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The functional-unit molecular size of the Na+/bile acid cotransport system and the apparent target size of the bile-acid-binding proteins in brush-border membrane vesicles from rabbit ileum were determined by radiation inactivation with high-energy electrons. The size of the functional transporting unit for Na(+)-dependent taurocholate uptake was determined to 451 +/- 35 kDa, whereas an apparent molecular mass of 434 +/- 39 kDa was measured for the Na(+)-dependent D-glucose transport system. Proteins of 93 kDa and 14 kDa were identified as putative protein components of the ileal Na+/bile acid cotransporter in the rabbit ileum, whereas a protein of 87 kDa may be involved in passive intestinal bile acid uptake. Photoaffinity labelling with 3- and 7-azi-derivatives of taurocholate revealed a target size of 229 +/- 10 kDa for the 93 kDa protein, and 132 +/- 23 kDa for the 14 kDa protein. These findings indicate that the ileal Na+/bile acid co-transport system is in its functional state a protein complex composed of several subunits. The functional molecular sizes for Na(+)-dependent transport activity and the bile-acid-binding proteins suggest that the Na+/bile acid co-transporter from rabbit ileum is a homotetramer (AB)4 composed of four AB subunits, where A represents the integral 93 kDa and B the peripheral 14 kDa brush-border membrane protein.

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Glimepiride is a novel sulfonylurea for the treatment of type II-diabetic patients exhibiting different receptor binding kinetics to B-cell membranes with 8-9-fold higher kof f rate and 2.5-3-fold higher kon rate compared to glibenclamide (see accompanying paper (Miiller, G. et al. (1994) Biochim. Biophys. Acta 1191, 267-277)). To elucidate the molecular basis for this differential behaviour of glimepiride and glibenclamide, direct photoaffinity labeling studies using fl-cell tumor membranes were performed.

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