In order to investigate the source of free amino acids found in the gut lumen during absorption of dipeptides, as well as evaluating the role of brush border peptidases in the mucosal hydrolysis of dipeptides during absorption, rates of dipeptide disappearance and appearance of hydrolytic products were measured during perfusion of rat jejunum and ileum in vivo with buffered and unbuffered 10 mM solutions of glycl-L-phenylalanine (Gly-Phe) and L-phenylalanyl-glycine (Phe-Gly). Mucosal brush border peptidase activity was then measured in the perfused segments in vitro at luminal pH and at two substrate concentrations. In addition cytosol peptidase activity in the perfused segments was measured at pH 7-4 and at 10 mM substrate concentrations. In the jejunum, there was a relationship between rates of free phenylalanine appearance in vivo (Phe-Gly greater than Gly-Phe) and rates of brush border (Phe-Gly greater than Gly-Phe) rather than cytosol (Gly-Phe greater than Phe-Gly) peptidase activities. No constant relationship between free phenylalanine appearance and hydrolysis of the dipeptides by either brush border or cytosol peptidases was observed in the ileal studies. These findings suggest that, in the jejunum, hydrolytic products originate from the surface of the cell whereas, in the ileum, hydrolytic products originate from both the intracellular compartment as well as from the surface of the mucosal cell. In the jejunum, in vitro rates of brush border hydrolysis of Gly-Phe were always less than in vivo disappearance rates, whereas rates of Phe-Gly brush border hydrolysis always exceeded luminal disappearance rates. These data imply that Gly-Phe is predominantly transported intact and hydrolysed by cytosol peptidases, In contrast, brush border peptidases play an importnat role in the mucosal hydrolysis of Phe-Gly.
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