C-peptide (57-87), human

C-peptide is cleaved from proinsulin, stored in secretory granules, and eventually released into the bloodstream in amounts equimolar with those of insulin. The measurement of the C-peptide is an important test for the β-cell function. In the red blood cells of type 2 diabetic patients, Na+,K+ ATPase activity is strongly related to blood C-peptide levels. C-peptide signal transduction in human renal tubular cells involves the activation of phospholipase C and PKC-d and PKC-varepsilon, as well as RhoA, followed by phosphorylation of ERK1/2 and JNK and a parallel activation of Akt. C-peptide shows specific binding to a G-protein-coupled membrane binding site, resulting in Ca2+ influx, activation of mitogen-activated protein kinase signalling pathways and stimulation of Na+, K+ ATPase and endothelial nitric oxide synthase.