Brominated detergents as tools to study protein-detergent interactions

In order to study protein-detergent short-range interactions, we analyzed the quenching by brominated detergents of reticulum sarcoplasmic (SR) Ca2+-ATPase intrinsic fluorescence. For this purpose, 7,8-dibromododecyl beta-maltoside and 2-O-(10,11-dibromoundecanoyl)sucrose, brominated analogs of two non-ionic detergents, the frequently used dodecylmaltoside and the newly synthesized 2-O-lauroylsucrose respectively, were prepared. Rayleigh scattering measurements showed that the brominated detergents efficiently and rapidly solubilized SR vesicles like their non-brominated analogs although at slightly higher concentrations. Similarly, each analog had a slightly higher critical micellar concentration than its parent detergent. The partition coefficient K (expressed as the ratio of the molar fraction of detergent in the SR lipid phase to that in the aqueous phase, at pH 7.5 and 20 degrees C) was similar for brominated and non-brominated dodecyl maltoside (3.5-4 X 10(5)) and slightly lower for dibromoundecanoylsucrose (approximate to 10(5)) than for lauroylsucrose (approximate to 2 X 10(5)). At detergent concentrations too low to solubilize the membrane, the brominated detergents rapidly inserted (within seconds) into SR vesicles. In this concentration range, Ca2+-ATPase fluorescence quenching steadily increased with detergent concentration. When the membrane was saturated with detergent, the residual fluorescence was about half of its initial value, indicating significant protein-detergent contacts, possibly due to a slightly higher affinity of Ca2+-ATPase for these detergents than for phospholipids. For higher detergent concentrations, solubilizing the membrane, the fluorescence continued to decrease with detergent concentration, with no evidence fur a dramatic change in the average hydrophobic environment of the protein during the transition from bilayers to a soluble state. For still higher detergent concentrations, above that necessary for membrane solubilization, the fluorescence was further quenched to a residual relative value of about 20%, corresponding to further delipidation of the protein surface, in agreement with previous results [de Foresta, B., le Maire, M., Orlowski, S., Champeil, P., Lund, S., Moller, J. V., Michelangeli, F. & Lee, A. G. (1989) Biochemistry 28, 2558-2567]. Fluorescence quenching for solubilized Ca2+-ATPase was quickly reversed upon addition of excess non-brominated detergent. The effects of the four detergents on the Ca2+-ATPase hydrolysis of p-nitrophenyl phosphate were similar and correlated with the protein-detergent contacts evidenced above. In conclusion, both these brominated detergents appear to be promising tools to study protein-detergent interactions at the hydrophobic surface of a membrane protein, either in a membrane or in solubilized complexes.