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We have studied intestinal metabolism and transport, which is considered to be sequential (1) or competitive (2) process in absorption (Scheme 1). (1) Disaccharide (maltose, cellobiose, lactose) conjugates of p-nitrophenol were hydrolyzed to p-nitrophenyl β-glucosides (p-NPβglc) on the mucosal side. p-NPβglc was transported by Na+/glucose cotransporter (SGLT1). Transport clearance of p-NPβglc formed from cellobiose and lactose conjugates of p-NP were higher than that from maltose or of p-NPβglc itself. These results suggest that SGLT1 is cooperatively coupled with lactase/phloridzin hydrolase catalyzing hydrolysis of cellobiose and lactose conjugates. There might be cooperative relationship between peptidase and H+/oligopeptide cotransporter or amino acid transporter as well. (2) Kyotorphin (KTP) was too unstable in intestine to be absorbed. KTP appeared on the serosal side in the presence of peptidase inhibitors. Meanwhile, cyclic KTP was stable in intestine to be absorbed. Absorption clearance of cyclic KTP was higher than the overall transport clearance of KTP, which was calculated according to the metabolic inhibition model. Competitive process was observed in intestinal absorption of α-naphthol as well. These results indicate that metabolism degradation and membrane transport are competitive. Unless drug is stabilized against metabolic enzyme, intestinal absorption of drug can not be improved even if membrane transport is increased.

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Role of Transporter and Metabolic Enzyme in Sequential and Competitive Process of Intestinal Absorption: Study on SGLT1, Disaccharidase and Peptidases.