Conversion of Thyroxine to Triiodothyronine by Cultured Human Cells

120

A B S T R A C T Studies on peripheral metabolism of simultaneously administered 125I-labeled L-thyroxine (['I1]T4) and "1'I-labeled L-triiodothyronine ([.1.I]T3) were performed in five normal subjects, in four patients with untreated hypothyroidism, and in 3 hypothyroid patients made euthyroid by the administration of T4. The fractional turnover rate (Xos) of thyroid hormones irreversibly leaving the site of degradation and the volumes of pool 1 (serum, Vi), of pool 2 (interstitial fluid, V2), and of pool 3 (all tissues, V3) were obtained by using a three-compartment analysis. In addition to the turnover studies, the ratios for the in vivo To to T3 conversion were determined by paper chromatographic study in sera obtained 4, 7, and 10 days after the injection. The rate (K12) of the extrathyroidal conversion of T4 to T3 was also estimated by the compartment analysis. The T3 distribution volume (V3) of pool 3, in which T3 is utilized and degraded, was about 60% of total distribution volume (V = V1 + V2+ V3) in normal subjects, whereas only about 25% of the extrathyroidal T4 pool was in the intracellular compartment, indicating that T3 is predominantly an intracellular hormone. The V3/V ratios of T3 were almost constant in a normal ratio of about 60% in hypothyroid patients before and after treatment. On the other hand, the V3/V ratios of T4 were elevated in patients with untreated hypothyroidism, and they returned almost to the normal range after administration of T4. Thus, the V3/V ratio of T4 bore a significant inverse relation to the free T4 concentration in all subjects (r = -0.68, P < 0.05). The findings suggested a redistribution of T4 into the cellular compartment in patients with untreated hypothyroidism. In normal subjects values for K12 averaged 3.00±0.68% per day of the extrathyroidal T4 pool, and the amount of T3 generated by the conversion of T4 (mean-SD, 17±5 Mg/day) was found to contribute approximately 70% of the daily T3 production (mean +SD, 24±5 /Lg/day). The patients with untreated hypothyroidism had elevated Ku2 (mean+SD, 4.16+0.72% per day). Because of diminution of extrathyroidal T4 pool, the amount of T3 converted from T4 was markedly diminished in the untreated patients (mean+SD, 4+3 jug/day), and it agreed closely with T3 production rate (mean±+SD, 3±2 Mg/day). In the patients treated with T4 K,2 returned almost to the normal range (mean±SD, 2.59+0.47% per day), and the amount of T3 arising from deiodination of T4 (mean±+SD, 24±6 Mg/day) corresponded closely to T3 production rate (mean+SD, 21±5 jug/day). Furthermore, a highly significant correlation was evident in the plots of serum T3 concentrations against the amounts of T3 generated by the conversion of T4 in all subjects (r = + 0.85, P < 0.001). The results indicated that the amount of T3 formed by the extrathyroidal conversion of T4 is a major determinant of serum T3 concentration in normal subjects and in patients with hypothyroidism before and after treatment.

Section: 'mentioning

confidence: 48%

Estimation of thyroxine and triiodothyronine distribution and of the conversion rate of thyroxine to triiodothyronine in man.

Inada¹,

Kasagi²,

Kurata³

et al. 1975

120

“…The conclusive demonstration of in vivo peripheral conversion of T4 to T3 in man (1) revived interest in various aspects of the peripheral metabolism of T4. Many workers have demonstrated that different tissues actively deiodinate T4 to T3 (19)(20)(21)(22)(23)(24), but liver and kidney are the most active (25)(26)(27)(28)(29)(30)(31)(32)(33). The conversion is enzymatic, because it is temperature, pH, and substrate concentration dependent (29)(30)(31)(32)(33).…”

Section: Introductionmentioning

confidence: 99%

The Role of Sulfhydryl Groups on the Impaired Hepatic 3′,3,5-Triiodothyronine Generation from Thyroxine in the Hypothyroid, Starved, Fetal, and Neonatal Rodent

