JBC, Vol. 251, Issue 10, 2945-2951, May, 1976
Synthesis of human placental lactogen and human chorionic gonadotropin by polyribosomes and messenger RNA's from early and full term placentas
M. Chatterjee, B. S. Baliga and H. N. Munro
Synthesis of human placental lactogen (hPL) and of human chorionic
gonadotropin (hCG) by membrane-bound and free polyribosomes from early and
from full term human placentas was investigated by in vitro release of the
nascent hormone peptides, followed by immunoprecipitation and
electrophoresis in sodium dodecyl sulfate gels, and by specific binding of
125I-labeled hPL antibody to nascent peptide chains. In addition, messenger
RNA'S Were extracted from total, free, and membrane-bound placental
polyribosomes and their capacities for hPL and hCG synthesis were measured
in a heterologous cell-free system prepared from wheat germ. Membrane-bound
polyribosomes from full term placentas were several times more active in
the synthesis of both peptide hormones than were free polyribosomes. By
binding 125I-labeled hPL antibody to nascent chains on the polyribosomes,
it was determined that hPL is made by clusters of seven to nine ribosomes.
About 8% of the nascent peptide chains released by incubation of
polyribosomes from full term placentas was accounted for by hPL, and 2% by
hCG. In contrast, no chains of hPL were released by polyribosomes from
10-week placentas, whereas 11% of the total released chains were accounted
for by hCG. When messenger RNAs prepared from the polyribosomes of 20- and
40-week placentas were used to stimulate protein synthesis in a wheat germ
system, hPL accounted for 0.4 and 2%, respectively, of total protein
synthesis, while hCG was 8 and 2%, respectively. This confirmed the
relative proportions observed for nascent chains on the polyribosomes of
early and late placentas. Unexpectedly, translation of mRNA from free
polyribosomes yielded as much hPL and two-thirds as much hCG as did
translation of mRNA from bound polyribosomes. We conclude, that the
decreased blood levels of hCG and increased blood levels of hPL with
advancing gestational age reflect the relative in vitro rates of synthesis
of these hormones by placental polyribosomes, the abundance of which is
determined by availability of their respective messenger RNAs at different
times in gestation.
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