Cholecystokinin-33 (CCK-33): GI peptide hormone research overview

Cholecystokinin-33 (CCK-33) is the 33-amino acid form of cholecystokinin, a peptide hormone that coordinates pancreatic enzyme secretion, gallbladder contraction, intestinal motility, and satiety signaling. First isolated from porcine intestinal mucosa by Viktor Mutt and Johannes Erik Jorpes in 1968, CCK-33 was the original characterized molecular form of cholecystokinin and established the biochemical identity of a hormone that had been functionally described since the 1920s. Understanding CCK-33 requires distinguishing it from the other members of the CCK molecular family—notably CCK-58, the largest circulating form, and CCK-8, the minimal bioactive octapeptide on which the FDA-approved diagnostic agent sincalide is based.

What is CCK-33?

CCK-33 is a 33-amino acid peptide (MW approximately 3,918 Da) derived from the CCK preprohormone encoded by the CCK gene on chromosome 3p22.1. The peptide is synthesized primarily in I-cells of the duodenal and jejunal mucosa and in neurons of the central and peripheral nervous systems. Like all bioactive CCK forms, CCK-33 contains a sulfated tyrosine residue (at position 27 relative to the C-terminus, equivalent to Tyr-7 in the CCK-8 numbering) and a C-terminal phenylalanine amide. Both the sulfation and the C-terminal amidation are required for full biological activity at the CCK-1 (also called CCKA) receptor.

The discovery of CCK-33 as the primary molecular form of cholecystokinin came from a heroic effort by Swedish biochemists Viktor Mutt and Erik Jorpes at the Karolinska Institutet, who published the amino acid sequence in 1968 after processing the proximal meter of small intestine from approximately 20,000 hogs—collecting roughly 20 kilometers of intestinal tissue—to yield milligram quantities of purified peptide. This work resolved the structure of a hormone whose gallbladder-contracting function (cholecystokinin means 'bile-sac mover') had been known since A.C. Ivy and E. Oldberg described it in 1928, and whose pancreatic secretagogue activity (pancreozymin) had been recognized in parallel. Mutt and Jorpes's 1968 paper established CCK-33 as the canonical form of the hormone.

The CCK molecular family is now known to encompass multiple bioactive forms generated by tissue-specific post-translational processing: CCK-83, CCK-58, CCK-33, CCK-22, CCK-8, and CCK-5. CCK-58 is the predominant circulating form in humans and other mammals, and is produced by endopeptidase cleavage of the larger precursor. CCK-33 and CCK-22 are generated by subsequent cleavage of CCK-58. CCK-8 (the C-terminal octapeptide) retains full biological potency at both CCK-1 and CCK-2 receptors. The relative proportions of circulating CCK molecular forms vary by species, tissue compartment, and physiological state.

Mechanism of action

CCK-33, like all CCK forms containing the bioactive C-terminal sequence, signals through two G protein-coupled receptors: CCK-1R (CCKAR) and CCK-2R (CCKBR). CCK-1R is expressed prominently on pancreatic acinar cells, gallbladder smooth muscle, vagal afferent neurons, and gastric smooth muscle. It is coupled to Gq, activating phospholipase C and driving intracellular calcium release, which in pancreatic acinar cells stimulates exocytosis of digestive enzyme-containing zymogen granules. CCK-2R is more broadly distributed, including in the gastric mucosa (where it stimulates gastrin-secreting G cells) and the central nervous system, where it mediates satiety, anxiety-related behaviors, and nociception.

The longer CCK forms (CCK-58, CCK-33, CCK-22) have higher affinity for CCK-1R relative to CCK-2R compared with CCK-8, due to the N-terminal extension providing additional binding contacts. This receptor selectivity profile has biological relevance: CCK-58 and CCK-33 may preferentially drive pancreatic secretion and gallbladder contraction (CCK-1R-mediated), whereas shorter forms like CCK-8 engage CCK-2R more efficiently, influencing gastric acid secretion and central CCK signaling. The sulfation at Tyr-27 is critical for CCK-1R binding; unsulfated CCK has approximately 1000-fold lower affinity for CCK-1R, though unsulfated forms retain CCK-2R activity.

Beyond receptor activation, CCK-33 exerts neural effects via vagal afferent neurons expressing CCK-1R. Post-meal CCK release stimulates vagal afferents in a paracrine manner from intestinal I-cells, transmitting satiety signals to the nucleus of the solitary tract and hypothalamus. This vago-vagal reflex loop is a critical component of short-term meal termination. CCK also potentiates insulin secretion from pancreatic beta cells (the incretin-like potentiation of CCK) and inhibits gastric emptying, thereby coordinating nutrient delivery to the small intestine with digestive capacity.

What the research shows

The CCK research literature is extensive and spans GI physiology, pancreatic biology, obesity and appetite regulation, and neuroscience. Key areas include: pancreatic exocrine function (CCK as the primary stimulant of pancreatic enzyme secretion following protein and fat ingestion); cholecystopathology (CCK's role in gallstone formation and biliary dyskinesia, including the mechanistic basis for using CCK analogs diagnostically); pancreatic growth (CCK is a trophic factor for pancreatic acinar cells); and satiety (CCK's role in meal termination via vagal afferent signaling). A comprehensive 2025 review in Journal of Internal Medicine (Rehfeld, PMID 40557463) summarizes contemporary CCK biology, receptor pharmacology, and clinical implications across multiple disease states.

CCK-33 and CCK-58 have been studied in comparison with CCK-8 to understand structure-activity relationships and receptor selectivity. Research published in PMC4496260 (Atrial Natriuretic Peptide, for comparison) and CCK-specific pharmacology literature indicates that the N-terminal extension of CCK-33 relative to CCK-8 confers higher potency at CCK-1R and a longer plasma half-life due to slower proteolytic processing. CCK-58's half-life in human plasma is longer than CCK-8, and it demonstrates a more sustained postprandial response. These kinetic differences have implications for interpreting the biological roles of different CCK forms in human physiology.

