More Labeled Thyroxines Within Reach Coupling reaction gives compound labeled in carbon or iodine atoms in 20% yield 140TH
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A new way to prepare additional tailormade, labeled thyroxines now makes it possible for chemists to study the metabolism of this hormone in greater detail. Dr. H. J. Cahnmann and Dr. Tetsuo Shiba of the National Institutes of Health have made several labeled thyroxines using a known synthetic route that may work just as well for a wide variety of radiothyroxines. The only labeled form of the hormone that is commercially available has two labeled iodines in the phenolic ring. Other workers have reported syntheses for other forms, but these methods are tedious or give low yields. Dr. Cahnmann says. Having thyroxines labeled in a variety of positions in either ring or in the side chain, he adds, would be very important in metabolic or other studies. To make these compounds, the NIH scientists use a reaction discovered by scientists at the Warner-Lambert Research Institute. They add 4-hydroxy3,5-diiodophenylpyruvic acid to a solution of 3,5-diiodotyrosine in a borate or phosphate buffer, about pH 7. Oxygen is bubbled through the reaction mixture, while the pH is kept con-
stant by the addition of base. The sodium salt of thyroxine precipitates from the mixture. Yields are about 20(/( on a gram scale, and 12 to 20% on a microgram scale. So far, Dr. Cahnmann and Dr. Shiba have made thyroxines with labeled iodines in the C-3' and C-5' positions (the one commercially available) or in the C-3 and C-5 positions. They have also prepared the compound with a carbon-14 phenolic ring and one with the nonphenolic ring and side chain of C-14. The same coupling reaction will make other labeled thyroxines if the labeled starting materials are available. Thus, chemists may make uniformly carbon- or iodine-labeled thyroxine, or thyroxine labeled with both radioactive elements. Uniformly tritium-labeled thyroxine will also be within reach. The ready availability of many more labeled thyroxines will make the job of tracing its metabolic pathway much easier. One of the routes in thyroxine metabolism involves the loss of one or both iodine atoms of the phenolic ring. These are the atoms that are labeled in the only radiothyroxine easily available up to now, so thyroxines with other labeled atoms are important in studying the fate of the hormone after deiodination.
NIH Chemists Use This Reaction .
. . . To Make These Labeled Thyroxines
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