Myelin basic protein in Alzheimer disease neuronal fractions and mammalian neurofilament preparations.

We previously reported a marked increase of a 20,000 molecular weight (MW) protein, P20, in some neuronal fractions and whole cortical homogenates isolated from affected cortex in Alzheimer disease; P20 comigrated electrophoretically with an unidentified, major 20,000 MW protein present in human neurofilament (NF) fractions. We now report that the 20,000 MW protein is a major constituent of rodent as well as human NF fractions and that it comigrates by one- and two-dimensional gel electrophoresis with purified myelin basic protein (MBP). Peptide mapping and staining with amido black confirmed the identity of the 20,000 MW protein of mammalian NF fractions as MBP. One- and two-dimensional gel electrophoresis of neuronal perikaryal fractions from human cortex indicated that the increased P20 protein in Alzheimer neuronal fractions comigrates with human MBP. Deliberate contamination of cortical samples with adjacent subcortical white matter (i.e., myelin) prior to neuronal separation did not result in an increase of P20 in the neuronal fraction. On the basis of these and additional experiments, we conclude that the increase of a 20,000 MW protein in neuronal fractions and whole homogenates from affected cortex in Alzheimer disease represents MBP of intracortical origin.