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On the measurement of cholecystokinin.

A hormone is a chemical transmitter that is secreted from one part of the body and circulates in the bloodstream to reach a distant target in another part of the body where it exerts its biologic effect. At its inception, this was a revolutionary concept that gave birth to an entire physiological and, later, medical discipline. Early on, hormones were discovered by their biologic actions. However, the physiology of hormones could not be assessed without quantification. Fundamental to the study of hormones was the ability to measure concentrations in the blood. Although biological assays have been the cornerstone of endocrinologic measurements, the development of the radioimmunoassay (RIA) completely changed the field of endocrinology (1). The attractive features of RIA include (a) applicability to most hormones, (b) ease of performance, and (c) relatively low cost, as well as high degrees of (d) accuracy, (e) sensitivity, and (fl specificity.

Cholecystokinin (CCK) was discovered in 1928 on the basis of the ability of intestinal extracts to stimulate gallbladder contraction in dogs (2). Later it was recognized that CCK was a potent stimulant of pancreatic enzyme secretion (3). However, it was not until 1966, when CCK was purified, that the primary sequence was determined (4,5). Early estimates of CCK-like activity in blood were based on biological assays such as pancreatic secretion or gallbladder contraction. However, these estimates were fraught with confounding problems that existed in whole animals, such as the effects of other hormones or neural influences. To circumvent these problems, a sensitive and specific in vitro bioassay was developed (6), but this method is labor-intensive and cumbersome and is not readily available to clinical laboratories. Attempts to develop an RIA for CCK had to overcome a number of unique challenges. Among these were (a) the multiple molecular forms of CCK, (b) amino acid sequence similarity between CCK and gastrin, (c) low blood concentrations of CCK, (d) limited peptide availability, and difficulties with (e) isotope labeling and (f) peptide synthesis. It should not be surprising then, that solutions to these many problems were slow in coming.

CCK was originally identified as a 33-amino acid peptide (CCK-33); however, since its discovery, multiple molecular forms have been described. In several species, biologically active forms ranging in size from CCK-83 to CCK-8 have been found to exist in intestine, brain, and blood (7,8). All forms are derived from a single CCK gene by posttranslational or extracellular processing (9). The biologically active portion of the molecule is its amidated carboxyl terminus. All forms of CCK larger than CCK-8 are biologically active. Therefore, to measure physiologically relevant CCKs, assays must detect the carboxyl terminus of all molecular forms. A major difficulty in developing CCK assays has been its structural similarity to gastrin. CCK and gastrin comprise a family of gastrointestinal peptides that share an identical carboxyl-terminal pentapeptide sequence. To develop a specific CCK RIA that does not cross-react with gastrin, the antisera should recognize the tripeptide sequence at the amino terminus of CCK-8, which is common to all forms of CCK but is dissimilar to gastrin.

Over the last 20 years, a number of RIAs and a bioassay for CCK have been developed (6,10-24). Although most have not undergone the extensive validation described in the current study by Rehfeld (25) in this issue, there is general agreement that CCK concentrations in the circulation are relatively low (in the picomolar range). In contrast, plasma concentrations of gastrin are 20-100 times higher. Thus, even slight antibody cross-reactivity with gastrin poses a substantial problem for accurately measuring blood concentrations of CCK. Accordingly, the sensitivity and specificity of an accurate CCK assay must be extremely high.

Even after the discovery of CCK-33, only limited amounts of material were available for raising antibodies. The largest bioactive form of CCK that has been described is CCK-83 (26). Abundant forms of CCK in tissue and blood include CCK-58, CCK-33, CCK-22, and CCK-8. Unfortunately, large forms of CCK are difficult to purify and still are not readily available. As such, CCK-83 and CCK-58 have not been used as standards for most assays, and the cross-reactivity with many antibodies is unknown. It has even been suggested that larger forms of CCK are less immunoreactive than smaller forms, perhaps because of tertiary structure (27).

Sulfation of the tyrosine residue at position seven from the carboxyl terminus of CCK is critical for biological activity. Because of this, synthesis of moderately large forms of CCK had not been possible until recently, and these peptides are still not commercially available.

The final problem in the development of a CCK RIA has been difficulty with isotopic labeling of the peptide. Oxidative methods to label CCK tended to destroy biological activity of the molecule through oxidation of the methionine residue in position three from the carboxyl terminus. Oxidation of CCK reduces its biological activity 100- to 1000-fold.

