Progress report. Radioimmunoassay of intestinal hormones.

Alimentary diseases are common yet often their aetiology is unknown. This ignorance reflects a basic lack of understanding of normal gut function. One area hitherto little studied is intestinal endocrinology although the bowel is known to be an important endocrine organ. Histologists have agreed that there are at least 11 well defined endocrine cell types in the mucosa1, presum- ably all producing different hormones. Why is progress in this field so slow? The answer lies in the great technical difficulty in extracting labile peptide hormones from cells scattered sparsely in the gut mucosa and surrounded by highly destructive proteolytic enzymes. The problems of low concentration and rapid degradation are so formidable that nearly 60 years of continuous work were required to purify the first hormone, secretin2, and even then many thousands of hogs had to be sacrificed to yield a few milligrams of hormone. The more classical endocrine tissues of the body are gathered together as glands and extracts can easily be made to test hormone actions. In the case of gut hormones their actions have to be elucidated first. Only then is it possible to purify a single peptide from the mass of material extractable from the gut, using its assayable biological property as the reference. If a hormone lacks a well defined biological action it cannot be purified. Thus histologists can still point to several endocrine cells in the gut unassociated as yet with any known hormone.