Effects of triiodothyronine-induced hyperthyroidism on lipid peroxidation, erythrocyte resistance and iron-binding and iron-oxidizing antioxidant properties of plasma in the rabbit.

The effect of hyperthyroidism on some oxidative stress parameters is reported. The hyperthyroid state was induced by intraperitoneal injection of triiodothyronine (T3)(10 microg/kg body weight) for 14 days in two groups of female rabbits (3 and 12 months old). The T3 injection caused increase by 1.5-fold to 1.7-fold in T3 serum level, and 2-fold to 3-fold decrease (age-dependent) in body weight gain at the end of experimental period. The induced hyperthyroidism caused a significant increase in the serum concentration of the lipid peroxidation end-product malondialdehyde and lowered erythrocyte resistance to oxidative stress when subjected to the free radical generator 2,2'-azobis(2-amidinopropane) hydrochloride in vitro. The half maximum haemolysis time (HT50) decreased in the both experimental groups of rabbits, by about 12 min in the 3-month-old animals and 27 min in the 12-month-old animals. The study showed for the first time that hyperthyroidism enhances the ability of plasma to protect against iron-binding and iron oxidizing organic radicals. The scavenging property and antioxidant capacity of plasma against iron-binding inorganic radicals also increased. Measurement of erythrocyte resistance to oxidative stress and the protective ability of plasma against oxygen radicals discriminates the thyroid hormone modulatory effects in defence mechanisms against lipid peroxidation.

Species differences in the susceptibility of erythrocytes exposed to free radicals in vitro.

The susceptibility of human, cow, pig, sheep and rabbit erythrocytes to free radicals (peroxyl radicals) generated in vitro by 2,2'-azo-bis(2-amidinopropane) hydrochloride (AAPH) was evaluated by means of a haemolysis test and expressed as the time to 50% of maximal haemolysis (HT50). The most sensitive to damage by free radicals appeared to be the erythrocytes of pigs and sheep, their HT50 values being (mean +/- SEM) 85.1 +/- 1.28 min and 89.0 +/- 1.31 min, respectively. Human erythrocytes and those of cows and rabbits were about twice as resistant, their HT50 values being (mean +/- SEM) 174.3 +/- 1.53 min, 181.2 +/- 1.22 min and 183.4 +/- 2.54 min, respectively. Pig and sheep erythrocytes used in the haemolysis test provided an indication of the antioxidant status in a shorter time (2.5 h versus 4.5 h) than those of the other species studied. The results indicate that the HT50 test may be a convenient alternative to the osmotic resistance test for defining the antioxidant resistance of erythrocytes.

Fever induced oxidative stress: the effect on thyroid status and the 5'-monodeiodinase activity, protective role of selenium and vitamin E.

The thyroid hormones metabolism is considerably altered in many pathological processes including fever. Experiments performed on rabbits (n=62) showed that increase in the rectal temperature by 1 degrees C (after turpentine oil sc injections) decreased 5'-monodeiodinase activity, the enzyme responsible for deiodination of thyroxine to the most active thyroid hormone 3,3',5-triiodothyronine (T3), in the liver by 25% and in the kidney by 20%. Triiodothyronines concentration in serum decreased during fever from 1.57+/-0.12 to 0.52+/-0.02 nmolT3/l and from 0.17+/-0.01 to 0.07+/-0.02 nmol rT3/l. The increase in the body temperature intensified lipid peroxidation processes (malondialdehyde level increased from 1.2 times in kidney, and 1.4 times in the liver homogenates to 1.6 times in serum). The antioxidants (vitamin E and selenium) supplementation decreased lipid peroxidation processes during fever and partly restored the 5'-monodeiodinase activity. The present study confirmed our previous observations in vitro that lipid peroxidation (free radical formation) influences the 5'-monodeiodinase activity in tissues and alters the thyroid hormones metabolism.

Erythrocyte osmotic fragility test as the measure of defence against free radicals in rabbits of different age.

