The Ontogeny of Iodothyronine 5'-Monodeiodinase Activity inRanacatesbeiana Tadpoles*

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Abstract
Generation of T3 from T4 in vivo cannot be demonstrated in tadpoles until just before metamorphic climax (MC). Possible explanations include the absence of the necessary 5''-monodeiodinase (5''D) process, and/or the presence of an active T3 and T4 5-deiodinase (5D) system before MC. In the present study, 5''D activity was determined in the 12,000 .times. g supernatent fraction of tissue homogenates prepared from tadpoles in premetamorphosis (PM) and MC (induced by immersion in 2 .times. 10-8 M T4) by measuring the 125I- formed from [125I] rT3 or [125I]T4 in the presence of 20 mM dithiothreitol. Eadie-Hofstee plots of data were used to determine maximum velocity and Km. During PM, 5''D activity was undetectable in liver, tail, heart, and kidney, minimal in brain and gut, and could be quantitated only in skin. During MC, 5''D activity was undetectable in liver, heart, and kidney, but was present in tail tissue and was increased more than 5-fold in skin and gut. The increased activity was due to a change in maximum velocity, with no change in Km. In its properties, the tadpole 5''D system was comparable to the type II system found in some mammalian tissues. Thus, it exhibited Km values for T4 and rT3 in the nanomolar range, the preferred substrate was T4, and activity was unaffected by propylthiouracil in the presence of 10 mM dithiothreitol. Under comparable incubation conditions, T3 5D activity was detected in most tissues during PM; the highest activity was found in liver, gut, and kidney. 5D activity was barely detectable in MC. From these studies it is suggested that accumulation of T3 generated from t4 in the tadpole is minimal before MD due to the predominance of 5D activity. During MC, accumulation of T3 is possible because of the substantial increase in 5''D activity together with a marked drop in 5D activity. The principle T3-generating organs appear to be gut, skin, and tail tissue.