<p>This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.</p> <p>New results present C-peptide as a biologically active
peptide hormone in its own right. Although C-peptide is
formed from proinsulin and cosecreted with insulin, it is a
separate entity with biochemical and physiological characteristics
that differ from those of insulin. There is direct
evidence of stereospecific binding of C-peptide to a
cell surface receptor, which is different from those for insulin
and other related hormones. The C-peptide binding
site is most likely a G–protein–coupled receptor. The association
constant for C-peptide binding is approximately
3 × 10<sup>9</sup>M<sup>-1</sup>. Saturation of the binding occurs already at a
concentration of about 1 nM, which explains why C-peptide
effects are not observed in healthy subjects. Binding of C-peptide
results in activation of Ca<sup>2+</sup> and MAPK-dependent
pathways and stimulation of Na<sup>+</sup>,K<sup>+</sup>-ATPase and eNOS
activities. The latter 2 enzymes are both deficient in several
tissues in type 1 diabetes. There is some evidence that
C-peptide, and insulin may interact synergistically on the
insulin signaling pathway. Clinical evidence suggests that
replacement of C-peptide, together with regular insulin
therapy, may be beneficial in patients with type 1 diabetes
and serve to retard or prevent the development of long-term
complications.</p>
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Exp Diabesity Res. 2004; 5(1):15-23