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) chloroformmethanol solutions containing more than 0.2 g/100 ml of proteolipid yielded a weightaverage aggregate weight of 2.9 X 108 and a radius of gyration of 645 A. The intrinsic viscosity of the solutions was 0.32 dl/g and the Huggins constant was 1.085. Lightscattering measurements in 88.5% formic acid-0.5M sodium formate yielded a weightaverage aggregate weight of 7.1 X 106 and a radius of gyration of 241 A. 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 X lo4 Ultracentrifugal analysis in 1.5: 1 (v/v) methylene chloridemethanol showed two broad peaks with s values of s ~ . ~ % = 25.05 S, s2% = 19.79 S for the minor peak and s1.5%.2% = 1.86 S for the major peak. Optical rotatory dispersion studies revealed large changes in bo 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/ lW 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.