Identifying the problems in developing a CCK assay is one thing, successfully overcoming those problems is another. In the current issue of Clinical Chemistry, Rehfield has undertaken the ambitious task of developing an accurate RIA for measuring blood concentrations of CCK. He has tackled each of the problems listed above. Selection of a proper antigen for raising specific CCK antisera was a critical initial step. A variety of antigens, including natural porcine CCK-33 and synthetic nonsulfated CCK-33 and -29, as well as synthetic sulfated CCK-4, -12, and -13, were all used as immunogens. Two types of tracers were used, including CCK-33 labeled by nonoxidative conjugation and Bolton-Hunter-labeled CCK-8. Characteristics of the antisera that were critically evaluated included the titer, affinity, specificity, and homogeneity of binding kinetics. Seventy-eight rabbits, 29 guinea pigs, and 8 mice were immunized. The most successful immunization strategy used an immunogen consisting of a CCK-12 analog corresponding to 0-sulfated CCK-10 extended at the N-terminus with a diglycine bridge for carrier coupling. The resulting rabbit antiserum (Ab. 92128) was of high titer (>500 000) and bound sulfated CCK-8, CCK-22, CCK-33, and CCK-58 with nearly equimolar potency and with essentially no cross-reactivity with gastrin. This latter point was demonstrated in three ways. First, Ab. 92128 did not detect gastrin in chromatographically purified plasma extracts. Second, there was no correlation between CCK and gastrin plasma concentrations in humans. And third, infusion of gastrin-17 into human subjects did not increase plasma CCK immunoreactivity. With this RIA, plasma CCK concentrations averaged ~1 pmol/L under basal conditions and increased to ~5 pmol/L after ingestion of a meal. These estimates are in the range of other accepted assays.

The discovery of CCK-58 in tissue and the circulation required special treatment to preserve forms larger than CCK-33 (28). It was suggested that immediate acidification of plasma was necessary to prevent the in vitro degradation of CCK. The current study confirmed that larger forms of CCK were detectable only after plasma was acidified and that only CCK-22 and smaller forms were present in neutral extracts, thus confirming that CCK-58 is a major component of CCK-like immunoreactivity in plasma. Unfortunately, large forms of CCK are not available as standards for RIAs, and it still remains to be determined whether the immunoreactivity and biological activity of CCK-58 are equivalent to other molecular forms. In future studies, these determinations and the ability to compare CCK values by various RIAs will depend on the manner in which plasma is collected and extracted, the antibody that is used, and the epitope to which it is directed. These challenges and the lack of CCK RIAs for general use are hurdles that remain to be overcome. Nevertheless, this report by Rehfeld represents an exhaustive and careful attempt to develop the best characterized CCK RIA to date.

References

(1.) Yalow RS, Berson SA. Assay of plasma insulin in human subjects by immunologic methods. Nature 1959;184:1648.

(2.) Ivy AC, Oldberg E. A hormone mechanism for gallbladder contraction and evacuation. Am J Physiol 1928;65:599-613.

(3.) Harper AA, Raper HS. Pancreozymin, a stimulant of secretion of pancreatic enzymes in extracts of the small intestine. J Physiol 1943;102:115-25.

(4.) Jorpes E, Mutt V. Cholecystokinin and pancreozymin, one single hormone? Acta Physiol Scand 1966;66:196-202.

(5.) Mutt V, Jorpes J. Structure of porcine cholecystokinin-pancreozymin. Eur J Biochem 1968;6:156-62.

(6.) Liddle RA, Goldfine ID, Rosen MS, Taplitz RA, Williams JA. Cholecystokinin bioactivity in human plasma: molecular forms, responses to feeding, and relationship to gallbladder contraction. J Clin Invest 1985;75:1144-52. ditorial

(7.) Eysselein VE, Eberlein GA, Schaeffer M, Grandt D, Goebell H, Niebel W, et al. Characterization of the major form of cholecystokinin in human intestine: CCK-58. Am J Physiol 1990;258:G253-60.

(8.) Paloheimo LI, Rehfeld JF. A processing-independent assay for human procholecystokinin and its products. Clin Chim Acta 1994;229:49-65.

(9.) Deschenes RJ, Haun RS, Funckes CL, Dixon JE. A gene encoding rat cholecystokinin. J Biol Chem 1985;260:1280-6.

