Detergents as probes of hydrophobic binding cavities in serum albumin and other water-soluble proteins

As an extension of our studies on the interaction of detergents with membra nes and membrane proteins, we have investigated their binding to water-solu ble proteins. Anionic aliphatic compounds (dodecanoate and dodecylsulfate) were bound to serum albumin with high affinity at nine sites; related nonio nic detergents (C12E8 and dodecylmaltoside) were bound at seven to eight si tes, many in common with those of dodecanoate. The compounds were also boun d in the hydrophobic cavity of beta -lactoglobulin, but not to ovalbumin, I n addition to the generally recognized role of the Sudlow binding region II of serum albumin (localized at the IIIA subdomain) in fatty acid binding, quenching of the fluorescence intensity of tryptophan-214 by 7,8-dibromodod ecylmaltoside and 12-bromododecanoate also implicate the Sudlow binding reg ion I (subdomain IIA) as a locus for binding of aliphatic compounds. Our da ta document the usefulness of dodecyl amphipathic compounds as probes of hy drophobic cavities in water-soluble proteins. In conjunction with recent x- ray diffraction analyses of fatty acid binding as the starting point we pro pose a new symmetrical binding model for the location of nine high-affinity sites on serum albumin for aliphatic compounds.