Deiodination activity in extrathyroidal tissues of the Atlantic hagfish, Myxine glutinosa

We measured low substrate (<1 nM) thyroid hormone (TH) deiodination activit ies in liver, muscle, intestine, and brain microsomes of Atlantic hagfish f asted for 2 weeks and found extremely low thyroxine (T-4) outer-ring deiodi nation (T4ORD) and inner-ring deiodination (T4IRD) as well as 3,5,3'-triiod othyronine (T-3) IRD activities. T3ORD, 3',5'-triiodothyronine (rT(3)) ORD and rT(3)IRD activities were undetectable. Hagfish deiodinating pathways re sembled those of teleosts in requiring a thiol cofactor (dithiothreitol, DT T) and in their inhibition by established deiodinase inhibitors and by TH a nalogues. However, under optimal pH and DTT conditions intestinal T4ORD act ivity exceeded that of liver about 10-fold. This contrasts with the situati on in teleosts but resembles that reported recently in larval and adult lam preys, suggesting the intestine as a primary site of TH deiodination in low er craniates. (C) 2000 Wiley-Liss, Inc.