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on polyclonal antibodies also do not distinguish active from inactive metabolites and
may actually be more likely to detect biologically inactive metabolites simply by
virtue of being able to detect more breakdown products. Furthermore, if a molecule
is cleaved into two fragments, each of these fragments may be detected by the assay,
resulting in an apparent and paradoxical increase in the concentration of the molecule
with ongoing degradation.
Assays based on mass spectrometry can theoretically overcome some of these
problems by identifying and quantifying the intact molecule and all of its metabolites.
However, interpretation of mass spectrometic results requires knowledge of the
degradation pathways for the molecule and the bioactivity of the various breakdown
products. For kisspeptin, these pathways have yet to be fully characterized.
Further complicating the measurement of kisspeptin in human blood samples is
the fact that kisspeptin undergoes ongoing degradation even after samples are col-
lected. Ramachandran et al. observed that kisspeptin concentrations, measured with
a radioimmunoassay based on a polyclonal antibody raised against kisspeptin-54,
declined gradually over time in human plasma and very rapidly in serum stored at
room temperature after sample collection [
9
]. Similarly, kisspeptin-10 added to
human plasma degrades with a half-life of 55 s in vitro [
10
,
11
]. The principal
breakdown product of kisspeptin-10 is a 9-amino acid peptide that results from
N-terminal truncation of the kisspeptin decapeptide [
10
]. While the biological activ-
ity of this 9-amino acid metabolite has not been tested formally, it is likely to retain
some biological activity because peptides consisting of just eight or even fi ve of the
C-terminal amino acids of kisspeptin are capable of stimulating the kisspeptin
receptor [
5
,
12
].
Pharmacokinetics of Kisspeptin-10
The pharmacokinetics of kisspeptin-10 have been examined using a radioimmuno-
assay based on a polyclonal sheep antibody that recognizes both kisspeptin-54 and
kisspeptin-10 and has minimal cross-reactivity with other peptides of the RF-amide
family [
13
,
14
]. After a single intravenous bolus of kisspeptin-10 in men and
women, kisspeptin immunoreactivity peaked at the next measured time point
(10 min after injection). By 20 min, kisspeptin immunoreactivity was detectable
only with high doses of kisspeptin (3 nmol/kg or higher) [
14
].
The half-life of kisspeptin-10 was determined more precisely after the end of an
infusion of kisspeptin-10. Kisspeptin-10 immunoreactivity decayed with a half-life
of 4 min in men, women in the early to mid-follicular phase, and women in the late
follicular phase in the days before ovulation [
14
]. As noted above, a liquid chroma-
tography/mass spectrometry assay that specifi cally detects kisspeptin-10 has been
used to demonstrate that kisspeptin-10 has an in vitro half-life of 55 s in human
plasma at 37°C [
10
,
11
]. The most plausible explanation to reconcile these two
results is that the radioimmunoassay used in the in vivo studies also detects
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