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Endocrinology Vol. 138, No. 1 254-258
Copyright © 1997 by The Endocrine Society

ARTICLES

Rainbow Trout Liver Expresses Two Iodothyronine Phenolic Ring Deiodinase Pathways with the Characteristics of Mammalian Types I and II 5'-Deiodinases1

Aurea Orozco, J. Enrique Silva and Carlos Valverde-R

Centro de Neurobiología (A.O., C.V-R.), Universidad Nacional Autonoma de Mexico, Apartado Postal 70–228, Ciudad Universitaria, México Distrito Federal 04510; and Department of Medicine (J.E.S.), Division of Endocrinology, McGill University, Montreal, Quebec H3T 1E2, Canada

Address all correspondence and requests for reprints to: Aurea Orozco, Centro de Neurobiología, Universidad Nacional Autonoma de Mexico, Apartado Postal 70 228, Ciudad Universitaria 04510, D.F., México. E-mail: aureao{at}servidor.unam.mx


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Deiodinases are major determinants of thyroid hormone tissue availability and disposal. The knowledge of the expression of these enzymes in lower species is important to understand evolutionary and ontogenetic aspects of thyroid hormone action and metabolism. Here we have studied outer ring deiodination in the trout liver using both reverse T3 (rT3) and T4 as substrates. The use of rT3 disclosed two enzymatic components with the characteristics of mammalian types I and II 5'-deiodinases. The high rT3-Km type I 5'-deiodinase activity (180 nM) has a low cofactor requirement (5 mM dithiothreitol) and is relatively sensitive to propylthiouracil inhibition, whereas the low rT3-Km activity was akin to the outer ring deiodination of T4 in these regards. The use of T4 exhibited only a single type of activity with a low Km (0.63 nM), a relatively high cofactor requirement (25 mM dithiothreitol), and propylthiouracil-resistance. Teleosts constitute a unique example of type II activity expression in the liver of an adult vertebrate. Furthermore, the Vmax of this enzyme is as high as that found in comparable homogenates from hypothyroid mammalian tissues, whereas the Vmax of the type I activity is lower than that of mammalian liver. These findings are in consonance with the peculiar kinetics of T3 in trout liver, kinetics remarkably similar to those of the mammalian pituitary, cerebral cortex, and brown adipose tissue, which also preferentially express type II deiodinase.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
THE THYROID gland secretes predominantly T4, which functions primarily as a prohormone because it is largely converted to T3 in peripheral tissues. T3 is about 10 times more active than T4 and is produced in an organ-specific manner by enzymatic outer ring deiodination (ORD) of its precursor. In mammalian species, ORD is catalyzed by two distinct enzymes, well characterized in operational terms and tissue distribution: the type I and type II 5'-deiodinases (1, 2, 3).

The complement of deiodinases present in vertebrate liver varies with the species and developmental stage of the organism. In fetal rats, for example, the liver expresses both type II and type III (an inner ring deiodinase), whereas, after birth, type I activity predominates (Aceves, C., unpublished data from our laboratory). Chicken embryo liver exhibits types I and II (4), whereas, in developing tadpoles, only type III is expressed (5). However variable the expression of deiodinases could be during ontogeny, adult liver of vertebrate species expresses predominantly, if not exclusively, type I deiodinase activity. In fish liver, it seems that two ORD processes are present, but it is not clear whether they correspond to the types I and II mammalian deiodinase activities (6, 7, 8). In trout, for example, enzyme kinetic analyses have demonstrated vastly differing Km values for hepatic T4-ORD. Whereas initial studies reported Km values of 190 nM (6) and 13 nM (7), a subsequent report by Mac Latchy and Eales (8) has suggested that two types of deiodinase activity, with Km values of 0.098 nM and 10 nM, coexist. Although the latter could correspond to type II ORD, the former, with such a low Km, is unprecedented. Furthermore, these authors find that the low-Km ORD is sensitive to inhibition by 6-N-propyl-2-thiouracil (PTU), whereas the higher Km process is relatively resistant to this agent, exactly the opposite of the observations with mammalian low- and high-Km 5'-deiodinases. The failure to obtain a consistent pattern of 5'-deiodinase activities in the trout may well be caused by methodological limitations. Thus, only T4 has been used as substrate, and in a range of concentrations that has not been adequate to detect separate enzymatic pathways such as the mammalian types I and II. Besides, even when tested in a broad range of concentrations, T4 is not a good substrate to demonstrate mammalian type I in assays that measure iodine release, as we will show here. Likewise, factors such as the temperature dependency of the enzymatic processes and the thiol cofactor concentrations have not been systematically controlled.

