A novel form of gastric inhibitory polypeptide (GIP) isolated from bovine intestine using a radioreceptor assay. Fragmentation with staphylococcal protease results in GIP1-3 and GIP4-42, fragmentation with enterokinase in GIP1-16 and GIP17-42

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Abstract
A novel form of gastric inhibitory polypeptide (GIP), later also referred to as glucose-dependent insulinotropic polypeptide, was isolated from bovine upper intestine. The purification was monitored by a recently developed radioreceptor assay, specific for GIP, using membrane preparations from hamster .beta.-cell tumors. A combination of ion-exchange and reverse-phase high-performance liquid chromatography was used in the isolation which resulted in homogeneous bovine GIP. Bovine GIP was, like porcine GIP, composed of 42 amino acid residues. The sequence was: Tyr-Ala-Glu-Gly-Thr-Phe-Ile-Ser-Asp-Tyr-Ser-Ile-Ala-Met-Asp-Lys-Ile-Arg-Gln-Gln-Asp-Phe-Val-Asn-Trp-Leu-Leu-Ala-Gln-Lys-Gly-Lys-Lys-Ser-Asp-Trp-Ile-His-Asn-Ile-Thr-Gln, which differs from that of the previously characterized porcine GIP by having isoleucine instead of lysine at position 37. Upon proteolytic digestion of GIP with the staphylococcal V8 protease and with enterokinase, 2 fragments were formed in each case, corresponding to GIP1-3, GIP4-42 and GIP1-16, GIP17-42, respectively.