(10.) Schlegel W, Raptis S, Grube D, Pfeiffer EF. Estimation of cholecystokininpancreozymin (CCK) in human plasma and tissue by a specific radioimmunoassay and the immunohistochemical identification of pancreozyminproducing cells in the duodenum of humans. Clin Chim Acta 1977;80:30516.

(11.) Marshall CE, Egberts EH, Johnson AG. An improved method for estimating cholecystokinin in human serum. J Endocr 1978;79:17-27.

(12.) Byrnes DJ, Henderson L, Borody T, Rehfeld JF. Radioimmunoassay of cholecystokinin in human plasma. Clin Chim Acta 1981;111:81-9.

(13.) Calam J, Ellis A, Dockray GJ. Identification and measurement of molecular variants of cholecystokinin in duodenal mucosa and plasma. J Clin Invest 1982;69:218-25.

(14.) Maton PN, Selden AC, Chadwick VS. Large and small forms of cholecystokinin in human plasma: measurement using high pressure liquid chromatography and radioimmunoassay. Regul Pept 1982;4:251-60.

(15.) Chang TM, Chey WY. Radioimmunoassay of cholecystokinin. Dig Dis Sci 1983;28:456-68.

(16.) Jansen JBMJ, Lamers CBHW. Radioimmunoassay of cholecystokinin in human tissue and plasma. Clin Chim Acta 1983;131:305-16.

(17.) Himeno S, Tarui S, Kanayama S, Kuroshima T, Shinomura Y, Hayashi C, et al. Plasma cholecystokinin responses after ingestion of liquid meal and intraduodenal infusion of fat, amino acids, or hydrochloric acid in man: analysis with region specific radioimmunoassay. Am J Gastro 1983;78: 703-7.

(18.) Izzo RS, Brugge WR, Praissman M. Immunoreactive cholecystokinin in human and rat plasma: correlation of pancreatic secretion in response to CCK. Regul Pept 1984;9:21-34.

(19.) Becker HD, Werner M, Schafmayer A. Release of radioimmunologic cholecystokinin in human subjects. Am J Surg 1984;147:124-9.

(20.) Ohgo S, Takemura J, Oki Y, Nishizono F, Ishikawa E, Yoshimi T, et al. Radioimmunoassay of cholecystokinin in plasma. Clin Chem 1988;34: 1579-84.

(21.) Hocker M, Schmidt WE, Creutzfeldt W, et al. Determination of plasma cholecystokinin (CCK) concentrations by bioassay and radioimmunoassay in man. A critical evaluation. Regul Pept 1992;37:255-69.

(22.) Ballinger AB, Clark ML. L-Phenylalanine releases cholecystokinin (CCK) and is associated with reduced food intake in humans: evidence for a physiological role of CCK in control of eating. Metabolism 1994;43:735-8.

(23.) Paloheimo LI, Rehfeld JF. Quantitation of procholecystokinin and its products in plasma by processing-independent analysis. Clin Chim Acta 1995; 238:21-33.

(24.) Rehfeld JF. The molecular nature of cholecystokinin in plasma. An in vivo immunosorption study in rabbits. Scand J Gastroenterol 1994;29:110-21.

(25.) Rehfeld JF. Accurate measurement of cholecystokinin in plasma. Clin Chem 1998;44:991-1001.

(26.) Eberlein GA, Eysselein VE, Davis MT, Lee TD, Shively JE, Grandt D, et al. Patterns of prohormone processing: order revealed by a new procholecystokinin-derived peptide. J Biol Chem 1992;267:1517-21.

(27.) Reeve JR, Jr, Eysselein VE, Rosenquist G, Zeeh J, Regner U, Ho FJ, et al. Evidence that CCK-58 has structure that influences its biological activity. Am J Physiol 1996;270:G860-8.

(28.) Eberlein GA, Eysselein VE, Hesse WH, Goebell H, Schaefer M, Reeve JR Jr. Detection of cholecystokinin-58 in human blood by inhibition of degradation. Am J Physiol 1987;253:G477-82

Rodger A. Liddle

Department of Medicine

Box 3913

Duke University Medical Center

Durham, NC 27710

Fax 919-684-8857
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Title Annotation:Editorial
Author:Liddle, Rodger A.
Publication:Clinical Chemistry
Date:May 1, 1998
Words:1991
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