THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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secretion of a peptide hor:none.
Thus, the first. placenÈal protein hormonê to be
recognized and characterized to some degree was human chorionic
gonadotropin (hCG). Initially in the 1930s, it was called
"prolanrr because of its availability in copious guantit,ies
from urine of pregnant \,romen. It noll¡ has been studied in
terms of its chemistry, physiology, and immunologíca1 properties,
ii) CHEMISTRY Of' hCGt
,
hCG ís a glycoprotein of 39,000 molecular r.r¡eight
(Got et al, 1960; Bahl- et aL, 19721 , about 308 of its weight
is carbohydrate, with N-acetylneurami nl c acid (NANA r siaLic
acid) and fucose as nonreducing terminal units (Bahl , 1969a).
The high sialic acid content. accounts for -its low isoelectríc
point of 2.95. variations in sialíc acid content have been
claimed to account for electrophoretic heterogeneity seen
also on isoelectríc focusing (8e11 et al , 1969; !{eise et al ,
1973) .
Structural studies mad.e on hCG obtained from human
pregnancy urine show it to consist of t$ro dissimilar" non-
covalentl-y-bound subunits alpha and beta, which can be dissociated
by urea and have been isolated and purified (Sî¡aminathan and
Bahl , 1970; Canfield et al , I97O, I97L¡ Ra thnam and Saxena,
I972r. A striking feature of the amino acid composition' of
hCG is its high content of proÌine, second only to that of
collagen, indicative of a low alpha-helical content consistent
with the circular dichroic spectra of t,he molecule (Hilgenfelt
et aI , 19741 . A large amount of serine is also present,