IN-VIVO PLACENTAL TRANSPORT OF GLYCINE AND LEUCINE IN HUMAN PREGNANCIES

L-[1-C-13]Glycine and L-[1-C-13]leucine were infused as a bolus into 1 2 pregnant patients carrying normal fetuses before fetal blood samplin g at gestational ages ranging from 20 to 37 wk. Maternal venous sample s were obtained every 2-3 min for 15 min after the bolus infusion, Fet al samples were obtained from the umbilical vein within 15 min of the bolus. Amino acid plasma enrichments (molar percent enrichment) were d etermined by gas chromatography-mass spectroscopy and their concentrat ions by ion exchange chromatography. The ratios of glycine and leucine transfer were assessed from fetal/maternal enrichment ratios for each amino acid. We now report that over the gestational age range of 20-3 7 wk, under relatively undisturbed fetomaternal conditions (fetal bloo d sampling), human placental glycine transfer is limited, with a glyci ne/leucine ratio = 0.16 +/- 0.02. We hypothesize that, in human pregna ncies, the relative rates of in vivo transplancental transport of amin o acids can be assessed indirectly utilizing fetal blood sampling and stable isotope methodology. The application of this approach to leucin e and glycine demonstrates that the transfer of leucine is rapid (demo nstrable in seconds), whereas that of glycine is more limited.