Characterization of canine intestinal cholecystokinin-58 lacking its carboxyl-terminal nonapeptide. Evidence for similar post-translational processing in brain and gut

CCK-8 and CCK-58 differ in their effects on nocturnal solid meal pattern in undisturbed rats

Various molecular forms of CCK reduce food intake in rats. Although CCK-8 is the most studied form, we reported that CCK-58 is the only detectable endocrine peptide form in rats. We investigated the dark-phase rat chow intake pattern following injection of CCK-8 and CCK-58. Ad libitum-fed male Sprague-Dawley rats were intraperitoneally injected with CCK-8, CCK-58 (0.6, 1.8, and 5.2 nmol/kg), or vehicle. Food intake pattern was assessed during the dark phase using an automated weighing system that allowed continuous undisturbed monitoring of physiological eating behavior. Both CCK-8 and CCK-58 dose dependently reduced 1-h, dark-phase food intake, with an equimolar dose of 1.8 nmol being similarly effective (-49% and -44%). CCK-58 increased the latency to the first meal, whereas CCK-8 did not. The intermeal interval was reduced after CCK-8 (1.8 nmol/kg, -41%) but not after CCK-58. At this dose, CCK-8 increased the satiety ratio by 80% and CCK-58 by 160%, respectively, compared with vehicle. When behavior was assessed manually, CCK-8 reduced locomotor activity (-31%), whereas grooming behavior was increased (+59%). CCK-58 affected neither grooming nor locomotor activity. In conclusion, reduction of food intake by CCK-8 and CCK-58 is achieved by differential modulation of food intake microstructure and behavior. These data highlight the importance of studying the molecular forms of peptides that exist in vivo in tissue and circulation of the animal being studied.

The endoproteolytic maturation of progastrin and procholecystokinin

The homologous brain-gut propeptides, procholecystokinin (proCCK) and progastrin, both undergo extensive posttranslational maturation in specific neuroendocrine cells. The process comprises multiple endoproteolytic cleavages at mono- and dibasic sites, in addition to exoproteolytic trimmings and amino acid derivatizations. Knockout of prohormone convertases (PCs) in mice and studies in cell lines indicate that PC1, PC2 and, to a minor extent, PC5, are responsible for most of the endoproteolytic cleavages of both prohormones. Progastrin in antral G-cells is cleaved by PC1 at two di-Arg sites, R36R37 and R73R74, whereas, PC2 only cleaves at the single di-Lys site, K53K54. Pituitary corticotrophs and intestinal TG-cells, both of which express gastrin, do not cleave K53K54 due to lack of PC2. In proCCK five monobasic (R25, R44, R50, K61 and R75) as well as a single dibasic site (R85R86) can all be cleaved by both PC1 and PC2. But the cleavage differs in a cell-specific manner in that PC1 is responsible for the entire endoproteolytic cleavage in intestinal endocrine I-cells, except for perhaps the K61 site. In contrast PC2 is responsible for most endoproteolysis of proCCK in the cerebral CCK-neurons, which do not express PC1 in significant amounts. Moreover, PC5 appears to contribute to a minor extent to the neuronal proCCK and to the antral progastrin processing. This review emphasizes that prohormone convertases play a decisive but substrate and cell-specific role in the biosynthetic maturation of gastrin and CCK.

Canine vagus nerve stores cholecystokinin-58 and -8 but releases only cholecystokinin-8 upon electrical vagal stimulation

Cholecystokinin-58 has been shown to be the major form of cholecystokinin (CCK) released to the circulation upon lumenal stimulation of the small intestine in humans and dogs. In anesthetized dogs, electrical vagal stimulation evokes pancreatic exocrine secretion that is in part mediated through the release of CCK. We studied the molecular form of CCK stored in canine vagus nerves and that released into circulation upon electrical vagal stimulation. Gel filtration and radioimmunoassay of the water and acid extracts of canine vagus nerves indicated CCK-8 (35%) and CCK-58 (65%) as the major molecular forms in the vagus nerve. Both forms of CCK isolated from the vagal extracts were equally bioactive as the standard CCK-8 and CCK-58, respectively, in stimulation of amylase release from isolated rat pancreatic acini. Analysis of plasma collected after electrical vagal stimulation indicated that CCK-8 is the only form released into the circulation. The release of CCK-8 upon electrical vagal stimulation was not affected by application of lidocaine to the upper small intestinal mucosa, suggesting that it was released from vagal nerve terminals.

Isolation and characterization of sulphated and nonsulphated forms of cholecystokinin-58 and their action on gallbladder contraction

Cholecystokinin (CCK) exists in multiple molecular forms with different polypeptide lengths and the absence or presence of sulphation. We have isolated sulphated and nonsulphated forms of CCK-58 from porcine intestine and have determined their bioactivities in a guinea-pig gallbladder contraction assay. Both forms co-eluted in cation-exchange chromatography and in several rounds of reverse-phase (RP)-HPLC, but separated upon RP-HPLC using a water/acetonitrile system with heptafluorobutyric acid as counter ion. Nonsulphated CCK-58 was the form detected by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry because of desulphation in that process. The biological activity of CCK-58 and CCK-33 is equipotent, although the kinetics of the response differ. Sulphated CCK-58 was found to be 35 times more potent than nonsulphated CCK-58. In contrast, sulphated CCK-8 is 150 times more potent than nonsulphated CCK-8, and for sulphated and nonsulphated CCK-33, the activities differ by a factor of 100. This type of correlation indicates that the N-terminal end of CCK-58 partially compensates for the decrease in activity arising from the lack of sulphated tyrosine. Given its fairly high bioactivity, nonsulphated CCK-58 may have a physiological significance.

