A radioimmunoassay for the myelin-associated glycoprotein.

The myelin-associated glycoprotein (MAG) was purified from rat brain and radioiodinated with Bolton-Hunter reagent for use in a double-antibody radioimmunoassay. The conditions of the assay were adjusted to measure between 2 and 30 ng of MAG. The antigenic sites of MAG in tissue samples were exposed by solubilization in 1% (wt/vol) sodium dodecyl sulfate (SDS), and the final assay was done in a mixture of 0.25% SDS and 0.25% Triton X-100. The presence of the Triton X-100 overcame the inhibitory effect of SDS alone on the immune reactions. Application of the assay to whole homogenates of developing rat brain revealed the expected increase of MAG with the progression of myelination. Adult brain homogenate and purified myelin contained 2.7 and 7.4 micrograms of MAG/mg protein, respectively. Sciatic nerve contained a lower level of MAG, and cross-reacting material was not detected in nonneural tissues. This assay makes possible for the first time the analysis of MAG in whole tissue without prior myelin isolation or glycoprotein separation.