these methods. ### Carcass Analysis For this method, individual fetuses are sacrificed at several points during gestation, and chemical analysis is performed on the carcass. The total amount of nitrogen, amino acid, or protein at that point in gestation can be determined. Data from individual fetuses are pooled to obtain an accretion curve. This …