Factors affecting the electrophoretic analysis of myelin proteins: application to changes occurring during brain development.

Several factors affecting the sodium dodecylsulfate (SDS)-electrophoretic analysis of myelin proteins were investigated. It was observed that complete solubilization of proteins with SDS-containing solutions required either that the lyophilized myelin sample was pretreated with organic solvents or that reducing agents were added to the solubilizing solution. The SDS-electrophoretic pattern of myelin proteins was strongly dependent upon the ionic strength of the resolving gel since the mobility of the large basic protein appeared to increase as the ionic strength was decreased. Attempts were made to measure ratios of the major myelin proteins as a function of pretreatment of myelin with organic solvents followed by visualization of the protein bands with different stains. These studies revealed that lowered ratios of the proteolipid to each of the basic proteins was obtained if the myelin was delipidated with ehter-ethanol mixtures followed by staining with either Fast green or naphthol blue black (amido black). The most consistent results were obtained when myelin was not delipidated, solubilized in an SDS solvent containing dithiothreitol as the reducing agent, and gels were stained with Coomassie blue. Studies on developmental changes in the protein composition of myelin using this technique revealed that as mouse myelin matured the membrane became increasingly enriched in the small basic protein with the ratios of proteolipid/large basic protein/small basic protein changing from about 2:1:1 at 8-10 days to 2:1:2 at 16-17 weeks. Furthermore, the ratios of proteolipid/large basic protein remained essentially the same at all ages examined, suggesting that these proteins may be incorporated into the membrane at about the same rate during maturation.