ISOLATION OF MYELIN BASIC-PROTEIN FROM WHOLE TISSUE-EXTRACTS BY SELECTIVE PH-DEPENDENT SOLUBILIZATION

A previous study of the selective solubility of myelin basic protein ( MBP) of tissue extracts at pH 9.0 has raised issues of its quantitativ e recovery, and the differential solubility of its charge isomers. The pH-dependent solubility of proteins of acid extracts of delipidated t issue of bovine spinal cord was therefore reexamined. MBP of whole ext racts was completely soluble up to pH 8.0 only, and less so by 25 % at pH 9.0, and 43 % at pH 10.0. The proteins other than MBP were virtual ly insoluble between pH 5.0 to 6.0, and 9.0 to 10.0. The solubility of the main charge isomers I to III of MBP of 18.5 kDa was found not to be affected by pH. Either pH 5.0 or 9.0 is therefore suitable for the selective isolation of MBP from whole tissue extracts, only pH 5.0 pro viding for the complete recovery of MBP. The pH-dependent solution beh aviour was also examined following the separation of proteins of whole extracts by anion exchange chromatography at pH 10.4. Purified MBP an d several related minor cationic components of lower molecular weight were soluble throughout. In contrast, the anionic proteins were only p artly soluble between pH 4.0 to 10.0, i.e. by 4 to 20 %. The results a re consistent with specific protein-protein interactions of the protei ns of whole extracts, either enhancing the solubility of non-MBP prote ins, e.g. at pH 7.0, or impairing that of MBP between pH 8.0 to 10.0.