Binding Properties of Radiolabeled Basic Protein of Myelin: Reassessment in Relation to Diagnostic Immunoassays

The binding of radioiodinated basic protein of myelin ([¹²⁵I]BPM) to a sheep lymphocyte cell pellet, sheep erythrocytes, and cell‐free tubes used in the assay was investigated as a possible diagnostic procedure in multiple sclerosis. [¹²⁵I]BPM apparently bound to 5 × 10⁶ sheep lymphocytes incrementally with no plateau, up to 300 ng, and also to sheep erythrocytes: when cells were transferred to fresh tubes, over 90% of the radioactivity remained (in the original tube, regardless of the tube surface. Various substances, including BPM and unrelated basic proteins, competitively inhibited the binding of basic protein of myelin to sheep cells and assay tubes. Binding was inhibited by sera from patients with multiple sclerosis, but equally so by normal sera. The large capacity of BPM to bind nonspecifically could limit its use in the above type of binding assay and would need to be allowed for in conventional radioimmunoassays.