Interaction of 1-anilino-8-naphthalene sulphonate with human serum low-density lipoprotein

Abstract

The binding of the fluorescent probe 1-anilino-8-naphthalene sulphonate to human serum low-density lipoprotein has been studied under varying conditions of pH and in different concentrations of CaCl₂ or NaCl. The effect of different alkali metal cations on 1-anilino-8-naphthalene sulphonate binding by low-density lipoprotein was studied by using LiCl, NaCl, KC1 or CsCl in the medium. It has been found that 1-anilino-8-naphthalene sulphonate fluoresces strongly when bound to low-density lipoprotein in 5 mM Tris buffer, pH 7.4; the fluorescence was enhanced on addition of salt, the divalent ion Ca²⁺ having a much more pronounced effect than the monovalent cations. Among the alkali metal cations the enhancement of fluorescence decreased in the order Li⁺ > Na⁺ >K⁺ >Cs⁺. Reciprocal as well as Scatchard plots showed only one type of binding site, the number per molecule of low-density lipoprotein being about 200, in 1.4 mM CaCl₂. The fluorescence increased with lowering of pH down to the lowest pH (2.45) studied. The effects of pH and of Ca²⁺ on the fluorescence suggest the binding site to be the phospholipid.