Solubilization and partial characterization of a phosphoprotein phosphatase from human myelin.

The phosphoprotein phosphatase (phosphoprotein phosphohydrolase, EC 3.1.3.16) solubilized from human central nervous system myelin has been shown to possess a comparatively high degree of specificity towards myelin basic protein, a constituent of the membrane and most likely its natural substrate, rather than the mixed histones. The enzyme has a pH optimum of 7.5. Hydrolysis of both the substrates is stimulated by dithiothreitol and is almost completely dependent upon the presence of divalent metal ions. The maximum rate of dephosphorylation of basic protein is attained in the presence of 125 micrometer Mn2+ whereas a much higher concentration of Mg2+ (50--100 mM) is required for the optimal dephosphorylation of histones. The dephosphorylation of basic protein was also stimulated by Triton X-100 (0.15%, v/v) and was shown to result from a 3-fold increase in the V of the reaction catalyzed by the phosphatase. The apparent Km values for basic protein and histones were unaffected by the presence of Triton X-100 and were found to be approx. 1 and approx. 160 micrometer, respectively. Under optimal conditions of assay, the phosphatase cleaved approx. 32 and approx. 0.7 nmol of orthophosphate.min-1.mg-1 of protein from basic protein and histones, respectively.