Differences in the solubilizing effectiveness of the sodium dodecyl sulfate complexes of various proteins

The detailed reversible binding isotherms of sodium dodecyl sulfate (NaDodSO4) with 13 different initially native proteins are reported; the data were obtained at 20 degrees C and pH 7.1, ionic strength 0.033, with amounts bound with some proteins up to 1.1 g per g of protein. Although the isotherms of some of the proteins do not vary widely, extreme variations between certain classes are found. Thus, for example, hemoglobin and myoglobin both have high affinities and high binding capacities, while gamma-globulin, apoferritin, and transferrin have low initial affinities, and change drastically at higher concentrations. The protein-NaDodSO4 complexes solubilize the water-insoluble dye dimethylaminoazobenzene (DMAB) as effectively as micelles of pure NaDodSO4 when only small amounts (0.2 to 0.5 g/g of NaDodSO4) are bound. In most cases this effectiveness falls progressively as larger amounts are bound, and may even cease altogether at limits characteristic of the individual protein. With some of the latter, a second region of renewed solubilization occurs when substantially higher amounts of NaDodSO4 are present. In all cases, solubilization by ordinary micelles in normal amount occurs when the free NaDodSO4 concentration exceeds the critical micelle concentration, but the binding of NPADodSO4 to the protein also increases, in competition with formation of micelles. With some, but not all proteins the NaDodSO4 bound at concentrations above the cmc also solubilizes DMAB. In such cases the solubilizations by the protein-NaDodSO4 complexes and by the simple micelles are additive. The significance of the differences in binding and solubilizing encountered among these proteins is discussed in terms of surface structure, cooperativity of binding, and protein composition. No certain correlations with content of most amino acids, subunit structure, solubility, and hydrophobicity have been found, but there is a weak inverse dependence of solubilizing effectiveness on molecular size and indications of a strong dependence on content of cationic groups.