Waters Symposium: Immunoassay
Radioimmunoassay of Gastrointestinal Hormones Eugene Straus Department of Medicine, SUNY Health Science Center at Brooklyn, Box 1196, Brooklyn, NY 11203-2098
Historical Background In 1833, William Beaumont published his Experiments and Observations on the Gastric Juice and the Physiology of Digestion. He had tracked, cajoled, forced, and studied his reluctant patient, Alexis St. Martin, whose gastric fistula was Beaumont’s window into the wonders of digestion, until he had formed the modern study of digestive processes and created the great tradition of the physician investigator. It was the first piece of good Yankee science, and the world took notice. Ivan Pavlov was stimulated by Beaumont, and he worked to understand how the primary digestive processes, particularly the secretory activities of the glands, were regulated and integrated. By 1902 Pavlov’s doctrine of “nervism”, which attributed virtually all regulatory functions to the nervous system, was generally accepted. But on 16 January of that year, on their “great afternoon”, two brothers-in-law at University College in London, William Bayliss and Ernest Starling, denervated a dog’s pancreas and achieved regulation of water and bicarbonate secretion by injecting extracts of duodenal mucosa into the dog’s veins. There was another regulatory mechanism. Starling declared that they had discovered “a chemical reflex”. They began describing their findings before the Royal Society within a week (1, 2). They called their chemical messenger secretin, and some three years later, at the suggestion of William Hardy, Starling coined the term “hormone”. Standing on the shoulders of giants, John Sydney Edkins of St. Bartholomew’s Hospital Medical School set out to find a “gastric secretin”. In 1905 he reported the discovery of what he called “gastrin” to the Royal Society (3). But by 1920 there was general agreement that gastrin did not exist. Edkins’s crude extracts contained histamine, and because trypsinization failed to abolish its stimulatory activity (as that process had neutered secretin), the scientific community concluded that histamine was stimulating acid secretion in Edkins’s denervated dog stomachs. Poor Edkins. He had been careful; he had noted the vasodilator activity in both his oxyntic and antral mucosal extracts, and the fact that only the antral extracts stimulated acid secretion. We now know that in the concentrations likely to have been found in Edkins’s extracts, histamine does not stimulate the dog stomach to secrete acid, and there are no trypsin-sensitive bonds in the structure of gastrin. I believe that Edkins died dejected, in a state of scientific rejection. But postmortem rehabilitates! In 1928 the cross-circulation experiments of Ivy and Oldberg extend the earlier observations of Okada, who, working in Starling’s laboratory, had found that intestinal acidification resulted in contraction of the gallbladder (4, 5). Active material was extracted from duodenal mucosa and was named cholecystokinin (CCK). Fifteen years later Harper and Raper demonstrated that intravenous injection of intestinal
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extracts stimulated the secretion of pancreatic enzymes (6 ). They named the hormonal material responsible for this activity “pancreozymin” (PZ). In 1966 Jorpes and Mutt purified the crude material and demonstrated that CCK and PZ were the same peptide (7). It is now conventional to use the term CCK. Recent Developments The application of radioimmunoassay (RIA), more recent understanding, and methods relating to peptide chemistry and purification—some of which have been the subjects of previous Waters symposia—changed everything, especially in the field of gastrointestinal regulatory peptides. Now the flow of scientific events reversed from the classic approach of Bayliss and Starling, which had defined endocrinology. No longer did we begin with physiologic action, such as the secretion of water and bicarbonate, and proceed to the identification of the hormone, in that case secretin. In the new mode we purify, sequence, and synthesize peptides and then look for their physiologic roles. The work of obtaining quantities of pure material, which required thousands of animals and occupied great chemists for many weeks, can now be done with few animals by a graduate student in an afternoon. And so the three original gastrointestinal hormones, still at the center of the intestinal hormonal firmament, have been joined by a cornucopia of regulatory peptides and candidate hormones. Still, from the point of view of clinical chemistry, there is only one that need routinely be measured. Insulin is the only absolutely essential peptide hormone. Only the peptide itself can compensate for its deficiency. Parathyroid hormone and ACTH are relatively essential, as their deficiency is incompatible with successful life, but in their absence the products of their target organ secretions will do nicely. In the case of the gastrointestinal tract, it may be entirely removed and the individual suffers from the loss of the vast absorptive surface, but there is no lack of an essential humor. From this we could predict that there would be no major syndrome of gastrointestinal hormone deficiency. Nonetheless, syndromes of gastrointestinal hormone excess are possible and, indeed, exist. The most important of these is the Zollinger–Ellison syndrome (ZES) in which gastrinsecreting tumors arising in the pancreas or duodenal wall cause marked hypersecretion of gastric acid, resulting in gastrointestinal ulceration and other complications (8). In fact, the major, if not the only, clinical indication for gastrointestinal hormone measurements is to confirm or rule out the diagnosis of ZES with gastrin determinations. Of course it is to be recognized that as in other clinical syndromes, RIA changed our clinical appreciation of ZES by allowing the diagnosis in the more common but less dramatic presentations
Journal of Chemical Education • Vol. 76 No. 6 June 1999 • JChemEd.chem.wisc.edu
Waters Symposium: Immunoassay
of the disease. A fasting plasma gastrin determination should be obtained whenever peptic ulcerations are multiple or in unusual locations, when the history includes accompanying diarrhea, and when complications of obstruction, bleeding, or perforation are present. Gastrin should be measured when the patient fails to respond to medical treatment, or if there is hypercalcemia or a history of multiple endocrine neoplasia (MEN-1), and most importantly, before surgery for ulcer disease. Mean fasting plasma or serum gastrin concentrations are below 50 pg/mL in studies employing the assays reported by Yalow and me (9). In general, commercial assay systems report normal fasting gastrin concentrations to be
