Molecule guides
Diabetes and Metabolism Research: Peptide Families, Antisera and Assay Kits
Insulin and C-peptide, GLP-1 and GIP, glucagon and the adipokines — how the metabolic section is organised and which format answers which question.
Three clusters of reagents dominate metabolic peptide research: the hormones of the islet together with their processing fragments, the incretin axis that has transformed the field over the last ten years, and the adipokines and hepatokines tying fat and liver to glucose handling. Our diabetes topic page brings together the peptide standards, antisera, tracers and immunoassay kits used throughout that literature. Below we set out which families the topic holds, why each is ordered, and which reagent format suits which question.
What the diabetes topic covers
Something belongs here when its parent peptide comes from the pancreatic islet, acts on the handling of glucose or lipid, or serves as an indicator of metabolic state in cell and animal models. That brings in hormones from beta, alpha, delta and PP cells; gut-derived incretins together with their analogues; signalling peptides released by adipose tissue and liver; and the myokines that tie exercise physiology to metabolic phenotyping. Considerable overlap exists with obesity — leptin, ghrelin and amylin appear in both — and with cardiovascular research via apelin and adrenomedullin. Everything listed is for in-vitro use and approved animal research only.
Hormones of the islet and processing markers
- Insulin — available in human, rat, mouse, bovine and porcine sequences, alongside proinsulin and analogue standards. Species is not a detail here: rodents carry two insulin genes, and assays calibrated against human insulin give misleading values on rodent samples.
- C-peptide — released mole for mole with insulin but cleared more slowly, which makes it the better index of endogenous secretion whenever a model also receives exogenous insulin.
- Glucagon — the counter-regulatory hormone of the alpha cell. Assays for it are famously prone to cross-reaction with other proglucagon products, so antibody specificity is the specification that matters.
- Amylin (IAPP) — secreted together with insulin and also the amyloid-forming peptide of islet amyloid, so laboratories order both soluble standards and aggregation-competent preparations.
- Somatostatin and pancreatic polypeptide — products of delta and PP cells, used as standards and as immunohistochemical markers of islet composition.
- Preptin — derived from proIGF-2 and co-secreted by beta cells, supported by a smaller but active literature.
The incretin axis
This is the topic's fastest-moving area, and the reason many laboratories come to the catalog in the first place:
- GLP-1 — supplied as the active GLP-1(7–36)amide and GLP-1(7–37), the inactive (9–36) metabolite, and the antagonist exendin(9–39). The incretin pathway hub charts the whole axis.
- GIP — glucose-dependent insulinotropic polypeptide, the second incretin and now central to dual-agonist pharmacology.
- Exendin and lixisenatide — the agonist lineage originating in the Gila monster and the clinical molecules descended from it, stocked as reference standards.
- Oxyntomodulin and glicentin — the remaining proglucagon products, where telling them apart from glucagon is the principal analytical challenge.
- GLP-2 — the intestinotrophic sibling peptide, investigated in gut adaptation rather than in glycaemic control.
- Secretin and PYY — gut hormones commonly measured alongside the incretins during nutrient-challenge protocols.
Adipokines, hepatokines and myokines
- Adiponectin — the insulin-sensitising adipokine, offered as fragments and standards, though with the caveat that the native protein circulates as multimers which fragment standards cannot replicate.
- Leptin and resistin — the classical adipokines connecting fat mass to insulin action.
- Ghrelin and obestatin — acylated and unacylated ghrelin differ in activity, and because the octanoyl group is labile, how a sample is handled determines what is actually measured.
- FGF-19 and FGF-23 — endocrine FGFs controlling bile-acid and phosphate metabolism respectively, both now standard entries in metabolic phenotyping panels.
- Betatrophin (ANGPTL8) — claims about beta-cell proliferation were retracted, yet the protein remains an actively studied regulator of lipid metabolism, illustrating neatly that catalog inclusion tracks research use rather than settled biology.
- Irisin and FNDC5 — the exercise-induced myokine and its precursor, where the specificity of available antibodies has been publicly disputed.
- Visfatin, vaspin and nesfatin — adipose-derived peptides with literatures spanning metabolism and appetite.
