ANTIGENIC DETERMINANTS OF HUMAN CHORIONIC -GONADOTROPIN (HCG) - IDENTIFICATION OF EPITOPES AND THEIR SIGNIFICANCE FOR BIOLOGICAL FUNCTIONS,FOR THE ESTABLISHMENT OF IMMUNOMETRIC ASSAYS AND THE DEVELOPMENT OF AFERTILITY REGULATING VACCINE
S. Dirnhofer et al., ANTIGENIC DETERMINANTS OF HUMAN CHORIONIC -GONADOTROPIN (HCG) - IDENTIFICATION OF EPITOPES AND THEIR SIGNIFICANCE FOR BIOLOGICAL FUNCTIONS,FOR THE ESTABLISHMENT OF IMMUNOMETRIC ASSAYS AND THE DEVELOPMENT OF AFERTILITY REGULATING VACCINE, Wiener Klinische Wochenschrift, 105(11), 1993, pp. 314-319
Analysing immunologically relevant structures of human chorionic gonad
otropin (hCG), a glycoprotein hormone, has bearing on a variety of cli
nical features. These structures are of importance for the establishme
nt of immunometric assays for the diagnosis and monitoring of pregnanc
y and hCG producing tumours. Moreover, the development of fertility re
gulating vaccines by the use of one of the subunits of hCG (hCGbeta) o
r a synthetic peptide corresponding to the carboxyl-terminal peptide t
hereof (hCGbetaCTP, beta 109-145) requires detailed knowledge of the a
ntigenic properties of hCG. It appeared that 16 different antigenic ep
itopes are localized on the protein backbone of hCG and that the carbo
hydrate moieties do not contribute significantly to the immunological
properties of this hormone. Reduced and carboxylmethylated subunits of
hCG, i.e., hCGalpha and hCGbeta, express only 1 alpha- and 2 beta-epi
topes, the majority of antigenic determinants being destroyed by this
chemical procedure. Epitopes of hCG are thus predominantly dependent o
n the intact tertiary or quaternary structure of the protein. The urin
ary low molecular weight fragment of hCGbeta (hcGbetacf), a predominan
t molecular fraction in the urine of pregnant women or patients with v
arious tumours expresses 7 out of 9 epitopes of hCGbeta (beta 1-beta 7
), whereas two antigenic determinants, beta 8 and beta 9, are localize
d on the hCGbetaCTP. Certain portions of 5 antigenic determinants were
defined by means of synthetic peptides corresponding to hCGalpha- and
hCGbeta-sequences: part of the alpha4-epitope is localized within the
sequence hCGalpha 17-22; the sequence hCGalpha 33-42 has a share in t
he formation of the alpha 6-epitope which is specific for free hCGalph
a; the beta 5-epitope is localized within region hCGbeta 45-52 and the
two epitopes present on hCGbetaCTP, beta 8 and beta 9, map to aminoac
id sequences hCGbeta 135-145 and hCGbeta 108-120, respectively. With r
espect to hormone-receptor-interaction it was demonstrated that antibo
dies to epitopes on hCGbetaCTP are not able to immunoneutralize hCG. M
oreover, it appeared that 3 epitopes are still accessible to antibody
binding on receptor-bound hCG, namely beta 3, beta 5 and beta 8. The p
ractical significance of the results of this investigation lies in the
creation of specific immunometric assays for the measurement of hCG,
its free subunits and metabolic products thereof (hCGcf, hCGbetaCTP),
as well as in understanding hormone-receptor-interaction. Furthermore,
we provide evidence for risks and shortcomings of various approaches
by national and international organizations to the development of fert
ility regulating vaccines.