myelin-basic-protein and zinc-chloride

Myelin basic protein is known to be released into the circulation following traumatic injuries or demyelination within the central nervous system, resulting in the generation of potentially immunogenic myelin basic protein material. In this investigation we have studied the binding of bovine and human myelin basic protein to human alpha2-macroglobulin, which was found to be the only major myelin basic protein-binding protein in human plasma. Myelin basic protein bound to all three conformational forms of alpha2-macroglobulin studied, i.e., native alpha2-macroglobulin, methylamine-treated alpha2-macroglobulin, and chymotrypsin-treated alpha2-macroglobulin. Zinc chloride (1 mM) or 1 mM iodoacetamide partly blocked the complex formation between myelin basic protein and alpha2-macroglobulin, while 1 mM magnesium chloride, 1 mM calcium chloride, or 1 mM EDTA had no effect on binding. Chymotrypsin and trypsin can degrade myelin basic protein to fragments which do not bind to alpha2-macroglobulin. However, when myelin basic protein was complexed with any of the conformational forms of alpha2-macroglobulin, no significant release of Na[125I]-labeled myelin basic protein occurred after proteinase treatment. The results suggest that binding of myelin basic protein to alpha2-macroglobulin may protect extracellular compartments in vivo from immunogenic myelin basic protein fragments and alpha2-macroglobulin may participate in the specific clearance of myelin basic protein from the circulation.

Topics: alpha-Macroglobulins; Blood Proteins; Calcium Chloride; Chlorides; Cysteine; Edetic Acid; Endopeptidases; Humans; Iodoacetamide; Magnesium Chloride; Myelin Basic Protein; Oxidation-Reduction; Protein Binding; Protein Conformation; Protein Denaturation; Solubility; Zinc Compounds

X-ray diffraction was used to record the effects of metal cations on the structure of peripheral nerve myelin. Acidic saline (pH 5.0) either with or without added metal cations caused myelin to swell by 10-20 A from its native period of 178 A. The X-ray patterns usually showed broad reflections, and higher orders were either weak or unobserved. With added ZnCl2, however, the swollen myelin gave diffraction patterns that retained sharp reflections to approx. 15 A spacing. Alkaline saline (pH 9.7) containing ZnCl2 produced a reduction of the myelin period by approx. 5 A which was at least twice as much as that produced by other metals. To examine the underlying chemical basis for these unique interactions of Zn2+ with myelin, we carried out parallel X-ray experiments on sciatic nerve from the shiverer mutant mouse, which lacks the major myelin basic proteins. Shiverer myelin responded like normal myelin to ZnCl2 in acidic saline; however, in alkaline saline shiverer myelin showed broadened X-ray reflections which indicated disordering of the regularity of the membrane arrays, and additional reflections were recorded which indicated lipid phase separation. This breakdown may come about by the binding of Zn2+ to negatively-charged lipids which could be more exposed due to the absence of myelin basic proteins. Electron density profiles were calculated on the assumption that, except for changes in their packing, the myelin membranes were minimally altered in structure. For both normal and shiverer myelins, treatments under acidic conditions resulted in swelling at the extracellular apposition and a slight narrowing of the cytoplasmic space. This swelling is likely due to adsorption of protons and divalent cations. Interaction between Zn2+ and myelin P0 glycoprotein could preserve an ordered arrangement of the apposed membrane surfaces. Alkaline saline containing ZnCl2 produced compaction at the cytoplasmic apposition in both normal and shiverer myelins possibly through interactions with a portion of P0 glycoprotein which extends into the cytoplasmic space between membranes.

Topics: Animals; Cations, Divalent; Chlorides; Extracellular Space; Hydrogen-Ion Concentration; Mice; Mice, Neurologic Mutants; Myelin Basic Protein; Myelin Sheath; Sciatic Nerve; Zinc; Zinc Compounds