Thyroid hormone deiodination pathways in brain and liver of rainbow trout, Oncorhynchus mykiss

Outer-ring (5') deiodination (ORD) and inner-ring (5) deiodination (IRD) of L-thyroxine (T-4) and 3,5,3'-triiodo-L-thyronine (T-3) were studied in whole-brain microsomes of rainbow trout and compared with liver deiodination. Brain T(4)ORD activity (apparent K-m = 1.2-2.5 nM; V-max = 0.10-0.14 pmol/hr/mg microsomal protein) was less than T(4)IRD activity (apparent K-m = 4.9; V-max = 0.32) and T(3)IRD activity (apparent K-m, = 5.2-5.4; V-max = 1.1-2.0); T(3)ORD activity was negligible. All three brain deiodinase pathways shared the following properties: pH optima between 7.0 and 7.3, activity enhanced by dithiothreitol (10 mM), weak inhibition by 6-n-propyl-2-thiouracil and iodoacetate, but stronger inhibition by aurothioglucose. Based on competitive inhibition, the substrate preference for brain T(4)ORD was T-4 = tetraiodothyroacetic acid (TETRAC) > 3,3',5'-triiodo-L-thyronine (rT(3)) > 3,5,3'-triiodothyroacetic acid (TRIAC) much greater than T-3 > 3,5-diiodo-L-thyronine (3,5-T-2). A comparable substrate preference profile was obtained for liver T(4)ORD (K-m 1 nM). Both T(4)IRD and T(3)IRD in brain had similar substrate preference profiles (rT(3) > 3,5-T-2 > T-4 > T-3) which differed from that of T(4)ORD. Negligible T(4)IRD and T(3)IRD activities existed in liver. We conclude that for rainbow trout (i) T(4)ORD systems in brain and liver are similar, and consistent with a common enzyme that does not match exactly either mammalian type I or II deiodinases, (ii) brain T(4)IRD and T(3)IRD enzymes share several common properties, and correspond functionally to the mammalian type III deiodinase, and (iii) under normal physiological conditions the predominant deiodinase pathways in brain (T(4)IRD and T(3)IRD) are poised toward T-4 and T-3 degradation, while that in liver (T(4)ORD) is poised toward T-3 generation. (C) 1996 Academic Press, Inc.