Peroxidation of the unsaturated bonds of membrane lipids increases fragility and cellular lysis of red blood cells. Erythrocyte susceptibility to the free radicals (peroxyl radicals) generated in vitro by 2,2'-azo-bis(2-amidinopropane) hydrochloride (AAPH) was evaluated and expressed as 50% maximal haemolysis time (HT50) in 3 groups of rabbits of different age. Erythrocytes of 1.5-month-old rabbits were more sensitive to free radicals than those of 3.5- and 6-month-old ones. In the three groups, significant negative correlation (r = -0.8 to -0.98) between the lipid peroxidation rate (thiobarbituric acid reactive substances; TBARS concentration) in blood plasma and the erythrocyte resistance to free radicals was found. This result suggests that the plasma antioxidant defence system is interrelated with that of the red blood cells and that the erythrocytes can be a good model for studies of oxidative stress. The simple haemolysis test reflecting the free radical defence can be useful for evaluating the antioxidant properties of various compounds.

Evidence for the presence of 5'-deiodinase in mammalian seminal plasma and for the increase in enzyme activity in the prepubertal testis.

Thyroid hormones are critical for structural and functional development of the testis and Sertoli cells are considered true target cells for triiodothyronine (T3). However, the role of thyroid hormones in the adult testis seems to be minimal and the mechanism by which they affect testicular function is not known. Due to the existing blood-testis barrier the concentration of thyroid hormones in seminal plasma is kept lower than in blood plasma. We have found that T3 may reach the testis not only from the circulation but also from local enzymic conversion of thyroxine to T3. The presence of the enzymic activity responsible for thyroxine 5'deiodination and for generating T3 locally was also found in boar's seminal plasma. The seminal plasma 5'-deiodinase (5'-D) appeared to be predominantly the propylthiouracil (PTU)-insensitive type II isoenzyme found, so far, in tissues where it plays a role in paracrine signalling. It contains selenocysteine in its molecule (inhibition by aurothioglucose), and has an apparent Km for reverse-T3 as substrate of 0.36 nM and a Vmax 23.8 fmol I-/mg protein/min. Because the seminal plasma 5'-D is partially, but uncompetitively, inhibited by PTU, the presence in seminal plasma of two 5'-D isoenzymes (type I and II) cannot be excluded. The 5'-D activity in testes increased significantly between week 3 and 4, and this increase was concomitant with increase in testicular size. The relationship between testicular weight gain and age showed a similar characteristic change and corresponded to the change in 5'-D activity. Unlike in rodents, the testis of the prepubertal pig has thyroid hormone receptors in Sertoli cells, and suggests that in growing piglets, testicular 5'-D is a key factor regulating local supply of biologically active T3, and is an essential factor in testicular paracrine function. The present results are the first demonstration and characterization of the 5'-deiodinase in seminal plasma.

Presence of thyroxine deiodinases in mammary gland: possible modulation of the enzyme-deiodinating activity by somatotropin.

Thyroid hormones (TH) and somatotropin (ST) play critical role in lactation. One explanation of their multiple physiological actions is based on the functional interrelationships among ST, TH, and thyroxin deiodinase (5'D). This enzyme is present in the mammary tissue, milk cellular components, and whole milk and is responsible for intramammary production of triiodothyronine (T3). In rats in which the 5'D isozymes in the mammary gland and in the liver are similarly of type I (5'D-I), an enhancement of mammary 5'D-I causes a reduction of hepatic 5'D-I activities. This opposite rearrangement in the mammary and hepatic deiodinating activities is thought to be a factor of a homeorhetic response characterized by an increased and compartmentalized energy expenditure of the mammary gland. In the cow, the mammary 5'D is the type II (5'D-II) deiodinase. The 5'D-II, owing to its high catalytic efficiency, secures T3 production, making tissues relatively independent from the circulatory levels of TH and from variations in the hepatic 5'D-I activity. No significant alterations of 5'D-II isozymes were found during a low T3 syndrome. Location of tissue deiodinases in the cow, the 5'D-II in the mammary gland, and the 5'D-I in the liver make it so that T3 production in these two tissues can be dissociated in time to secure better local requirement for T3 supporting lactation. To date, attempts to evidence that the alterations in iodothyronines blood levels and in tissues' 5'Ds activity during lactation are due to ST action have not received clear experimental support in either cows or rats.

