Cation-dependent extraction of basic protein from isolated human myelin. Independence of endogenous acid proteolysis.

Cation-dependent acid protease activity associated with isolated human myelin was inhibited by pepstatin A, or enzymatic reactions were suppressed altogether by maintaining samples at 0 degrees C rather than 37 degrees C to examine whether or not they are involved in the extraction of myelin basic protein (MBP) by increased ionic strength. These measures largely abolished the degradation of MBP by acid protease activity associated with myelin, whereas the extraction of protein was only slightly diminished. Electrophoresis revealed that soluble protein was exclusively accounted for by undegraded MBP. Acid proteolysis, therefore, appears not to be involved in the cation-mediated removal of MBP from myelin. It is suggested that this mechanism may account for the appearance of undegraded MBP in body fluids, as well as for its pathologically increased degradation once it has become soluble.