Radioiodinated biotin derivatives for in vitro radioassays

The synthesis of two radioiodinated biotin derivatives with the biotin-ureido group intact is described. This synthesis was performed by coupling (pH 8.5, 20-22 degrees C, 90 min) N-hydroxysuccinimidobiotin to tyramine, which was radioiodinated prior to this using a modified chloramine-T method. Two derivatives were produced, the N-[beta-(4-OH-3-125I-phenyl)ethyl] and the N-[beta-(4-OH-3,5-di 125I-phenyl)ethyl] biotin amides, depending on the amount of tyramine used in the radioiodination reaction. The final products were separated by thin layer chromatography (n-butanol: 2N NH4OH: ethanol, 3:1:1, v/v/v). The radioiodinated derivatives that were synthesized or their resulting mixture were found to complete with biotin for the avidin-binding sites; thus, they were capable of being used as tracers in biotin radioassays. The specific activity of their mixture was high-greater than 350 Ci/mmol-and they were stable for 2 mo at 4 degrees C.