- INSL5 — an enteroendocrine peptide of the colon belonging to the relaxin superfamily, studied in gut-to-metabolism signalling.
Formats found in the diabetes topic
Synthetic peptide standards act as assay calibrators and as receptor agonists or antagonists. Most of the workload in this topic falls on ELISA, EIA and RIA kits, since quantifying insulin, C-peptide, GLP-1, glucagon and adipokines in plasma and islet supernatant is routine work — and the kit you pick determines the quality of your data. Polyclonal antisera cover islet immunohistochemistry and Western detection. Labelled peptides provide I-125 tracers for classical RIA and fluorescent GLP-1 analogues for imaging receptor trafficking. Magnetic beads support multiplex metabolic panels.
Choosing within a family
Four questions come up again and again. Species comes first and matters most: assays calibrated on human insulin or leptin systematically misreport rodent samples, so check species reactivity on every item. Processing state comes second — total against active GLP-1, acylated against unacylated ghrelin, intact against C-terminal FGF-23. Settle the analyte first, then the kit. Third comes cross-reactivity, which defines the proglucagon family's difficulties: a glucagon assay that also sees glicentin will inflate glucagon values in gut-rich samples. Fourth, decide between a kit and separate peptide plus antibody: for measuring a concentration in a sample, a validated kit is quicker and more reproducible, while mechanism, localisation or custom assay development calls for the components individually.
Handling notes specific to this topic
Two problems cause most of the avoidable failures. DPP-4 cleaves active GLP-1 and GIP within minutes in collected blood, so plasma destined for active-form assays must receive a DPP-4 inhibitor at the point of collection — no choice of reagent can make up for omitting it. Acyl-ghrelin sheds its octanoyl group unless samples are acidified and treated with protease inhibitors. Otherwise the usual rules hold: sealed lyophilised vials at −20 °C, gentle reconstitution, single-use aliquots and no repeated freeze–thaw. Amylin and other aggregation-prone sequences demand particular attention to which solvent is used — see peptide solubility, the storage guide and the COA guide.
Where to look next
Browse the complete diabetes topic, the pancreas and islets hub, or the incretin pathway hub. Related guides include the obesity and appetite section, which shares the adipokine families, and the assay kit type guide for deciding between ELISA, EIA and RIA formats.
Questions
Why is C-peptide measured rather than insulin?
It is secreted in equimolar quantities with insulin yet cleared much more slowly, and it escapes first-pass hepatic extraction, so it reflects endogenous secretion more accurately. It is also unaffected by any insulin given experimentally, which an insulin assay cannot separate from the hormone the animal produces itself.
How do total and active GLP-1 assays differ?
An active assay picks up only GLP-1(7–36)amide and (7–37), whereas a total assay also counts the (9–36) metabolite produced by DPP-4, returning values several times higher. Neither is incorrect; they answer different questions. Measuring the active form also demands a DPP-4 inhibitor in the collection tube, or the analyte is gone before analysis.
Will a human insulin ELISA work on mouse plasma?
Usually not without validation. Mouse insulin 1 and insulin 2 differ from the human sequence at several positions, and antibodies calibrated on human insulin recognise them variably and often poorly. Choose a rodent-specific kit, and confirm whether it reports both mouse isoforms or just one.
Why do glucagon assays have a poor reputation?
Proglucagon is cleaved differently in the pancreas and the gut, yielding glucagon, glicentin, oxyntomodulin and major proglucagon fragment, all of which share sequence. Antibodies directed at the mid-region cross-react widely. Sandwich assays demanding both the N- and C-terminus of glucagon are far more specific, so check the epitope specification before buying.
What is the right way to collect ghrelin samples?
For acyl-ghrelin, use EDTA with a protease inhibitor, acidify without delay and keep everything cold. The octanoyl ester on Ser3 hydrolyses rapidly at neutral pH, converting acyl into des-acyl ghrelin and distorting the ratio. Total ghrelin assays tolerate more but measure something different.
Can these reagents be used to study diabetes in people?
They cannot. Everything in the research catalog is intended for in-vitro assays and approved animal research. Some of these molecules have approved therapeutic equivalents, but catalog material is not produced to pharmaceutical standards.