Background The fetus is highly dependent on the placenta for its nutrient and oxygen supply. Normal placental metabolism plays a critical role in placental functioning and the energy demanding process of nutrient transport.
Objective To quantify the in vivo synthesis rate of mixed placental proteins in human pregnancies during different periods of gestation.
Methods Pregnant women received three continuous stable isotope infusions ([1-13C]leucine, [1-13C]phenylalanine, [U-13C5]valine) starting 4, 3, and 2 hours prior to cesarean section, respectively. Placental tissue samples were washed and frozen. Enrichments of amino acids incorporated in placental proteins were determined and the mean enrichment of the free amino acids in maternal and umbilical plasma was used as precursor. The product/precursor enrichment ratios were plotted against the duration of the respective tracer infusion. The % of the total placental protein pool that is daily renewed (fractional synthesis rate, FSR) was calculated using the slope of this trend line.
Results We included 11 pregnant women who gave birth to 16 non-identical infants (8 term, 1 singleton at 31 wks, 1 triplet at 35 wks, and 1 quadruplet at 28 wks). Infants were born solely on maternal indication. FSR decreased slightly with gestational age (see fig).
Conclusions The placenta has a high protein turnover rate, comparable to liver, which diminishes at the end of gestation. This rate may be necessary for optimal functioning and active nutrient transport to the fetus.