C-peptide

Where it comes from

Made in

Beta cells of the pancreatic islets, one molecule for every molecule of insulin

Sent out by

With insulin, from the same granules, whenever blood glucose rises

Acts on

• Kidney
• Peripheral nerves
• Small blood vessels

What it does

• Holds proinsulin in the shape that lets the two insulin chains fold and link
• Signals of its own are reported in nerve and kidney cells, but no settled function and no agreed receptor ¹
• Lasts about five times longer in blood than insulin, so it reports the pancreas’s output faithfully

In medicine

• Measured to tell how much insulin a pancreas still makes: low or absent in type 1 diabetes, preserved in type 2
• Tells injected insulin, which carries no C-peptide, from the body’s own in a hypoglycaemia work-up
• Used to decide who needs insulin and who may stop it

Beyond the body’s own use

• Replacement C-peptide was trialled for diabetic nerve damage. In 250 people with type 1 diabetes given a long-acting form weekly for a year, nerve conduction and the clinical neuropathy score did not improve over placebo; only the vibration threshold did, and development stopped ²
• That is as far as the idea has gone, and the missing receptor is the main reason: without one, there is no way to say what a dose should do ¹
• It is a measurement, not a treatment. What is asked of it now is how long a pancreas keeps making any insulin after a type 1 diagnosis ¹