Pitfalls in Studies ofin VitroDeiodination of Thyroxine

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Abstract
Labeled L-thyroxine (T4) was incubated with mixtures of potassium ferricyanide and ferrocyanide in varying molal ratios in order to evaluate the minimal oxidation-reduction potential required to deiodinate T4 at a physiological pH. After paper chromatog-raphy in butanol-ethanol-ammonia (BEA), the percentage of I131 behaving as T4 decreased as the proportion of ferricyanide (and the red-ox potential) of the incubation mixture increased. These results were not altered by addition of plasma to the solutions prior to paper chromatog-raphy. The tentative conclusion that deiodination of T4 at pH 7.4was a function of the red-ox potential of the reaction mixture was, however, totally invalidated by further experiments which showed that results obtained after paper chromatography in BEA did not represent phenomena actually occurring in the system under study, being greatly influenced by the method of analysis. Artif actual deiodination of T4 arises when one of the components of the solution under study becomes an active deiodinating agent during the analytical procedure. The presence of such gross errors is not disclosed by the control blanks, and usual "protective" measures against artif actual deiodination may no longer be effective. Success in detecting this source of errors lies chiefly in our awareness of their possible presence. The possibility that artifacts of the general type exemplified here are interfering with results obtained in studies of in vitro deiodination by tissue preparations and biochemical model systems is stressed.