The three major iodothyronine deiodinases have recently been cloned (9, 10, 11, 12). These enzymes share, in common, having selenocysteine in the active center and a substantial level of homology in seemingly critical domains of the molecules; however, as predicted from studies based on biochemical identification, they differ rather markedly in catalytic properties, tissue distribution, and physiological responses. The accurate characterization of these enzymes in lower species is important to understand phylo- and ontogenetic aspects of thyroid hormone metabolism. Even though molecular biology could provide invaluable clues in these regards, the establishment of functional correlates among the members of this enzyme family is essential. We have therefore undertaken the systematic study of ORD processes in lower species. The goal of the present studies was to characterize iodothyronine ORD in the trout liver in an effort to clarify the confusion derived from previous studies. Our results demonstrate that this tissue expresses two ORD processes whose characteristics are quite similar to those reported for the mammalian type I and type II. Furthermore, the liver of the adult trout expresses abundant type II activity, a finding which may be relevant to previous observations on the metabolism of iodothyronines in this and related species.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Reagents
Nonradioactive iodothyronines were obtained from Henning Co. (Berlin, Germany). PTU was obtained from Sigma Chemical Co. (St. Louis, MO). Outer ring (125 I)-labeled T4 and rT3 (sp. act. 1200 and 1174 µCi/µg, respectively) were purchased from New England Nuclear (Boston, MA). Dithiothreitol (DTT) was obtained from Calbiochem (La Jolla, CA). Bradford reagents for protein determination were from Bio-Rad (Hercules, CA).

Animals
Rainbow trout ranging from 300–600 g were obtained from El Zarco hatchery located in the State of Mexico; Cb' (w2) (w) ig (semi-cold climate 5–12 C) (13). Fish were held under natural photoperiod and fed ad libitum with commercial trout pellets (Purina). Fish were killed by a blunt blow to the head, and the liver was rapidly removed and frozen in dry ice-acetone until used. Collected livers (n = 40) were homogenized in 1:10 (wt/vol) ice-cold homogenizing buffer (10 mM HEPES, 0.25 M sucrose, 10 mM EDTA, pH 7). Aliquots of pooled homogenates were quick-frozen in dry ice-acetone and stored at -70 C until assayed.

Deiodination assay
Enzyme activities in pooled homogenates were measured in triplicate under varying conditions by a modification of the radiolabeled iodide release method (14). The reaction mixture (total vol, 100 µl), containing liver homogenate diluted to the optimal protein concentration, was incubated with approximately 100,000 cpm 125I-T4 or 125I-rT3, plus variable concentrations of nonradioactive thyronine, DTT, and PTU as indicated in the corresponding experiment. Also, 1 µM of unlabeled T3 was added to the mixture to minimize the IRD of the substrates. The reaction was stopped with 50 µl of a cold solution containing 50% normal bovine serum and 10 mM PTU, immediately followed by 350 µl of 10% trichloroacetic acid. After centrifugation (3,000 rpm x 10 min), the supernatant was decanted onto a 1-ml Dowex-50X2 column equilibrated in acetic acid. The (125I-), product of the deiodination, was eluted with 2 ml of 10% acetic acid and counted in a gamma counter. Data are expressed in femtomoles 125I released per milligram of protein per hour. Protein content was measured by Bradford’s method (15). Basic characterization of enzyme activity included the analysis of the effects of protein concentration (15–600 µg), time (15–60 min), and temperature of incubation (12, 24, and 37 C). Subsequent analysis included substrate and cofactor dependency and effect of inhibitors. Analysis of the raw data included Lineweaver-Burk plots, and the graphical resolution of the kinetic data into two components was performed as described by Hofstee (16).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Definition of assay conditions
Because of the limitations inherent in using only one substrate to detect both types of ORD, we have used both, 125I-rT3 or 125I-T4. Protein-, time-, temperature- and pH-dependency were defined with 2 nM of either substrate. Enzyme activity was linear up to 2 mg/ml of protein per reaction mixture and optimal pH was 7 (data not shown). Optimal rates of deiodination were obtained at 23–25 C (Fig. 1Go), which is within the usual temperature range for this specie (17). Incubation time was fixed at 1 h for convenience.



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  		Figure 1. Effect of assay incubation temperature on hepatic 5'-deiodinase activity. The optimal assay incubation temperature (0–37 C) was tested using 2 nM 125I-rT3 or 125I-T4 and 25 mM DTT. For both substrates, enzyme activity was higher at around room temperature. Incubations were done in triplicate for 1 h using ~100 µg protein and at pH 7.

