An extended GIP peptide is expressed in human gut cells. (A) Alignment of human GIP (residues Y52 to K107) with corresponding residues from 17 other vertebrates showed that the GIP open reading frame contains alternative basic cleavage sites for the generation of multiple GIP isoforms in primates, dogs, cats, cows, pigs, and horses (upper panel). The mature peptide region is indicated by a dark horizontal bar above the alignment. Residues that are conserved from teleosts to humans are indicated by a gray background. Putative basic cleavage sites are indicated by a red background. The position of the variable residue 103 is indicated by an arrow. Alternative post-translational processing of proGIP could lead to the generation of a 42-amino-acid mature GIP and an extended 55-amino-acid isoform (GIP55G or GIP55S) that differ at position 52 (lower panel). (B) GIP and GIP55 peptides are colocalized in the duodenum cells. Immunoreactive GIP and GIP55 were detected in select duodenum cells by immunofluorescent staining. The right panels showed the dark field and phased contrast images of merged immunofluorescent signals (800×). The white horizontal bar in each panel represents a distance of 30 μm. (C) GIP, GIP55G, and GIP55S suppressed exogenous glucose in fasting rats in vivo. Each of the three GIP peptides reduced glucose contents in the blood to basal levels at 1 h after injection of the peptide and glucose. Each data point represents the mean ± SEM of triplicate samples. Similar results were observed in five separate experiments.