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A study of the properties of the enzyme in rat liver that deiodinates 3,3'5'-triiodothyronine to 3,3'-diiodothyronine.

Studies were performed to assess the factors that influence the metabolism of [125I]rT3 in homogenates of rat liver. [125I]rT3 was metabolized with great rapidity; in 10% homogenates, rT3 was completely degraded within 30 min, giving rise to nearly equal proportions of two 125I-labeled metabolites, [125I]3,3′-diiodothyronine ([125I]3,3′-T2) and [125I]iodide. This process appeared to be enzyme mediated, since it was heat labile, temperature dependent, concentration dependent, and saturable with stable rT3. Aerobic preincubation markedly impaired the reaction, while anaerobic preincubation did not. These effects appeared to be due, respectively, to depletion and preservation of tissue glutathione (GSH) content during preincubation. Thus, the loss of activity seen after aerobic preincubation was prevented by the addition of either GSH itself or a NADPHgenerating system. GSH (5 HIM) stimulated [125I]rT3 degradation in dilute homogenates (1–5%) but not in concentrated homogenates (10–20%). Further evidence sug...