The synthesis of 125I-2-iodohistamine for radioimmunoassay.
Recently radioactively labeled iodohistamines have been claimed to have superior shelf-life to the iodophenols more commonly used in radioimmunoassay of small molecules. This claim is based largely on theoretical considerations; no systematic study has appeared. We found that iodination of histamine on a macroscale proceeds rapidly at pH 7-8.4 to yield principally 2-iodohistamine. With a large excess of iodine, substantial diiodination can be achieved. In 0.5 N sodium hydroxide solution, triiodination produces 1,2,5-triiodohistamine; the N-I bond, however, is somewhat labile. 125I-2,5-Diiodohistamine is also somewhat unstable, having a first order decomposition rate of 1.6 X 10(-3) day-1 (t1/2, 182 days), while 125I-2-iodohistamine shows a barely perceptible change in 60 days (7.5 X 10(-5) day-1). The assignment of the first iodine introduced to C-2 is based on a comparison of the NMR spectra of monoiodohistamine and histamine. Iodination with carrier-free iodine-125 using the Hunter-Greenwood procedure (chloramine-T) produces a 76% yield of mono- and a 19% yield of diiodo product which are easily isolable by a single TLC using silica gel in the solvent system, ethanol:ethyl ether:water, 5:5:2.
D055598:Chemical Phenomena; D002621:Chemistry; D006632:Histamine; D007457:Iodine Radioisotopes; D007553:Isotope Labeling / Q000379:methods; D011863:Radioimmunoassay
D007457:Iodine Radioisotopes; D006632:Histamine