Free amino acids in human fetal liver and fluids at 12-17 weeks of gestation

The concentration of 23 free amino acids was measured in homogenates of fet al liver and samples of fetal plasma from 20 pregnancies between 12 and 17 weeks gestation and compared with those found in matched samples of materna l plasma and amniotic fluid. A feto-maternal plasma concentration gradient was observed for 21 amino acids indicating that the feto-maternal amino aci d gradient across the placenta is established from very early in pregnancy. The amino acid concentration pattern was similar in fetal plasma and amnio tic fluid but different in fetal liver, supporting the concept that it is e ssentially placental transport and metabolism that provides the fetus with these molecules. The highest amino acid concentration was found for glutami ne in fetal plasma and glutamic acid in fetal liver. Very low concentration s of glutamic acid in fetal plasma suggest that this amino acid is actively taken up by the fetal liver. Citrulline, alpha-aminobutyric acid, methioni ne, arginine and tryptophan were not measurable in fetal liver tissue, indi cating that this organ has a limited role in utero in the metabolism of the se amino acids, Significant positive correlations were found between fetal plasma and amniotic fluid for the concentration of most amino acids whereas only the concentration of threonine was found to be positively correlated between fetal liver and plasma. These results suggest that during the secon d trimester passive diffusion through the unkeratinized fetal skin is the m ain pathway for amino acids between the fetal circulation and the amniotic cavity.