Prolonged intestinal afferent nerve discharge in response to cholecystokinin-58 compared to cholecystokinin-8 in rats

Intestinal afferents are sensitive to cholecystokinin (CCK) octapeptide. However, CCK-58 may be a more biologically relevant molecule. Therefore, recordings from jejunal mesenteric afferent bundles were secured for extracellular multi-unit recording and the responses to CCK-8 and CCK-58 compared. CCK-8 and CCK-58 (i.v.) evoked a dose-dependent, devazepide sensitive, increase in afferent nerve discharge. Peak discharge frequency was higher for CCK-8 at all doses (P < 0.05). However, response duration was more prolonged for CCK-58 (P < 0.05) at 200 pmol.kg(-1). This resulted in an overall increase in area under the curve of CCK-58 compared to CCK-8 (P < 0.05). CCK-58 stimulates afferent discharge in a different pattern than CCK-8 and, therefore, may have differential biological effects.

CCK-58: a novel reagent for studying the regulation of cholecystokinin bioactivity

CCK-58 has been shown to be the major circulating form of the hormone in the dog and human. To date, there have been no reports on its biological activity in vivo. We report here that CCK-8 and CCK-58 were equipotent in decreasing gastric motor function after bolus doses and in stimulating protein secretion after continuous infusion in urethane-anesthetized rats. The present results are the first on the in vivo activity of CCK-58, and indicate that because CCK-58 is equipotent to CCK-8, and because it is a major released and circulating form, it may be considered as a major contributor to the expression of cholecystokinin bioactivity.

Natural and synthetic CCK-58. Novel reagents for studying cholecystokinin physiology

CCK-58 is a unique reagent for testing how segments of a peptide far removed from its active site can influence the expression of its biological activity. Indications of tertiary structure have come from studies with natural peptide purified from canine small intestine. These studies gave clear indications that tertiary structure affects CCK-58 bioactivity, but the small quantities of CCK-58 available made it impossible to characterize completely how tertiary structure influenced bioactivity. Canine CCK-58 was synthesized manually using a solid support and was purified by reverse phase high pressure liquid chromatography (HPLC). Synthetic CCK-58 was characterized by isocratic reverse phase and gradient HPLC, amino acid analysis, mass spectral analysis, sequence analysis, and three bioassays. Synthetic and natural canine CCK-58 had the same elution profiles, amino acid composition, sequence, and mass. The two peptides were equipotent for the stimulation of pancreatic secretion. Natural canine CCK-58 was equipotent to CCK-8 for CCK "B" receptor binding, a further indication of the purity of the natural peptide. However, natural CCK-58 was more potent than CCK-8 for CCK "A" receptor binding and less potent than CCK-8 for stimulation of pancreatic secretion. These data support the concept that CCK-58 has a stable tertiary structure. This structure does not affect its binding to CCK "B" receptors, enhances its binding to low affinity CCK "A" receptors, and decreases its activity expressed through binding to high affinity CCK "A" receptors. The concept of a stable tertiary structure is also supported by the fact that many antibodies directed towards the carboxyl terminus of cholecystokinin react better with CCK-8 than CCK-58. The availability of synthetic CCK-58 will allow analysis of its tertiary structure by physical and chemical methods as well as studies on how peptide tertiary structure can affect receptor binding, receptor activation, metabolism in blood, degradation in interstitial fluid, and inactivation at the receptor. Evaluating all of these systems will help investigators understand the regulation of cholecystokinin activity by its major endocrine form, CCK-58.

Comparison of clearance and metabolism of infused cholecystokinins 8 and 58 in dogs

BACKGROUND

Cholecystokinin (CCK) 58 is the predominant molecular form of CCK in canine and human intestine and circulating blood. There is no report on the metabolism and clearance rate of CCK-58. The aim of this study was to compare the in vivo half-life and metabolism of CCK-58 with that of synthetic CCK-8.

METHODS

CCK-58 was purified from canine intestine by consecutive high-performance liquid chromatographic (HPLC) and fast protein liquid chromatographic steps. The peptides were given to 12 dogs as an intravenous (IV) bolus injection to determine the half-life of circulating CCK. Six dogs were given CCK-58 or CCK-8 as a constant IV infusion to determine plasma clearance rates and stability in circulating blood. Circulating molecular forms of CCK were determined by radioimmunoassay after extraction of CCK from plasma and characterization by HPLC.

RESULTS

The half-life of CCK-58 was 4.4 +/- 0.6 minutes compared with 1.3 +/- 0.1 minutes for CCK-8. Less than 5% of CCK-58 could be detected as smaller forms during constant IV infusion.

CONCLUSIONS

The longer half-life of CCK-58 compared with CCK-8 and the minimal conversion into smaller forms during constant IV infusion are consistent with the finding that CCK-58 is not only the major stored form but also the circulating form of CCK after endogenous stimulation in dogs.

N-terminal fragments of intestinal cholecystokinin: evidence for release of CCK-8 by cleavage on the carboxyl side of Arg74 of proCCK

From porcine duodenal mucosa we have identified three major procholecystokinin (proCCK) fragments: desoctaCCK-33, desnonaCCK-33 and desnonaCCK-39. (DesoctaCCK-33 means CCK-33 devoid of the 8 C-terminal amino acids, etc.). The fragments were purified by immunoaffinity chromatography and three steps of reverse phase HPLC monitored by a radioimmunoassay specific for the N-terminal part of CCK-33. The structures could be deduced from the proCCK sequence by N-terminal sequence determination and mass spectrometry. Whereas desnona-fragments of CCK have been described before, this is the first finding of a desoctaCCK, and it indicates that CCK-8 is released from the longer forms by endogenous cleavage of the Arg-Asp-bond. A carboxypeptidase B-like exopeptidase subsequently must produce the desnona-fragments by removing the arginine residue.