β-Endorphin and ACTH immunoassays were employed to examine the concentrations, distributions, and character of those peptides in rat gastrointestinal tissues. Sections of the gastrointestinal tract were obtained from fasted and fed animals and were extracted in 5 N acetic acid containing proteolytic enzyme inhibitors. Aliquots immunoassayed for β-endorphin and ACTH revealed highest concentrations to be present in the small bowel, with stomach and colon containing little immunoreactivity. Tissues from fasted animals contained more immunoreactivity than did those from fed animals. Gel chromatography showed the presence of large molecular weight forms of β-endorphin and ACTH in gut extracts. Concanavalin A affinity chromatography revealed that approximately 5% of gut immunoreactivity contained carbohydrate. Therefore, β-endorphin and ACTH immunoreactivities are present in the gut. The demonstration of large molecular weight and glycosylated forms of immunoreactivity suggests the presence of biosynthetic precursors of β-endorphin and ACTH. The increase in immunoreactivity in response to fasting suggests that these peptides play a role in gut physiology.
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