Extraction, gel filtration pattern, and receptor binding of porcine gastrointestinal glucagon-like immunoreactivity

Abstract

Different techniques for the extraction and initial purification of porcine gastrointestinal glucagon-like immunoreactivity (GLI) were compared with reference to yield, and preservation of number and pattern of GLI components. The conventional acid-ethanol technique combined with ethanol-ether purification gave high yields and a reproducible pattern of components. Large amounts of tissue were more easily extracted using another technique, based on extraction by boiling, extraction and precipitation with acetone, and--if necessary--salting out. By means of the latter two techniques mucosal tissue from all of the porcine gastrointestinal tract was extracted and subjected to gel filtration. Glucagon-like peptides were searched for using: 1. a radioimmunoassay which quantifies gut type glucagon (GTG), as well as pancreatic type glucagon (PTG), 2. a radioimmunoassay highly specific for pancreatic type glucagon (PTG), and 3. a radioreceptor assay based on binding of glucagon to porcine liver cell membranes. The oesophageal, the fundic, and the antro-pyloric parts of the gastric mucosa contained very small amounts of GLI. The cardiac gland region contained small amounts of a peptide indistinguishable from "true" glucagon. The duodenal mucosa contained small amounts of "true" glucagon and may be a smaller, glucagon-like peptide. The mucosa of the small intestine contained large amounts of both high and low molecular weight GTG and, in addition, PTG of high molecular weight and "true" glucagon. The colon also contained these components with "true" glucagon in high concentrations. Only small GTG and "true" glucagon were receptor-active, the former with less than its immunometric potency.