Preparation of highly 3H-labeled S-100 protein under nondenaturing conditions

A method for obtaining a tritium-labeled S-100 protein of high specific radio-activity (〉~ 10 Ci/mmol) under mild conditions is described. The method is based on the reductive methylation of lysine residues; the labeling procedurs does not appreciably alter the physical and chemical properties of 8–100 protein, as measured by studies of intrinsic fluorescence enhancement, ⁴⁵Ca binding, electrophoretic mobility, titrations of sulfydryl groups, and intramolecular crosslinking of S-100 via disulfide bond formation. Alternative labeling procedures based on chemical or enzymatie iodination with ¹²⁵I, invelving the use of powerful oxidizing agents, cause an irreversible exidation of the sulfydryl groups and affect the above-mentioned properties of the S-100 protein.