[Effect of the medium on the functional and structural properties of serum albumins. I. Effect of ionic strength on N-variant of human serum albumin].

The effect of ionic strength on the 1-anilino-8- naphtalene sulfonate (ANS) binding sites of the N-form of human serum albumin (HSA) was studied by means of the protein and ligand fluorescence. The parameters of the binding of ANS to HSA (the number of sites and the binding constants) were determined by two methods: by measuring the ANS fluorescence either (i) at increasing protein concentrations and constant ANS concentration, or (ii) at increasing ANS concentration and two constant protein concentrations (9.6 and 2.53 microM). An increase in the NaCl concentration results in a monotonous decrease of the ANS fluorescence yield of HSA-ANS complex, which could be interpreted in terms of two different effects: first, the direct collisional quenching interaction of Cl-ions with bound ANS located on the surface of HSA and, second, the ca. 10 per cent decrease of the number of bound ANS molecules due to the lowering of the ANS-HSA association constant values. The fluorimetric titration showed that at low ionic strength (0.008 M NaCl) HSA molecule has one strong ( lgKaI = 6.75-7.25) and two secondary sites with lower affinity ( lgKaII = 6.35). The increase in the NaCl concentration results in a decrease of the affinity for both kinds of binding sites (in 0.2 NaCl lgKaI = 6.3-6.7; lgKaII = 5.6-5.9). In contrast with NaCl, Na2SO4 induces only a limited decrease of the ANS fluorescence occurring within a narrow concentration range (from ca. 0.02 to 0.05 M Na2SO4, e. i. at ionic strengths from 0.07 to 0.15) and which can be described as a cooperative interaction of six SO4(2)-ions with a HSA molecule.(ABSTRACT TRUNCATED AT 250 WORDS)