THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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found products of molecular weights 2I ,OOO and 19r000; the
latter is thus smaller than authentic hpl, (21 ,600), and it,
is consequentJ.y difficult to accept the fo¡mer as the
precursor molecule. Furthermore. Chat.terjee et al (1926)
were unable to identify the larger hormone molecule erhen
placental nRNA vras translated by the r.rrheat-germ system.
They also found by means of binding of l25r_1.beL1ed anti_
hPt that the polyribosome clusters with hpL peptide chains
vrere compatible with a product of M.$I. 2l,0OO. However,
Berken et. aI (L977) have recently identified a sequence of
19 a.a. at the N-terminus of the hpI, peptide formed by
translating hpl mRNA in a v¡heat-germ system. This sequence
was similar in size and high leucine content to the signal
peptide proposed by Blobe1 and Dobberstein (1925) as a
co¡nmon feature of the precgrsor or secreted proteíns.
Thus, the occurrence of a precursor form of hpl, appeared
to be demonstrable under some special conditions of translation.
vii)
In contrast to total protein released
into the medium, which occurred at a constant rate, hpI.
release varied from one placenta to another (Suwa and
FrÍesen, 1969b). ThÍs may account for the variable hpÏ,
levels in the serum of different pregnant ì\romen. ControL
mechanisms specific to hpl. release may exist, as suggested
by the observation (Gaspard and Franchímont I I9Z2) that
placental sLices incubated in vitro rapidly decrease in t,their
ability to synthesize hpl while maintaining hCG secretion.