Radiohalogenation of small biomolecules

Halogenation of tyrosine, uracil, cytosine, uridine and histidine has been attempted—using both the enzymatic catalysis and chemical oxidation labelling methods—with I-125 and Br-80m. Chemical and enzymatic halogenation gave high yields of iodinated tyrosine, uracil, cytosine, uridine, and histidine, and brominated tyrosine, uracil, and cytosine. Reverse-phase high-pressure liquid chromatography separation and gel filtration were both investigated for use in the isolation and analysis of the labelled products. HPLC was most successful and had the advantages that: (1) the technique was fast enough to use with short-lived radiohalogens, i.e. t½ ≈ 20mins (such as I-128, t½ = 25 mins); (2) the products were pure and free from interfering impurities. It was found that the use of chemical oxidation and enzymatic catalysis labelling coupled with reverse phase HPLC separation provided a very rapid method of obtaining high specific activity of radiohalogenated compounds in aqueous solution completely free from buffering agents and non-halogenated parent compounds.