All cysteines of mouse ileal and hepatic sodiumdependent bile acid transporters (Isbt and Ntcp, respectively) were individually replaced by alanine. Replacement of Cys 106 in Isbt and Cys 96 in Ntcp, which are located closely in alignment, decreased taurocholate uptake. Although Cys 51 in Isbt is conserved in Ntcp, the replacement spoiled Isbt only. Both similarity and diŠerence in the arrangement of functional sites are suggested.

Anionic surfactants in water were determined by a spectrophotornetric method involving flow injection coupled with solvent extraction. Eight cationic dyes were examined with several extraction solvents. Of the pairs of cationic dye and extraction solvent investigated, Methylene Blue and 1,2-dichlorobenzene were the most efficient. The ion associate formed between an anionic surfactant and Methylene Blue was extracted into the organic phase, the absorbance of which was measured at 658 nm. The carrier stream was distilled water and the reagent stream consisted of a cationic dye, sodium sulphate and acetate buffer (pH 51. A phase separator with a poly(tetrafluoroethy1ene) porous membrane (0.8 pm pore size) was used to separate the organic phase. The sampling rate was 20 samples per hour. The calibration graphs were linear up to 3 X M (8.7 mg I-') or 7 X lo4 M (20.2 mg I-') of anionic surfactant when the injection volumes were 300 and 100 PI, respectively. The relative standard deviation ( n = 10) was 0.9% when 300 pI of 2.1 x 10-SM (610 pg 1-11 sodium dodecylsulphate were injected. The detection limit corresponding to a signal to noise ratio of 3 was 1 x 10-8 M (5 pg 1-1) for the injection of 300 pI of sodium dodecylsulphate. The proposed method was used to determine anionic surfactants in river water.