• The functional and molecular characterisation of the pig ileal NA+/bile acid co-transport protein

      Knight, Gillian Lynsey (University of BedfordshireUniversity of Bedfordshire, 2000-12)
      Bile acids are essential for the efficient digestion and absorption of lipids, and are re-absorbed by aNa+ /bile acid co-transport protein within the ileum. Investigation into the transport protein, using BBMV and Xenopus laevis oocytes, revealed that pig ileal Na +/bile acid co-transport protein was not strictly dependent of the presence of an inwardly directed Na + gradient, as previously determined. Cations which had ionic radii between 0.8 - 1.33A, most notably Na+, K+ and Ca2+, stimulated secondary active transport of taurocholate. Taurocholate uptake in the presence of inwardly directed cation gradients of Na + and K+, demonstrated Michealis Menten Kinetics, concentrative accumulation, competitive inhibition and was temperature sensitive. Preventing the translation of the gene encoding the Na+/bile acid co-transport protein, abolished secondary active transport in the presence ofboth Na+ and K+. Isolation and computer modelling of the gene which encoded the pig ileal Na +/bile acid co-transport protein, revealed that this protein was composed of 8 transmembrane domains and lead to the identification of proposed cation and bile acid binding sites. The ability of K+ to stimulate the Na +/bile acid co-transport protein could be of physiological importance in vivo, because of the depleted Na + concentration present within the ileum. Though, the rate of taurocholate transport in the presence of K+ is reduced when compared to Na+, the transport protein has very similar affinities for taurocholate in the presence of both cations and therefore could use K+ for efficient re-absorption of taurocholate. Therefore, for this study it was proposed that the Na+/bile acid co-transport protein had a preference for Na+ rather than a strict dependence as previously concluded.