THYROID-HORMONE DEIODINATION IN VARIOUS TISSUES OF LARVAL AND UPSTREAM-MIGRANT SEA LAMPREYS, PETROMYZON-MARINUS

Properties and activities of four potential thyroid hormone (TH) monod eiodinating pathways (T4ORD, L-thyroxine (T4) outer-ring (5') deiodina tion to 3,5,3'-triiodo-L-thyronine (T3); T4IRD, T4 inner-ring (5) deio dination to 3,3',5'-triiodo-L-thyronine (reverse T3); T3ORD, T3 outer- ring deiodination to 3,5-diiodo-L-thyronine; T3IRD, T3 inner-ring deio dination to 3,3'-diiodo-L-thyronine) were studied in microsomes of liv er, kidney, muscle, and intestine of unmetamorphosed larvae and nontro phic upstream-migrant (spawning-phase) sea lampreys, Petromyzon marinu s. T4ORD properties (pH optimum, dithiothreitrol cofactor requirement, apparent K-m, substrate preference and potency of potential inhibitor s) were similar in most respects to those described previously for tel eosts. T4ORD activity was detected in all larval tissues examined and was highest in intestine. In upstream migrants, T4ORD was also greates t in intestine, but low in muscle and kidney and undetectable in liver . T3ORD activity was not found in any tissue of either developmental s tage. T4IRD and T3IRD activities were negligible in larval tissues, bu t present in kidney and particularly intestine of upstream migrants. W e conclude that depending on developmental/physiological state, sea la mpreys possess low-K-m, outer-ring and inner-ring monodeiodinases, whi ch in most respects correspond functionally with those of teleosts. Ho wever, in contrast to teleosts, deiodination is particularly active in larval intestine, perhaps reflecting the release from the endostyle o f TH into the lumen of the alimentary canal. (C) 1997 Academic Press.