RADIATION-INACTIVATION ANALYSIS OF THE NA-ACID COTRANSPORT SYSTEM FROM RABBIT ILEUM( BILE)

The functional-unit molecular size of the Na+/bile acid co-transport s ystem and the apparent target size of the bile-acid-binding proteins i n brush-border membrane vesicles from rabbit ileum were determined by radiation inactivation with high-energy electrons. The size of the fun ctional transporting unit for Na+-dependent taurocholate uptake was de termined 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 comp onents of the ileal Na+/bile acid cotransporter in the rabbit ileum, w hereas 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 pro tein, 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 r abbit ileum is a homotetramer (AB)(4) composed of four AB subunits, wh ere A represents the integral 93 kDa and B the peripheral 14 kDa brush -border membrane protein.