Triiodothyronine (T3), insulin and characteristics of 5'-monodeiodinase (5'-MD) in mare's milk from parturition to 21 days post-partum.

It is generally accepted that hormones and tissue growth factors are supplied from mother to neonate via mammary secretion. Among the protein hormones, insulin and prolactin are considered as the most important milk components for neonates. The significance of the thyroid hormones, namely triiodothyronine (T3) generated locally by 5'-monodeiodinase (5'-MD) in the mammary tissues, for the mammary gland itself and for suckling neonates is still under consideration. In the present study the activity of the 5'-MD and the concentrations of T3 and insulin in mare's colostrum and milk during the first 21 days of lactation were measured. Post partum, T3 increased to its highest concentration around day 4 (1.14+/-0.08 nmol/L), then progressively decreased until day 7, reaching a relatively stable concentration of 0.71+/-0.06 nmol/L (overall mean for days 7-21). The colostral insulin concentration, highest on the day of parturition (401.0 +/-24.9 microU/mL), decreased to a nadir value (25.0+/-3.4 microU/mL) on day 5, after which it tended to increase. The mare's milk showed the presence of PTU-sensitive (type I) and PTU-insensitive (type II) 5'-monodeiodinases (5'-MD). Contrary to the classical type II 5'-MD, the mare's milk isoenzyme was inhibited non-competitively by aurothioglucose. A significant relationship (r=0.962, P < 0.01 ) between T3 concentration and 5'-MD activity, from the I st to the 6th lactational day was found, which may indicate a dependence of T3 concentration on the milk 5'-MD activity. The presence of 5'-MD of type II suggests that intra-mammary T3 generation may play a paracrine role supporting lactogenesis. Estimating that 1.8 microg of colostral T3 (0.456 microg/L) is consumed daily by a suckling foal, the T3 hormone action within the intestinal tract cannot be ruled out. This is the first paper to provide evidence of T3 and insulin concentrations, and of T4-5'-monodeiodinases activity in colostrum and milk of the mare.

Stimulatory effect of melatonin on the 5'-monodeiodinase activity in the liver, kidney, and brown adipose tissue during the early neonatal period of the rabbit.

The response of type I 5'-monodeiodinase activity (5'-MD) to a s.c. injection or oral administration of melatonin was studied in 3-, 5-, and 7-day-old rabbits. Melatonin-treated animals showed higher activity of the type I 5'-MD in the liver and kidney and of type II 5'-MD in brown adipose tissue (BAT). This respond to melatonin treatment was age dependent. The stimulatory effect of melatonin on renal 5'-MD activity was observed only in 3- and 5-day-old rabbits and in the liver and BAT during the first week of life. Oral melatonin administration tended to exert a more marked effect on enzyme activity than s.c. injection of the hormone. Changes in 5'-MD activities were accompanied by an increase in serum iodothyronine (T4, T3, and rT3) concentrations. The T3 and rT3 increases may result from the deiodinating processes by the type I 5'-MD and 5-MD, respectively, whereas the rise in the serum T4 was probably due to the stimulatory effect of melatonin on the secretory activity of the thyroid gland itself. These results are the first description of the effects evoked by melatonin treatment during the early neonatal period in newborns of the altricial type.

The protective role of some antioxidants and scavengers on the free radicals-induced inhibition of the liver iodothyronine 5'-monodeiodinase activity and thiols content.

