UBC Theses and Dissertations
The study of the chemical characterization of gastric inhibitory polypeptide (GIP) and the role of GIP in the enteroinsular axis Kwauk, Sam Tsung-Ming
The dual objectives of this thesis were to study the physiological role of GIP in the enteroinsular axis and to chemically characterize a side fraction identified in the purification of GIP. In the physiological studies, the dependency of the insulinotropic action of GIP on the prevailing state of glycaemia was confirmed in dogs using a system of steady state hyperglycaemia. GIP, from both exogenous and endogenous sources, was demonstrated to potentiate insulin release in the presence of moderate hyperglycaemia. Both glucose and fat administered enterally released immunoreactive-GIP (IR-GIP) and potentiated immunoreactive insulin (IRI) release during moderate hyperglycaemia (150 mg% above basal). Intravenous administration of GIP at 2.0 μg/kq.h was also capable of eliciting insulinotropic action during moderate hyperglycaemia. A mixture of ten amino acids was demonstrated to potentiate insulin release with mild hyperglycaemia (40 mg% above fasting) regardless of routes (intravenous, intraduodenal and oral) of administration. However, the release of IR-GIP was not demonstrated following the administration of the amino acid mixture. Arginine and alanine infused individually did not potentiate insulin release in the presence of mild hyperglycaemia. Intraduodenal hydrochloric acid infusion was also demonstrated not to release IR-GIP in the presence of mild hyperglycaemia. The interactions of GIP with tricarboxylic acid cycle intermediates and pyruvate were studied in euglycaemic conditions in dogs. Intravenous administration of individual metabolites (a-ketoglutaric acid, succinate and pyruvate) on an equimolar basis were shown not to be insulinotropic in the presence or the absence of concurrent GIP infusion (0.4 μg/kg.h). The presence of a minor peptide component in the stage III GIP was initially identified by thin layer chromatography. Confirmation of the presence of this minor component was obtained from polyacrylamide-urea gel. These techniques were unsuitable for preparative separation, as were conventional gel filtration and ion exchange separation. Further purification of GIP on high pressure liquid chromatography indicated the presence of a 5% minor peptide component which was eventually shown to contain two less amino acid residues (tyrosine and alanine) than GIP. The amino acid sequence of GIP III indicated the presence of a peptide component with an amino acid sequence different from GIP in that the first two amino acid residues of the N-terminal portion of the molecule (tyrosine and alanine) were missing. The lack of inhibitory activity to pentagastrin-stimulated acid secretion by synthetic GIP led to a reinvestigation of the amino, acid sequence of the molecule. The work in collaboration with Jornvall (Sweden) indicated an error, in that the original sequence included a second glutamine in position 30.
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