ABSTRACT
Early labelings were done “in bulk” using 100 to 400 mg of a protein and incorporating four to six atoms (or more) of iodine per molecule. The number of iodine atoms substituted per molecule of protein can be calculated from the starting molar ratio of the reactants in the iodination mixture in conjunction with the efficiency of the reaction. Efficiency is best determined by direct counting of the protein- bound and unbound radioactivity fractions following iodination. Iodine monochloride (IC1) has been in use as a halogenating agent since the 1840s. The amount of IC1 used should be small enough to avoid substituting an undesirably large number of iodine atoms and large enough relative to the quantity of I represented by the radioactivity. The major problem area of the IC1 method is the labeling of proteins in the microgram range, this being particularly true for the variants of the method which rely on isotope exchange.
