Mechanism of Protection by Anionic Detergents against Denaturation of Serum Albumin.

In 1948, Duggan and Luck (1) observed that certain organic anions in appropriate concentrations prevented the rise in viscosity of bovine serum albumin caused by 6 M urea. Among the anions studied, dodecyl sulfate was found to be the most effective. In 1957, Markus and Karush (2) showed that the rise in levorotation of human serum albumin caused by 6 M urea could also be prevented or reversed by the addition of small amounts of sodium decyl sulfate. Earlier, Karush and Sonenberg (3) found that serum albumin preferentially bound 14 molecules of alkyl sulfate. Pallansch and Briggs (4), extending the observations of the former authors, differentiated between two sets of binding sites: 10 sites of high binding energy and approximately 80 sites of much weaker affinity. The concentration range of decyl sulfate (up to 0.01 M) found to be effective against 6 M urea in the study of Markus and Karush corresponded to the saturation range of the first set of binding sites. However, no direct measurements of binding were made in that study. It should be noted that urea is not the only denaturing agent against which anionic detergents are effective. Duggan and Luck (1) observed protection against heat denaturation, and Klotz and Heiney (5) found dodecyl sulfate effective in reducing the high levorotation of bovine serum albumin caused by low pH. The present study was designed to provide information concerning the mechanism of protection. The approach used was to study the protection of human serum albumin against urea denaturation as a function of the amount of detergent bound, the concentration of urea, and the state of ionization of the cationic side chains. Optical rotation was used as the critical parameter for the evaluation of the extent of denaturation. In addition, other physical measurements were carried out on the albumin in an attempt to detect and characterize structural changes associated with the denatured and the protected state.