Distribution of myelin-associated enzymes and myelin proteins into membrane fractions from the brains of adult shiverer and control (+/+) mice.

Shiverer, an autosomal recessive mutation in the mouse, is characterized by a severe deficiency in CNS myelin. The concentrations of the myelin basic and proteolipid proteins in the brains of two-month-old shiverer mice, although high enough to be measured, were much lower than in the control (+/+) brains. In contrast, the specific activities of the myelin-associated enzymes, 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNP), 5'-nucleotidase, and carbonic anhydrase, were close to normal in the brains of the mutants. The activities of these enzymes and the concentrations of the myelin large basic and proteolipid proteins were compared in membrane fractions prepared, by differential and density gradient centrifugation, from the brains of shiverer and +/+ control mice. In myelin purified from the brains of shiverer mice the specific activities of 5'-nucleotidase and CNP were close to normal, and the specific activities of all three enzymes were normal in a crude myelin fraction from brains of the mutants. However, in the shi/shi brains abnormally high proportions of the three myelin-associated enzymes were distributed into the P3 (microsomal) fraction and into membrane fractions denser than myelin. The major myelin proteins, although low in total amounts in the mutants' brains, were distributed into the membrane fractions from control and shiverer brains in relative proportions similar to the relative proportions observed for the three enzymes. Thus, carbonic anhydrase, 5'-nucleotidase and CNP in the brains of shiverer mice are not truly dissociated from the major myelin proteins but are, rather, distributed for the most part into the same populations of membranes as are the residual, small amounts of the myelin basic and proteolipid proteins.