The influence of free radicals on iodothyronine 5'-monodeiodinase activity, the enzyme responsible for the deiodination of thyroxine to most active thyroid hormone 3,3',5-triiodothyronine (T3), was examined in rabbit's liver. Incubation of the liver homogenate with the xanthine oxidase based free radical generating system (FRGS) caused a reduction in 5'-monodeiodinase activity to the 53.9% of initial value taken as 100%, and on increase (52.9% over the control value) in the level of lipid peroxidation by-product malondialdehyde. The inhibitory effect of FRGS on 5'-monodeiodinase activity was blocked by free radical scavengers: catalase (91.2%), thiourea (88.8%), superoxide dismutase (85%) and by some antioxidants; Trolox (the water soluble alpha-tocopherol analog, 81.4%) and glutathione (77.7%). These results suggest that oxygen radicals, hydrogen peroxide and hydroxyl radicals were involved in the inhibition of the 5'-monodeiodinase activity. The same scavengers significantly decreased the malondialdehyde formation. In the presence of the FRGS the amount of total SH groups (the cofactor of the deiodination reaction) was decreased in the liver homogenate to 51% of the initial value, and a positive relationship between the total SH groups levels and the 5'-monodeiodinase activity in the presence of free radical scavengers was observed. It suggests, that active oxygen radicals generated by FRGS may inactivate 5'-monodeiodinase, at least in part, by reduction of thiol cofactors.

Divergent deiodination of thyroid hormones in the separated parts of the fetal and maternal placenta in pigs.

Previous work from this laboratory has shown that the thyroid gland of the fetal pig begins to function at about day 46-47 (0.40-0.415 fraction of gestational age). Sera from fetuses contain lower thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) concentrations than maternal sera, except for about 2 weeks before term. The fetal T4 metabolism is dominated by the 5'-monodeiodinating activity (5'-MD). In the present study we measured the iodothyronines content, and the outer (5'-MD) and inner (5-MD) monodeiodinases activity, in homogenates of the placenta. The pig placenta, which is of the epitheliochorial type, was separated into the fetal and the maternal part. The concentrations of T4, T3 and rT3 were lower, and the deiodinating activity of 5'-MD and 5-MD higher, in the fetal than in the maternal placenta. The fetal placenta not only deiodinated more actively T4 to T3 and T4 to rT3, but degraded T3 to 3,3'-diiodothyronine (3,3'-T2) more actively than rT3 to 3,3'-T2. Such divergent deiodinating activity of T4 to T3, T3 to 3,3'-T2 and rT3 to 3,3'-T2 might favor establishing a relatively high and constant rT3 concentrations in fetal and maternal placentas, and a lower T3 in the fetal placenta. The inner ring deiodinating activity (excluding a day before parturition) was always more active in the fetal placenta, while the outer ring deiodinations varied in this respect, depending on the gestation stage. These results support the hypothesis that in the fetal pig, enzymatic deiodination of thyroid hormones forms a barrier which reduces transplacental passage of the hormones and that the fetal part of the placenta is the primary factor in the mechanism regulating the hormonal transfer. In spite of the presence of the barrier, there is an adequate maternal supply of thyroid hormones to the fetus in early gestation, which suggests that the enzymatic mechanism is influenced in some way by the thyroid status of the fetus.

Ontogeny of the generation of diiodothyronines (3,3'-T2 and 3',5'-T2) from triiodothyronines in the liver and kidney during the fetal life of the pig.

The present study was conducted to obtain information on the generation of diiodothyronines from triiodothyronines (3,3'-T2 from T3, and rT3, and 3',5'-T2 from rT3) as a result of the activity of the tissue monodeiodinase enzymes (MD) in the liver and kidney of pig fetuses and their mothers between 32 and 113 days of gestation. T3-5-MD activity in the fetal kidney during the gestational period was stable and higher than in the liver and in the maternal kidney. In contrast, T3-5-MD activity of the liver was 3-4 times lower in fetal than in maternal tissue in the first half of pregnancy, and in the second half of pregnancy 3,3'-T2 production from T3 in the maternal liver was equal to, or lower than, that in the fetal liver. The activity of MD deiodinating rT3 (3,3'-T2 and 3',5'-T2 generation) increased significantly in fetal liver and kidney in the last 2-3 weeks of pregnancy and was higher than in maternal tissues. In both tissues examined the inner ring deiodinating activity (IRD) was 5-10 times lower as compared to the outer ring (ORD).

