Abstract
Spectrofluorometric studies using 1-anilino-8-naphthalenesulfonate (ANS) reveal that Z-fraction from rat liver has far the more hydrophobic binding sites than the other cytoplasmic binding fractions X and Y. Diagnostically important organic anions, such as sulfobromophthalein (BSP) and rose bengal (RB), are shown to displace ANS from Z-fraction by quenching the fluorescence of ANS and Z-fraction mixture without the change of quantum yield and emission maximum. The binding constants of these dyes to Z-fraction are determined by the competitive studies with ANS, and are coincident with those from other methods. Since the spectrophotometric technique reveals that the number of RB binding sites on Z-fraction is about twice as large as that of ANS binding sites, ANS and RB do not have necessarily the identical binding sites, though they must have partly common ones. The major binding force between organic anions and Z-fraction is supposed to be hydrophobic considering that the binding constant of Z-fraction for ANS is only about one-eight of that of bovine serum albumin (BSA) of which site is both hydrophobic and cationic. This property of sites accounts for the lower affinity for BSP of Z-fraction extracted from CCl4 chronically intoxicated rats, which our former study has made clear.
