Solution properties and structure of brain proteolipids

Bovine white matter proteolipids have been studied by several physical methods and have been found to exist as micelles in 2 : 1 (v/v) chloroform–methanol solution. The data would indicate the existence of a critical micelle concentration at 0.017–0.022 g/100 ml. The curve appears linear in the range 0.017–0.2 g/100 ml, but from the data at higher concentrations it would appear that a change in slope is occurring in the region 0.2–0.3 g/100 ml. Light-scattering measurements on 2 : 1 (v/v) chloroform–methanol solutions containing more than 0.2 g/100 ml of proteolipid yielded a weight-average aggregate weight of 2.9 × 10⁶and a radius of gyration of 64.5 Å. The intrinsic viscosity of the solutions was 0.32 dl/g and the Huggins constant was 1.085. Light-scattering measurements in 88.5% formic acid–0.5M sodium formate yielded a weight-average aggregate weight of 7.1 × 10⁶ and a radius of gyration of 241 Å. The intrinsic viscosity observed for this solvent system is 0.14 dl/g and the Huggins constant is 1.005. Osmotic pressure measurements in 2 : 1 (v/v) chloroform–methanol containing less than 0.2 g/ 100 ml of proteolipid yielded a number-average aggregate weight of 7.2 × 10⁴ Ultracentrifugal analysis in 1.5:1 (v/v) methylene chloride–methanol showed two broad peaks with, s values of s_1.5% = 20.05 S, s_2% = 19.79 S for the minor peak and s_1.5%,2% = 1.86 S for the major peak. Optical rotatory dispersion studies revealed large changes in b₀ with change in solvent and proteolipid concentration. The present data suggest that the mode of attachment of protein to lipid is primarily of a noncovalent type. The results of this investigation also suggest that the proteolipid micelle above 0.2 g/100 ml is cylindrical (prolate ellipsoid) in 2:1 (v/v) chloroform-methanol and approaches a more spherical shape in 88.5% formic acid. A structure for the proteolipid micellar complex above concentrations of 0.2 g/100 ml is proposed.