 
Enzyme characterization
As shown in Fig. 2Go, saturability of deiodination occurs at much higher concentrations with rT3 than with T4 (1 µM vs. 10 nM, respectively). This suggests fundamental differences in the deiodinative pathways of these two substrates. Based on these results, we examined rT3 deiodination between 0.5 and 2000 nM at various temperatures (12, 24, and 37 C) and at high DTT concentrations (25 mM) with the idea of distinguishing more than one enzymatic pathway. As shown in Fig. 3AGo, the enzyme seems to be more efficient around the physiological temperature (24 C). At this temperature, the Eadie-Hofstee analysis of the data (Fig. 3BGo) clearly shows two components, suggesting indeed the presence of two pathways of rT3 ORD. One of them has a high rT3 Km (180 nM) and a Vmax of 28 pmol 125I/mg·h, whereas the other has a lower Km (8 nM) and a Vmax value of 2 pmol 125I/mg·h In contrast, T4-5'-deiodinase generated a single component in the Eadie-Hofstee plot, with Km and Vmax of 0.63 nM and 200 fmol 125I/mg/h, respectively (Fig. 4Go). This experiment was done at 25 mM DTT (see below and Fig. 5Go). These results strongly suggest the presence of two pathways of ORD, one that can deiodinate both T4 and rT3 at low concentrations, and another that works at higher concentration that is demonstrable only with rT3 in our assay conditions.



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  		Figure 2. Effect of substrate concentration on hepatic 5'-deiodination of outer ring-labeled 125I-rT3 or 125I-T4 by trout liver homogenates. A wide range of rT3 and T4 concentrations (0.5–2000 nM) was tested with 25 mM DTT, at 24 C, and otherwise as indicated in the legend to Fig. 1Go.

 


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  		Figure 3. Kinetics of rT3-5'-deiodination by trout liver homogenates. Effect of temperature reaction. Except for temperature and substrate concentrations, other incubation conditions were as indicated in Fig. 2Go. A, Velocity-substrate concentration plot at the indicated temperatures; B, Eadie-Hofstee analysis of data obtained at 24 C. Two components were obtained with Km and Vmax values of 180 and 8 nM and 28 and 2 pmol/mg·h, respectively.

 


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  		Figure 4. Kinetics of T4-5'-deiodination by trout liver homogenates. Because of the low level of 125I release at higher concentrations (see Fig. 2Go), maximal 125I-T4 concentration tested was 9 nM; otherwise, the assay conditions were as in Fig. 2Go. The Eadie-Hofstee plot shows one component with a Km value of 0.63 nM and a Vmax of 0.200 pmol/mg·h.

 


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  		Figure 5. Effect of DTT on rT3 and T4-5'-deiodination by trout liver homogenates. The assay was performed using an isotopic solution of 200 nM 125I-rT3 or 1 nM 125I-T4 at indicated DTT concentrations; otherwise, assay conditions were as in Fig. 2Go. Each bar represents the mean ± SE of 125I release as percent of total counts (n = 3).

 
We then examined the effect of DTT on 5'-deiodinase activity at 200 nM rT3 and 1 nM T4. Figure 5Go shows that when rT3 is the substrate, the deiodinating activity is not modified by DTT concentrations above 2.5 mM, whereas T4 deiodination increases with DTT up to 25 mM. In experiments not shown, DTT concentration was increased to 35 mM without further increase in activity.

To complete the characterization of the two pathways, we then examined the effect of PTU. Conditions were chosen to see the higher Km activity with 200 nM rT3 and 5 mM DTT, and the low Km activity with 1 nM T4 and 25 mM DTT. PTU effectively reduced rT3 deiodination activity, by 50% at 1 mM and by 85–90% at 10 mM, whereas it failed to inhibit T4 ORD at these two concentrations (Fig. 6Go). On the other hand, the inhibition by PTU was rT3 concentration-dependent, because 1 mM PTU inhibited activity by less than 10% at substrate concentrations of 2 nM or less (data not shown), which is consistent with the T4 deiodinase being the same as the second deiodinase for rT3.