Harris¹,

Fang²,

Hinerfeld³

et al. 1979

A B S T R A C T The role of nonprotein sulflhydryl groups (NPSH) in the decreased in vitro hepatic 3',3,5-triiodothyronine (T3) generation from thyroxine (T4) in the starved, hypothyroid, fetal and 1-to 4-d-old neonatal rat and dwarf mouse was assessed. NPSH were measured in fresh 25% liver homogenates prepared in 0.1 M P04/10 mM EDTA buffer. As 516 4-d-old neonates, but had little or no effect in the fetal and hypothyroid rat and dwarf mouse. Liver homogenates stored for 6 mo at -20°C lost their capacity to generate T3 from T4. NPSH concentrations in the frozen homogenates decreased progressively with increasing storage and were absent by 6 mo. 5'-Deiodinase activity correlated with NPSH concentration in the stored homogenates (r = 0.95, P < 0.005). Addition of DTT partially restored hepatic T3 generation in the frozen homogenate. It is concluded that NPSH are important for the action of the liver 5'-deiodinase. The decreased hepatic T3 generation in the starved rat is associated with decreased NPSH but not with a decrease in the absolute quantity of 5'-deiodinase because provision of sulfhydryl groups restored hepatic T3 generation to normal. In contrast, the decreased hepatic T3 generation in the adult hypothyroid rodent and in the fetal rat is probably due to a decrease in the enzyme concentration per se. In the 1-and 4-d neonatal rat, the decrease in hepatic T3 generation is secondary to a decrease in NPSH and the deiodinating enzyme.

“…Thus, conversion of T4 to T3 has been demonstrated to occur in cultured human liver and kidney cells (5), in cultured human fibroblasts (6), and in freshly isolated human polymorphonuclear leukocytes (7,8). These studies involved the use of radioiodine-labeled T4 as substrate, with detection and quantification of the T3 generated by chromatographic analysis.…”

Section: Introductionmentioning

confidence: 99%

l-Triiodothyronine and l-Reverse-Triiodothyronine Generation in the Human Polymorphonuclear Leukocyte

Woeber¹,

Maddux²

1978

A B S T R A C T Extrathyroidal monodeiodination of Lthyroxine (T4) is the principal source of L-triiodothyronine (T,) and L-reverse-triiodothyronine (rT,) production. To define some of the cellular factors involved, we examined T3 and rT3 generation from added nonradioactive T4 in human polymorphonuclear leukocytes, using radioimmunoassays to quantify the T3 an-d rT3 generated. Under optimumii incubation conditions which included a pH of 6.5 in sucrose-acetate bulffer, the presence of dithiothreitol as a sulfhydrylgroup protector, and incubation in an hypoxic atmosphere, significant net generation of T3 and rT3 was observed. Of the several subcellular fractions studied, the particulate fraction obtained by centrifugation at 27,000 g was found to possess the highest T3-and rT3-generating activities per unit quantity of protein. With respect to T3 generation from substrate T4, the K,m was 5 ,MNI and the Vmax was 7.2 pmol/min per mg protein. Propylthiouracil, methimazole, and prior induction of phagocytosis inhibited both T3 and rT3 generation, but T3 generation was inhibited to a greater extent. rT3, in a concentration equimolar to that of substrate T4, did not alter T3 generation, but inhibited T3 generation when the molar ratio of rT3 to T4 approached 10:1.Under the incubation coniditions employed, particulate fractions of leukocytes obtained fromn five cord blood samiples displayed an essentially normal relationship b)etween T3-and rT3-generating activities, despite the distinctly divergent serum T3 and rT3 concentrations in these samples. From our findings, we draw the following conclusions: (a) the human polymorphonuclear leukocyte possesses the ability to generate T3 and rT3 from substrate T4; (b) the T3-and rT3-generating activities are associated principally with the 27,000 g particulate fraction and display enzymic characteristics with a sulfhydryl-group requirement; (c) T3-generating activity appears to be more susceptible to inhibitory influences than rT3-generating activity; and (d) in cord