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Thyroid hormone deiodination in various tissues of larval and upstream-migrant sea lampreys, Petromyzon marinus.

Properties and activities of four potential thyroid hormone (TH) monodeiodinating pathways (T4ORD, L-thyroxine (T4) outer-ring (5') deiodination to 3,5,3'-triiodo-L-thyronine (T3); T4IRD, T4 inner-ring (5) deiodination to 3,3',5'-triiodo-L-thyronine (reverse T3); T3ORD, T3 outer-ring deiodination to 3,5-diiodo-L-thyronine; T3IRD, T3 inner-ring deiodination to 3,3'-diiodo-L-thyronine) were studied in microsomes of liver, kidney, muscle, and intestine of unmetamorphosed larvae and nontrophic upstream-migrant (spawning-phase) sea lampreys, Petromyzon marinus. T4ORD properties (pH optimum, dithiothreitrol cofactor requirement, apparent K(m), substrate preference and potency of potential inhibitors) were similar in most respects to those described previously for teleosts. T4ORD activity was detected in all larval tissues examined and was highest in intestine. In upstream migrants, T4ORD was also greatest in intestine, but low in muscle and kidney and undetectable in liver. T3ORD activity was not found in any tissue of either developmental stage. T4IRD and T3IRD activities were negligible in larval tissues, but present in kidney and particularly intestine of upstream migrants. We conclude that depending on developmental/physiological state, sea lampreys possess low-K(m) outer-ring and inner-ring monodeiodinases, which in most respects correspond functionally with those of teleosts. However, in contrast to teleosts, deiodination is particularly active in larval intestine, perhaps reflecting the release from the endostyle of TH into the lumen of the alimentary canal.