Transfer of cyst(e)ine and methionine across the human placenta

Abstract

After intravenous loads in pregnant women, L-methionine, L-leucine, and L-ornithine were transferred from maternal to fetal plasma against a two- to threefold difference in initial concentration. Cyst(e)ine is unique among the free amino acids of plasma in that its basal concentration in maternal plasma was equal to or greater than that in fetal plasma. Furthermore, after intravenous loads with L-cystine or L-cysteine, total cyst(e)ine (cystine and cysteine) was transferred less readily to the fetal plasma. Although the concentrations of cystine in the fetal plasma continued to rise in the face of rapidly falling concentrations of cystine in the maternal plasma, at no time during the experiment did the concentration of cystine in fetal plasma exceed that in the maternal plasma. When D-cystine was administered intravenously to a mother, in amounts equimolar with L-cystine, the transfer of D-cystine was not measurable. © 1973 International Pediatric Research Foundation, Inc.