Amniotic and allantoic fluid concentrations of thyroxine, 3,3',5'-tri-iodothyronine, 3,3'-di-iodothyronine and 3',5'-di-iodothyronine in the pig during the period of gestation.

Thyroxine (T4), 3,3',5'-tri-iodothyronine (reverse T3; rT3) and di-iodothyronines (3,3'-T2 and 3',5'-T2) were measured in pig amniotic fluid (AF) and allantoic fluid (Al) between 32 and 113 days of normal pregnancy. Low but measurable quantities of T4 in AF and Al (2.1 +/- 0.3 and 3.2 +/- 0.5 nmol/l respectively) were found before the onset of fetal thyroid gland function, which indicates the maternal source of T4. The presence of rT3 (55.8 +/- 4.1 pmol/l in AF and 49.8 +/- 5.3 pmol/l in Al), 3,3'-T2 (45.5 +/- 0.6 pmol/l in AF and 49.2 +/- 9.2 pmol/l in Al) and 3',5'-T2 (20.8 +/- 2.6 pmol/l in AF and 24.0 +/- 2.2 pmol/l in Al) may be attributed to the monodeiodinase system already active in fetal pig tissues in early pregnancy, as demonstrated previously. T3 concentration was undetectable in both AF and Al. An approximately twofold increase in the levels of T4, rT3 and T2s in AF and Al at mid-gestation was observed. T4 and rT3 in AF showed a positive correlation with protein concentrations. AF rT3 concentration (but not T4) correlated with rT3 in the cord and maternal serum. The 3,3'-T2 and 3',5'-T2 in AF and Al showed parallel changes to rT3, while the rT3/3,3'-T2 and rT3/3',5'-T2 molar ratios remained constant. T4 concentrations in AF and Al were markedly lower than in corresponding maternal and fetal serum; the rT3 concentration in Al was equal to that in AF and two to four times lower than in fetal serum. In spite of differences between serum hormone patterns in the pig and human near term, iodothyronine concentrations in AF showed some similarities, mainly the following: undetectable T3, a strong correlation between rT3, T4 and AF total protein and the presence of 3,3'-T2 and 3',5'-T2 in measurable levels. Comparative data for Al, except the ones in the present study in the pig, are not available.

Peroral administration of triiodothyronine (T3) affects absorption of immunolactoglobulins in calves.

On the basis of our studies which demonstrated that T3 is a natural milk-borne component of cow mammary secretions, this study investigated the influence of T3 (and thyroxine, T4) on the serum Ig level (used here as an indicator of intestinal absorption). Forty healthy calves were given a single dose of either T3 or T4 with the first colostrum meal 6 h following birth. Blood samples were taken before and 42-50 h after hormone administration. The T4 treatment resulted in metabolic changes that were reflected by an increase in blood glucose, triglycerides (TG), non-esterified fatty acids (NEFA), and a decrease in total serum proteins (TP). Ig levels were reduced from 27.5 milligrams (controls) to 20.6 milligrams. The T3 treatment caused an increase in serum TG and FFA (p < 0.01), and, in contrast to T4, an increase in TP and Ig (p < 0.001). These results indicated that peroral administration of T3, but not T4, could exert a positive effect on the transfer of immunolactoglobulins in the neonatal calf intestine. The contrasting hormonal effects are likely attributable to different responses of intestinal cells to T3 and T4.

Antioxidant status of dairy cows supplemented prepartum with vitamin E and selenium.

