THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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sialic acid díd not alter the complenent-f ixing activity of
hCG (Mori, 1970).
c- Other Modifications-
Tyrosine residues are thought to be important to
protein structure and activity. A recent study employing
the react,ion of hCG with tetranitromethane (Hum et aL, !9741
suggests that Tyrosine-88 and tyrosinê-89 in the hCc-atpha
subunit are important to the biol-ogícal activity of hcc,
but not the tyrosine residues of the beta-subunit. (at
positions 58 and 81), and that the Tyrosine-65 of the alpha
is involved in holding the alpha and beta subunits together
in native conformation.
Finally, there is a report of trypsin digestion of
hCG down to a core of 6000 molecular weight, which stil.l re_
¿¿in, biological activity (Kikutaní et aln L965), similar
to the claim made for FSH digested with papain (Rathnam and
Saxena, 1970).
iii) CELT, OF ORIGIN AT{D BIOSYNTHESIS OF hCG¡
a- cell of orgin of hcG
Although early histochemical studies suggested secretion
of hCG by both cytotrophoblast and syncyti otrophoblast, the
advent of immunohistochemistry has provided more precise
evidence of its origin. Recent studies (Bossart et al , 1965;
Ðreskin et aI, 1970; Ikonikoff et at, 1973) using peroxidase-
label1ed antibody to hCG have shohrn that this hormone is
primarily located in the syncytiotrophoblast of both early