The state of association of the Na⁺-translocating reduced nicotinamide adenine dinucleotide:quinone oxidoreductase in detergent solution - An ultracentrifugation study

Na⁺:reduced nicotinamide adenine dinucleotide:quinone reductase (Na⁺-NQR) is a redox-driven sodium pump found in some bacterial respiratory chains. The oligomeric state of Na⁺-NQR from Vibrio cholerae was studied by sedimentation velocity and sedimentation equilibrium experiments in the analytical ultracentrifuge. Sedimentation velocity analysis of the purified enzyme in solutions of the nonionic detergent n-dodecyl-β-D-maltoside (Dm) revealed the presence of a nearly homogeneous protein population. From its sedimentation and diffusion coefficient, and considering reasonable amounts of Dm bound by the enzyme, it is shown that the component corresponds to monomeric Na⁺-NQR. This result is corroborated by sedimentation equilibrium experiments, performed under conditions of density matching for the bound detergent. No influence of NaCl on the sedimentation behaviour of Na⁺-NQR was detected. The amount of the protein-bound detergent was found to be about 0.57 g Dm per gram of protein.