Synthesis and applicability of a photolabile 7,7-azi analogue of 3-sulfated taurine-conjugated bile salts

For the identification of proteins involved in hepa- tobiliary transport, the photolabile derivative 7,7-ASLCT ( (7,7-azi-3a-sulfat~5~holan-24~yl)-2'-aminoethanesulfonate) was synthesized. 7,7-ASLCT is taken up into liver and ex- creted into bile completely unmetabolized at a rate between the excretion rate of SLCT ((3a-sulfato-5~cholan-24-oyl)-2'- aminoethanesulfonate) and SCCT ((7a-hydroxy-3a-sulfato-5B cholan-24-oyl)-2'-aminoethanesulfonate). The dependency of flux rate of uptake into freshly isolated hepatocytes on the concentration of 7,7-ASLCT in presence of Na+ (143 mM) and with Na+ depletion (1 mM) is best described by the assump tion of two simple transport systems, the kinetic parameters of which are similar to those of SLCT. As studied in the presence of Na+, 7,7-ASLCT and SLCT exhibit a clearly com- petitive cross-inhibition of uptake with inhibition constants that are similar to the corresponding half-saturation con- stants. s Photoaffinity labeling of isolated hepatocytes using 7,7-ASLCT (400 p~) resulted in the irreversible inhibition of the uptake of 7,7-ASLCT and SLCT to similar extents, con- firming the kinetic data that 7,7-ASLCT is a true competing substrate for the uptake of SLCT. Because in intact rat liver 7,7-ASLCT and SLCT mutually inhibit their biliary excretion, the photolabile derivative shares with SLCT the same path- ways in uptake and in excretion. Thus, 7,7-ASLCT should be appropriate for the study of hepatobiliary transport of sul- fated and taurineconjugated bile salts by photoaffinity label- ing.-Dietrich, A, W. Dieminger, S. Mac Nelly, W. Gerok, and G. Kun. Synthesis and applicability of a photolabile 7,7-azi analogue of 3-sulfated taurine-conjugated bile salts. J. Lipid Res. 1995. 36: 1729-1744.