Influence of N-dodecyl- N,N-dimethylamine N-oxide on the activity of sarcoplasmic reticulum Ca 2+-transporting ATPase reconstituted into diacylphosphatidylcholine vesicles: Effects of bilayer physical parameters

Abstract

Sarcoplasmic reticulum Ca-transporting ATPase (EC 3.6.1.38) was isolated from rabbit white muscle, purified and reconstituted into vesicles of synthetic diacylphosphatidylcholines with monounsaturated acyl chains using the cholate dilution method. In fluid bilayers at 37 °C, the specific activity of ATPase displays a maximum (31.5 ± 0.8 IU/mg) for dioleoylphosphatidylcholine (diC18 : 1PC) and decreases progressively for both shorter and longer acyl chain lengths. Besides the hydrophobic mismatch between protein and lipid bilayer, changes in the bilayer hydration and lateral interactions detected by small angle neutron scattering (SANS) can contribute to this acyl chain length dependence. When reconstituted into dierucoylphosphatidylcholine (diC22 : 1PC), the zwitterionic surfactant N-dodecyl- N,N-dimethylamine N-oxide (C12NO) stimulates the ATPase activity from 14.2 ± 0.6 to 32.5 ± 0.8 IU/mg in the range of molar ratios C12NO : diC22 : 1PC = 0 ÷ 1.2. In dilauroylphosphatidylcholines (diC12 : 0PC) and diC18 : 1PC, the effect of C12NO is twofold—the ATPase activity is stimulated at low and inhibited at high C12NO concentrations. In diC18 : 1PC, it is observed an increase of activity induced by C12NO in the range of molar ratios C12NO : diC18 : 1PC ≤ 1.3 in bilayers, where the bilayer thickness estimated by SANS decreases by 0.4 ± 0.1 nm. In this range, the 31P-NMR chemical shift anisotropy increases indicating an effect of C12NO on the orientation of the phosphatidylcholine dipole N +–P − accompanied by a variation of the local membrane dipole potential. A decrease of the ATPase activity is observed in the range of molar ratios C12NO : diC18 : 1PC = 1.3 ÷ 2.5, where mixed tubular micelles are detected by SANS in C12NO + diC18 : 1PC mixtures. It is concluded that besides hydrophobic thickness changes, the changes in dipole potential and curvature frustration of the bilayer could contribute as well to C12NO effects on Ca 2+-ATPase activity.