Possible relationships among dietary antioxidants, oxidative status, and placental retention were investigated in periparturient dairy cows. During 6 wk prepartum, 16 cows each were given daily by capsule 1000 IU of vitamin E, 3 mg of Se, both vitamin E and Se, or neither (control). alpha-Tocopherol in serum and fast-acting antioxidants in plasma increased, but, in red blood cells, thiobarbituric acid-reactive substances decreased during the last 6 wk before parturition in cows given vitamin E. These measurements were unaffected by supplementation of Se. Cows that had retained placenta > or = 12 h had lower fast-acting antioxidants in plasma and glutathione peroxidase in red blood cells up to 2 wk before calving than did cows that shed fetal membranes in < 12 h. Results suggest that inadequate dietary antioxidants may increase oxidative stress, production of lipid peroxides, and incidence of retained fetal membranes in dairy cows.

The appearance and activity of tissue thyroxine 5'- and 5-monodeiodinase during ontogenesis in the fetal pig.

The ontogeny of the fetal monodeiodinase systems, during the period before the onset of thyroid activity until birth, has been described only in the rat, rabbit and chick. We have studied 5'- and 5-monodeiodinase activities in the liver, kidney and placenta of pig fetuses, from day 32 of gestation up to birth, and on days 1, 3, 7 and 14 after birth. Fetuses (123) from 15 litters, 32 newborn piglets, 15 sows (mothers) and 10 non-pregnant pigs were used. The relationships between monodeiodinase activities, thyroxine (T4), tri-iodothyronine (T3) and reverse triiodothyronine (rT3) blood serum concentrations and sulphydryl groups (total (T-SH) and non-protein (NP-SH)), were measured. 5'-Monodeiodinase activity (5'-MD type I; converting T4 to T3) and 5-monodeiodinase (5-MD; converting T4 to rT3) were detectable in the liver at the onset of thyroid hormone biosynthesis, and before that time (day 32 of gestation) in the placenta. Liver 5'-MD activity was very low between days 32 and 84 of gestation, and then increased rapidly at about day 93, reaching a maximal value 2 days before term. Activity in the liver (but not in the kidney) was higher than that in maternal, newborn and non-pregnant adult tissue. The increase in NP-SH groups in the liver and kidney and T-SH groups in the placenta during the last 5 days of gestation coincided with the increase in 5'-MD activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxidative stress, antioxidants, and animal function.

Reactive oxygen metabolites generated during normal metabolism and metabolism stimulated by xenobiotics can enter into reactions that, when uncontrolled, can impair performance of dairy cows. Direct effects include peroxidative changes in membranes and other cellular components. Indirectly, competitive consumption of reducing equivalents can interfere with important metabolic functions and divert glucose from other pathways by inducing the monophosphate shunt. Normally, the body is protected by a wide range of antioxidant systems working in concert. Metal catalysts of oxidative reactions are removed in intracellular fluids by metal-binding macromolecules. Superoxide dismutases, glutathione peroxidase, and catalase within cells remove superoxide and peroxides before they react with metal catalysis to form more reactive species. Finally, peroxidative chain reactions initiated by reactive species that escaped enzymatic degradation are terminated by chain-breaking antioxidants, including water-soluble ascorbate, glutathione, and urate and lipid-soluble vitamin E, ubiquinone, and beta-carotene. To optimize performance, oxidative stress in high producing cows must be controlled by supplying all known antioxidant nutrients and by minimizing effects of substances that stimulate reactive oxygen metabolites.

Local generation of triiodothyronine by the mammary gland as a source of measurable quantities of the hormone in milk.

Simple diffusion or facilitated diffusion, by a carrier system represented by binding sites on plasma membranes, are thought to be responsible for thyroid hormones (TH) uptake by tissues. The concept implies the existence of an equilibrium ratio between plasma and cellular hormone content. Studies of TH presence in milk have shown that the passage of TH through mammary gland differs between T4, rT3, and T3, and only T3 exists in the milk in measurable amounts (without definite relationship between the levels of T3 in milk and the blood plasma). The present experiments have revealed that the whole milk or its cellular components (namely macrophages, lymphocytes and granulocytes) possess deiodinating enzyme system converting T4 into T3. The milk from animals displaying mastitis show lower T3 levels than controls. Of different enzyme systems, the outer ring deiodinase (5'D-II) dominates in mammary secretion. It plays an important role in galactopoiesis yielding biologically potent T3. The presence of TH deiodinating system(s) in mammary gland is thought to lower T4 and T3 in milk, already in small concentrations due to a poor permeability of the blood-mammary gland barrier for these compounds. On the other hand measurable amount of T3 in milk is due to a local enzyme mechanism generating T3 from T4. It has been concluded that the ultimate THs level in milk depends on the rate of transport of the hormones, their degradation and simultaneous generation of T3 from T4 in mammocytes.