CCK-1R has been a therapeutic target in obesity research, as CCK-1R agonism produces satiety and reduces food intake in animal models. However, clinical development of selective CCK-1R agonists for obesity has been challenging. CCK-2R antagonists have been investigated for peptic ulcer disease (to block gastrin-stimulated acid secretion) and for anxiety and pain modulation. No CCK-33 or CCK-58 analog has achieved regulatory approval as a therapeutic. The CCK field remains predominantly a research domain, with the major clinical application being sincalide (CCK-8 / kinetabs), used diagnostically.

CCK-33 versus sincalide (CCK-8): an important distinction

Sincalide is the pharmaceutical name for the C-terminal octapeptide of cholecystokinin — CCK-8 — which is FDA-approved under the brand name Kinevac for diagnostic use in gallbladder function testing (cholecystography) and to stimulate pancreatic secretion for aspiration studies. Sincalide is administered by intravenous injection in clinical settings and is a distinct compound from CCK-33. CCK-33 is the 33-amino acid parent peptide from which CCK-8 is derived through endopeptidase cleavage; it is not an approved pharmaceutical. The two forms share the same bioactive C-terminal sequence (CCK-8 is embedded within CCK-33 as its C-terminal octapeptide), which explains why both stimulate gallbladder contraction and pancreatic secretion. However, CCK-33's additional 25 N-terminal residues confer higher CCK-1R affinity and slower clearance, and their pharmacological profiles are not identical. Researchers studying CCK biology should be explicit about which molecular form is being examined.

Pharmacokinetics

Endogenous CCK is released postprandially, with plasma concentrations rising within 15–30 minutes of meal initiation. Fasting CCK-8 equivalent plasma concentrations are approximately 1–5 pmol/L in healthy adults; postprandial peaks reach 5–20 pmol/L. CCK-33 and CCK-58 circulate at lower molar concentrations due to larger molecular weight, but contribute substantially to total CCK biological activity given their higher CCK-1R affinity. The plasma half-life of CCK-8 is approximately 1–2 minutes; CCK-33 has a longer half-life (estimated 5–10 minutes) due to the requirement for N-terminal truncation before the core CCK-8 sequence is released for further degradation. CCK is cleared primarily by tissue peptidases and the liver.

Physiologic levels and pharmacologic dose data

Not medical advice. These are ranges reported in research literature, not personalized recommendations. Consult your physician.

Endogenous CCK: fasting plasma CCK concentrations (typically measured as CCK-8 immunoreactivity) are approximately 1–5 pmol/L; postprandial peak concentrations are 5–20 pmol/L in healthy adults. These are physiologic values, not administered doses. For pharmacologic reference: sincalide (CCK-8), the FDA-approved diagnostic form, is administered intravenously at 0.02 mcg/kg (for gallbladder ejection fraction testing). CCK-33 itself has been administered as an experimental research infusion in human physiologic studies at doses designed to produce physiologic-range plasma concentrations; these are research-use studies rather than therapeutic dose-finding trials. No CCK-33-based therapeutic has an approved dose.

CCK-33 as an administered peptide has appeared in human physiologic studies (e.g., studies examining pancreatic secretion, gallbladder function, or appetite signaling) at doses calibrated to approximate physiological postprandial concentrations, typically in the range of picomolar to low nanomolar plasma levels. These are experimental physiologic studies, not therapeutic dose-finding trials. CCK-33 has no approved therapeutic use.

Storage and handling

Research-grade CCK-33 peptide is typically supplied lyophilized and should be stored at −20 °C in a desiccated environment, protected from light. Due to the sulfated tyrosine residue, CCK-33 is susceptible to desulfation under strongly acidic or alkaline conditions, which abolishes CCK-1R potency. Reconstitution in neutral-pH aqueous buffer is recommended. Aliquoted solutions should be stored at −80 °C for long-term stability and at 2–8 °C for short-term use. Repeated freeze-thaw cycles should be minimized. As a 33-amino acid peptide, CCK-33 is more proteolytically stable than the smaller CCK-8 but remains susceptible to serum and tissue peptidases.

What CCK-33 is NOT

CCK-33 is not sincalide. Sincalide (Kinevac) is the FDA-approved pharmaceutical consisting of the C-terminal CCK-8 octapeptide, used diagnostically for gallbladder and pancreatic function testing. CCK-33 is the parent 33-amino acid peptide from which CCK-8 derives its C-terminal sequence; it is not an approved drug and has no approved clinical indication. CCK-33 is also not CCK-58, which is the largest and predominant circulating form of CCK in humans. CCK-33 is distinct from gastrin, a structurally related peptide hormone that shares the C-terminal pentapeptide sequence (Gly-Trp-Met-Asp-Phe-NH2) with CCK but is encoded by a different gene, produced in gastric G cells rather than intestinal I-cells, and primarily regulates gastric acid secretion. CCK-33 should not be confused with pentagastrin, a synthetic gastrin analog used diagnostically, nor with motilin, secretin, or GLP-1, which are unrelated GI hormones.

References

Primary sources include: Mutt and Jorpes' original CCK sequence determination (Biochem J, 1968); Rehfeld's 2025 comprehensive CCK biology review in Journal of Internal Medicine (PMID 40557463); the CCK and hormone concept historical review (PMC8052576); Pancreapedia's authoritative CCK reference; and the FDA Kinevac (sincalide) prescribing information for the pharmacologic CCK-8 comparator.