Uptake of polypeptide fragments of proteins by rat intestine in vitro and in vivo

Minute amounts of intact proteins were previously shown to be taken up from the intestine into the systemic circulation of mature animals; fragments were not detected. In this study, we sought evidence for uptake of fragments in both in vitro and in vivo experiments. Polypeptide fragments produced by pepsin digestion of bovine serum albumin (ranging in molecular weight from approximately 6000 to 25,000) were labeled with 125I. Everted jejunal gut sacs prepared from rat intestine were incubated with labeled fragments. After incubation, fluid exposed to the serosal surface was applied to a Sephadex G-50 gel permeation column. Radioactivity was detected in fractions corresponding to the elution position of the fragments. Transfer of fragments from the mucosal to the serosal surface was temperature-dependent. In in vivo studies, labeled fragments were infused into the jejunum of rats. Blood samples obtained from a mesenteric vein or the portal vein contained labeled fragments. After infusion of unlabeled fragments, nanogram amounts of immunoreactive fragments were detected by radioimmunoassay of mesenteric and portal venous blood. Thus, polypeptide fragments of a potential food protein were capable of being transferred across the mucosa in vitro and in vivo. Failure to detect fragments in the systemic circulation most likely results from their rapid clearance.