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  		Figure 6. Effect of PTU on rT3 and T4-5'-deiodination by trout liver homogenates. Assays were performed with 200 nM 125I-rT3 or 1 nM 125I-T4 at the indicated PTU concentrations; otherwise, assay conditions were as in Fig. 2Go. Each bar represents the mean ± SE of 125I release as percent of total counts (n = 3).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
To the best of our knowledge, this is the first study in teleosts in which rT3 has been used to characterize hepatic 5'-deiodinase activity. Because rT3 is a good substrate for both type I and type II deiodinases, it was more likely to allow the identification of the two types of deiodinase activities. The Eadie-Hofstee plot of data obtained with a broad range of concentrations of rT3 shows indeed two components with Km and Vmax values in the range reported for mammalian type I and type II deiodinases. Furthermore, at concentrations better reflecting the high Km component, enzyme activity has a low DTT requirement (Fig. 5Go) and is sensitive to inhibition by PTU (Fig. 6Go) in a manner akin to the mammalian type I (1, 2). The two components observed are similar to those found in rat cerebral cortex (18, 19) and in the pituitary (20), tissues that contain both type I and type II deiodinases. In contrast, ORD of 125I-T4 showed a steeper fall, with increasing T4 concentrations (Fig. 2Go), and when analyzed by the Eadie-Hofstee’s method, it exhibited only a single component with Km and Vmax values comparable with mammalian type II deiodinase. The demonstration of just one component should not be taken as an indication that a putative type I deiodinase in trout liver would not catalyze ORD of T4. On the contrary, the single component is a reflection of the relatively low type I-mediated T4 ORD and of the limitations imposed by the specific activity of 125 I-T4 to measure the release of 125 I when the predominant pathway of deiodination is the type II. Thus, it has been demonstrated previously that T4-ORD can be detected in trout liver at higher substrate concentrations if the activity is measured by the production of T3 by RIA (7, 21, 22), which would not be possible if the activity were measured by the release of 125 I, because, at high substrate concentrations, the fractional rate deiodination is very low (Fig. 2Go). In the rat liver, in contrast, type I-mediated T4 ORD can be demonstrated both by isotopic 5'-deiodination and by T3 RIA (23, 24). Nonetheless, as measured here, ORD of T4 is dependent on DTT and is resistant to PTU, as reported for mammalian type II in pituitary and brain (18, 19, 20, 25).

Previous studies regarding hepatic deiodinating systems in teleosts have all been conducted using T4 as substrate (6, 7, 8, 21, 22, 25, 26). The fact that total trout liver 5'-deiodinase activity saturates at low T4 concentrations (10 nM) is probably the explanation for the previous failure to clearly identify type I activity in teleostean liver and for the observation that ORD had a low (7, 26) or ultra-low T4-Km type I activity (8). Nevertheless, present results in trout liver and previous reports in tilapia kidney (22, 25) have clearly shown the presence of rT3-ORD activity with the kinetic characteristics of mammalian type I. Of note is the fact that, when used at high substrate concentrations, both rT3 (present results, 22, 25) and T4-ORD (6) consistently exhibit a conspicuous, albeit partial, resistance to PTU.

Present results show that the Vmax of T4-ORD in the euthyroid trout liver is 200 fmol 125I/mg protein·h. This value is comparable with that found in hypothyroid rat pituitary and higher than the values found in hypothyroid rat brain (27). Altogether, these data indicate that there is abundant type II deiodinase in the trout liver. Such a high specific activity and the large size of the liver compared with brain or pituitary are likely to be germane to the high T3-circulating levels in the trout, despite the lack of thyroidal T3 secretion, under normal physiological conditions (28, 29, 30). It should be noted that thyroidal T3 secretion is higher in fasted trout (31). Nonetheless, our data do not allow a quantification of the contribution of each liver pathway to the extrathyroidally produced T3 in the trout. Such analysis would require in vivo experiments beyond the scope of this paper. Furthermore, the presence of high quantities of a type II T4 deiodinase could be relevant to the also-peculiar kinetics of T3 in trout liver. Whereas rat liver rapidly equilibrates with plasma (32), trout liver seems to have two compartments, a fast one and a slow one (30). This latter compartment has not been explained but its kinetics of exchange with plasma are remarkably similar to the pituitary (33), cerebral cortex (34), and brown adipose tissue (35) in rats. The predominant ORD in these three tissues is type II. Moreover, calculations of free T3 in the nucleoplasma show that very high quantities of free T3 exist in the brain and, in general, in tissues with type II (36). These findings are consistent with the presence of a perinuclear compartment of high T3 concentration that is dependent on the activity of type II as postulated for cerebral cortex (35). Thus, it is tempting to speculate that the peculiar kinetics of exchange of trout liver T3 from plasma is related to the presence of abundant type II. Whether this internal compartment dilutes the incoming T3 from plasma or is the reflection of another kind of compartmentation is something that will have to be investigated.

Previous studies have reported the presence of type II activity in embryonic chicken liver (4), as well as in embryonic rat liver (Aceves C, unpublished data from our laboratory). The finding of abundant type II in adult trout liver makes these observations even more interesting because the expression of type II in embryonic bird and mammal livers may be reflecting an evolutionary pattern.


   Acknowledgments
 
We gratefully thank Dr. Donald L. St Germain for his invaluable comments and critical reviews of this manuscript.


   Footnotes
 
1 This study was supported in part by Grants CoNaCyT: 1981 PN, 4851-N 9406, and Scholarship 85753. Back

Received April 19, 1996.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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