AJP - Endocrinology and Metabolism, Vol 271, Issue 3 E535-E540, Copyright © 1996 by American Physiological Society
Mechanism of clearance and transfer of dipeptides by perfused human placenta
S. A. Adibi, S. Schenker and E. Morse
Department of Medicine, University of Pittsburgh School of Medicine, Pennsylvania 15213, USA.
Glycylglutamine (Gly-Gln) is stable source of glutamine for parenteral
nutrition. In the present study we have investigated whether this dipeptide
is transferred intact across the human placenta. Although after 90 min of
placental perfusion there was almost complete disappearance of Gly-Gln (100
microM) from the maternal compartment, only a small concentration of this
dipeptide (< 6 microM) appeared in the fetal compartment. To investigate
whether this transfer was due to transcellular transport, brush-border
membrane vesicles of the human placenta were probed with [3H]Gly-Gln, which
showed no uptake. To investigate whether hydrolysis was the mechanism of
disappearance of Gly-Gln, the perfusion study was repeated with
glycylsarcosine (Gly-Sar), which is resistant to hydrolysis. In sharp
contrast to Gly-Gln, after 90 min of perfusion nearly 80% of Gly-Sar
remained in the perfusate (half-life of 24 vs. 235 min). The rest of the
Gly-Sar was recovered intact in the fetal compartment. The addition of
Gly-Gln to the maternal compartment increased the accumulation of glycine,
but not glutamine, in both the maternal and fetal compartments. In
conclusion, our data suggest that 1) the mechanism of clearance of Gly-Gln
by perfused human placenta is largely hydrolysis, whereas that of Gly-Sar
is largely passive diffusion, and 2) the placenta has a greater preference
for glutamine than for glycine.
