Conversion of thyroxine (T4) to 3,5,3′-triiodothyronine (T3) in rat brain has recently been shown in in vivo studies. This process contributes a substantial fraction of endogenous nuclear T3 in the rat cerebral cortex and cerebellum. Production of T4 metabolites besides T3 in the brain has also been suggested. To determine the nature of these reactions, we studied metabolism of 0.2-1.0 nM [125I]T4 and 0.1-0.3 nM [131I]T3 in whole homogenates and subcellular fractions of rat cerebral cortex and cerebellum. Dithiothreitol (DTT) was required for detectable metabolic reactions: 100 mM DTT was routinely used. Ethanol extracts of incubation mixtures were analyzed by paper chromatography in t-amyl alcohol:hexane:ammonia and in 1-butanol:acetic acid. Rates of production of iodothyronines from T4 and T3 were greater at pH 7.5 than at 6.4 or 8.6 and greater at 37°C than at 22° or 4°C. Lowering the pH, reducing the protein or DTT concentrations, and preheating homogenates to 100°C all increased excess I− production but reduced iodothyronine production.
Michael M. Kaplan, Kimberlee A. Yaskoski