N-Homocysteinyl-Proteins - Homocysteine in Protein Structure/Function and Human Disease - page 73

5.4 Structural and Functional Consequences

The first indication that the incorporation of Hcy into peptide bonds can be detri-
mental to biological function came from now classical structure/function studies of
oxytocin, the first peptide hormone to be sequenced and synthesized [315]. Oxytocin
is secreted from the pituitary gland and acts as a neuromodulator in the brain. As one
of the two known hormones released by the human posterior pituitary gland that act
at a distance (the second is vasopressin), oxytocin is important in sexual reproduc-
tion, induces uterine contractions and stimulates milk production, and plays a key
role in social attachment and affiliation in nonhuman mammals. Intranasal adminis-
tration of oxytocin causes a substantial increase in trust among humans, thereby
greatly increasing the benefits from social interactions [316]. Administration of
oxytocin has also been shown to modulate emotion processing in healthy male
volunteers, which may contribute to the emerging role of the neuropeptide in
promoting affiliative and approach behaviors by reducing the salience of potentially
ambiguous and threatening social stimuli [317].
Oxytocin is a 1,007 Da cyclic nanopeptide CysTyr I le GlnA snCys Pro Leu GlyNH 2
[315] composed of a disulfide-bonded cyclic hexapeptide amide-linked to a
tripeptide amide. Cysteine residues 1 and 6 form an intrachain disulfide bond.
Cys1 residue possesses a free amino group and is joined to the rest of the cyclic
portion of the molecule through its carboxyl group, while Cys6 residue is connected
through its amino group to the rest of the cyclic portion of the molecule and through
its carboxyl group to the tripeptide ProLeuGly-NH 2 . Its structure has been deter-
mined by classical peptide chemistry methods and confirmed by chemical synthe-
sis. Chemically synthesized oxytocin has full biological activity of the natural
oxytocin isolated from the posterior pituitary gland [315]. The ability to prepare
synthetic oxytocin allowed elucidation of relationships of molecular structure to
biological function of the hormone.
One of the analogs prepared is Hcy-oxytocin in which Cys1 residue is replaced
by Hcy [247]. This substitution introduces one additional methylene (-CH 2 -) group
and generates an analog with a 21-membered disulfide ring, in contrast to oxytocin,
which possesses a 20-membered ring. This change in structure led to a loss of
characteristic pharmacological properties of oxytocin. The Hcy-oxytocin does not
exhibit avian depressor and rat pressor activity and has only a very low oxytocic
activity, less than 0.2 % of that possessed by oxytocin [247]. Loss of activity in
Hcy-oxytocin is caused by the increase in size of the ring and not by the change in
position of the free amino group relative to the disulfide bond. This was shown by
synthesis and functional examination of 1-

-mercaptobutyric acid-oxytocin, a
deaminated analog of Hcy-oxytocin, which turned out to have no detectable avian
depressor activity and 0.8 % of oxytocic activity [318]. 1

γ

-Mercaptopropionic-
oxytocin, a deaminated analog of oxytocin, is highly potent and has enhanced avian
depressor, oxytocic and rat antidiuretic activities, a decreased rat pressor activity,
and an unaltered milk-ejecting activity in lactating rabbits [319].

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