Insulin and C-Peptide Secretion

Insulin and C-peptide are released together when pancreatic beta cells process proinsulin, so measuring them reports on how much insulin the pancreas secretes. C-peptide is especially informative because, unlike insulin, it largely escapes first-pass extraction by the liver and is not affected by injected insulin.

Definition

Insulin and C-peptide are the two products of proinsulin cleavage, co-secreted in equimolar amounts by pancreatic beta cells; their measurement indexes beta-cell secretory activity, with C-peptide reflecting endogenous secretion independent of exogenous insulin.

Scope

The entry explains the shared origin of insulin and C-peptide, why C-peptide is a more faithful index of endogenous secretion, and how these analytes are measured. It is a reference-biochemistry topic and does not provide diagnostic thresholds or treatment guidance for any individual.

Core questions

• Why are insulin and C-peptide secreted in equal amounts?
• Why is C-peptide often a better marker of endogenous insulin secretion than insulin itself?
• What analytical challenges affect insulin and C-peptide measurement?

Key concepts

• Proinsulin processing
• Equimolar co-secretion of insulin and C-peptide
• Hepatic first-pass extraction of insulin
• Endogenous versus exogenous insulin
• Immunoassay standardization
• Beta-cell secretory capacity

Mechanisms

Beta cells synthesize proinsulin, which is cleaved into mature insulin and the connecting (C) peptide and stored together in secretory granules; stimulation releases the two in equimolar amounts. Insulin undergoes substantial and variable first-pass extraction by the liver and cannot be distinguished from injected insulin, whereas C-peptide is extracted minimally, has a longer half-life, and is not present in insulin preparations, so peripheral C-peptide more faithfully tracks the pancreatic secretory rate. These properties, balanced against assay standardization limits, make C-peptide a preferred index of endogenous beta-cell output (Polonsky & Rubenstein, 1983; Sacks et al., 2011).

Clinical relevance

Insulin and C-peptide measurements describe beta-cell secretory activity and insulin dynamics, concepts central to understanding insulin resistance and beta-cell function. This entry explains what the markers represent and their analytical limits; it is not a basis for diagnosis or treatment decisions for individuals.

History

The radioimmunoassay developed by Yalow and Berson made it possible to quantify circulating insulin and opened modern endocrinology. Recognition that C-peptide is co-secreted with insulin yet largely escapes hepatic extraction established it as a marker of endogenous secretion, with its pitfalls and limitations characterized by Polonsky and Rubenstein (Polonsky & Rubenstein, 1983).

Debates

How comparable are insulin and C-peptide assays across laboratories?

Immunoassays for insulin and C-peptide have historically lacked full harmonization, so absolute values can differ by method, motivating standardization efforts to make results comparable.

Key figures

• Kenneth Polonsky
• Arthur Rubenstein
• Rosalyn Yalow
• Solomon Berson

Seminal works

• polonsky-1983
• sacks-2011

Frequently asked questions

Why measure C-peptide instead of insulin to assess pancreatic secretion?

C-peptide is co-secreted with insulin in equal amounts but escapes most of the liver's first-pass extraction and is absent from insulin preparations, so it more faithfully reflects how much insulin the pancreas itself is making.

What does it mean that insulin and C-peptide are equimolar?

Each proinsulin molecule yields one insulin and one C-peptide molecule, so the pancreas releases them in equal numbers, which is why C-peptide can stand in for endogenous insulin secretion.