Investigation of thyroxine monodeiodinase activity in the liver, kidney and brown adipose tissue of foetal and neonatal rabbit.

The maturation of the 5'- and 5-monodeiodinase system in liver, kidney and brown adipose tissue of rabbits, during the foetal period (from 21 days of gestation to birth) and the neonatal period (from birth to 3 weeks of life) was studied. A sudden increase of 5'- and 5-monodeiodinase activity in liver and kidney 3 days before birth was observed, falling to a nadir at day 3 after birth. Foetal and neonatal serum T4, T3 and rT3 concentration were very low and rose progressively with age, reaching adult values at about day 21. In the foetal brown adipose tissue high 5'-monodeiodinase and low 5-monodeiodinase activity was found. The 5'-monodeiodinase decreased during the first days of life whereas the 5-monodeiodinase activity remained at a low stable level until day 3 when the activities of both enzymes increased. The increase of conversion rate of T4 to T3 and rT3 in liver and kidney well correlate with the triiodothyronines concentration in serum from day 3 after birth.

Ontogenic characterization of type I and II thyroxine 5'-monodeiodinases in brown adipose tissue from fetal and newborn rabbits.

The ontogenic changes in thyroxine 5'-monodeiodinase (5'-MD) activity were examined in brown adipose tissue (BAT) of fetal (26, 28 and 30 days of gestation) and newborn (1-14 days old) rabbits. A near-maximum stimulation of BAT 5'-MD activity was reached at 1 mM of dithiothreitol (DTT). The kinetics of 5'-MD inhibition by propylthiouracil (PTU) was uncompetitive in fetal and noncompetitive in neonatal tissue. In fetal tissue the Ki was 3.2 mM independently of DTT concentration, in newborns from 0.5 to 8.25 mM at 1-10 mM of DTT. An apparent Km constant for T4 to T3 conversion in BAT of fetal rabbit amounted to 0.2, 0.67, 1.9 and 2.0 microM in the presence of 1, 2, 5 and 10 mM DTT, respectively. In neonatal rabbits Km was constant (0.72 microM) in various DTT concentrations. The presence of 5'-MDI and 5'-MDII in BAT of fetal and neonatal rabbit was identified. Using iopanoic acid and amiodarone as inhibitors, a decrease in the 5'-MDII:5'-MDI ratio from 8.9 to 2.7 after birth was observed. Changes in 5'-MD activity with advancing age were not correlated with the SH group content in the BAT.

Placental outer and inner ring monodeiodination of thyroxine and triiodothyronines in the rabbit.

Both inner- and outer-ring iodothyronines deiodinating activity was found in homogenates of rabbit placentas. The T4 to rT3 and T3 to 3,3'-T2 deiodinating activity was already high on day 10 before delivery but decreased being about 7 times lowered on day 5. Once the T3 to 3,3'-T2 monodeiodination reached a low and a relatively steady level, the outer ring deiodination of T4 begun, reaching a peak value at about day 3 before term and then fell again. The fetal serum thyroid hormones levels were low, showing no significant variability during the period of observation. The results suggested that in the rabbit, representing animals in which the thyroid gland activity begins early in fetal life, there are two distinct phases of the placental monodeiodinating activity. The first is characterized by a high inner-ring deiodinating activity (yielding rT3) and is followed by the second phase with a high outer-ring deiodinating activity (yielding T3) declining just before term.