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Characteristics of type III iodothyronine deiodinase.

Type III iodothyronine deiodinase (ID-III) catalyzes the inner ring deiodination of T4 to rT3 and of T3 to 3,3'-T2, representing an important pathway for the inactivation of thyroid hormone. High activities of this "oncofetal" enzyme are found in rat brain, skin and fetal intestine, rat and human placenta, chick embryo liver, monkey hepatocarcinoma cells, human colon carcinoma cells, and tadpole liver. ID-III shows substrate preference for T3 over T4; Km values are approximately 10-fold lower for T3 than for T4 but Vmax values are similar. In contrast to the marked ontogenic pattern of ID-III in different tissues, the enzyme shows little change under pathophysiological conditions, such as fasting and thyroid dysfunction. Brain ID-III activity is decreased in hypo- and increased in hyperthyroidism, but the changes are small. Reaction of brain and placenta microsomes with BrAc 125I-T3 results in extensive labeling of a 32 kDa protein (p32). However, the relationship of p32 with ID-III is not clear, since labeling of p32 is also observed in tissues without ID-III activity and is not inhibited with a large excess of substrate.