Frith S D, Eales J G
Department of Zoology, University of Manitoba, Winnipeg, Canada.
Gen Comp Endocrinol. 1996 Mar;101(3):323-32. doi: 10.1006/gcen.1996.0035.
Outer-ring (5') deiodination (ORD) and inner-ring (5) deiodination (IRD) of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3) were studied in whole-brain microsomes of rainbow trout and compared with liver deiodination. Brain T4ORD activity (apparent Km = 1.2-2.5 nM; V(max) = 0.10-0.14 pmol/hr/mg microsomal protein) was less than T4IRD activity (apparent Km = 4.9; V(max) = 0.32) and T3IRD activity (apparent Km = 5.2-5.4; V(max) = 1.1-2.0); T3ORD 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 T4ORD was T4 = tetraiodothyroacetic acid (TETRAC) > 3,3',5'-triiodo-L-thyronine (rT3) > 3,5,3'-triiodothyroacetic acid (TRIAC) >> T3 > 3,5-diiodo-L-thyronine (3,5-T2). A comparable substrate preference profile was obtained for liver T4ORD (Km 1 nM). Both T4IRD and T3IRD in brain had similar substrate preference profiles (rT3 > 3,5-T2 > T4 > T3) which differed from that of T4ORD. Negligible T4IRD and T3IRD activities existed in liver. We conclude that for rainbow trout (i) T4ORD 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 T4IRD and T3IRD 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 (T4IRD and T3IRD) are poised toward T4 and T3 degradation, while that in liver (T4ORD) is poised toward T3 generation.
研究了虹鳟全脑微粒体中L-甲状腺素（T4）和3,5,3'-三碘-L-甲状腺原氨酸（T3）的外环（5'）脱碘作用（ORD）和内环（5）脱碘作用（IRD），并与肝脏脱碘作用进行了比较。脑T4ORD活性（表观Km = 1.2 - 2.5 nM；V(max) = 0.10 - 0.14 pmol/小时/毫克微粒体蛋白）低于T4IRD活性（表观Km = 4.9；V(max) = 0.32）和T3IRD活性（表观Km = 5.2 - 5.4；V(max) = 1.1 - 2.0）；T3ORD活性可忽略不计。三种脑脱碘酶途径具有以下共同特性：最适pH在7.0至7.3之间，二硫苏糖醇（10 mM）可增强活性，6-正丙基-2-硫氧嘧啶和碘乙酸有微弱抑制作用，但金硫葡萄糖抑制作用更强。基于竞争性抑制，脑T4ORD对底物的偏好顺序为T4 = 四碘甲状腺乙酸（TETRAC）> 3,3',5'-三碘-L-甲状腺原氨酸（反T3，rT3）> 3,5,3'-三碘甲状腺乙酸（TRIAC）>> T3 > 3,5-二碘-L-甲状腺原氨酸（3,5-T2）。肝脏T4ORD也有类似的底物偏好谱（Km为1 nM）。脑中的T4IRD和T3IRD具有相似的底物偏好谱（反T3 > 3,5-T2 > T4 > T3），与T4ORD不同。肝脏中T4IRD和T3IRD活性可忽略不计。我们得出结论，对于虹鳟：（i）脑和肝脏中的T4ORD系统相似，与一种既不完全符合哺乳动物I型也不完全符合II型脱碘酶的共同酶一致；（ii）脑T4IRD和T3IRD酶具有几个共同特性，在功能上对应于哺乳动物III型脱碘酶；（iii）在正常生理条件下，脑中主要的脱碘酶途径（T4IRD和T3IRD）倾向于T4和T3的降解，而肝脏中的（T4ORD）则倾向于T3的生成。
