Plasma Membrane Transport of Thyroid Hormones and Its Role in Thyroid Hormone Metabolism and Bioavailability

Abstract

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Comparative vibrational analysis of thyronine hormones using infrared and Raman spectroscopy and density functional theory calculations

“…Raman spectra between 100 and 4000 cm 1 were recorded with a Bio-Rad FT Raman spectrometer (4 cm 1 resolution, 1064 nm excitation) using a 180°backscattering geometry. 11 The solid pure hormones were deposited in quartz tubes of 0.25 cm inner diameter.…”

“…For T2 the B3LYP/SDD calculations with the COOH and NH 2 groups gauche to the˛ring yield an energy that is higher by 7.61 kJ mol 1 than the structure with the COOH group in an anti conformation. The optimization of such a configuration of T3 failed, whereas a stable structure was reached for T4 with an energy that is higher by 12.6 kJ mol 1 .…”

“…Therefore, we simplified the procedure by adopting a uniform factor equal to 0.9743 to scale the wavenumbers directly. Determination of this factor was based on the highest wavenumber ring modes˛(1) andˇ(1) that give rise to the strongest Raman bands between 1570 and 1600 cm 1 . 21 The factor was chosen such that the best agreement was achieved between the calculated and the experimental wavenumbers for the three iodothyronines.…”

“…The aromatic ring vibrations give rise to four medium and strong Raman bands between 1530 and 1620 cm 1 (Fig. 2).…”

“…Growing evidence indicates that saturable transport mechanisms mediate the greater part of thyroid hormone movement across the plasma membrane and play important roles in the regulation of thyroid hormone bioavailability. 1 To gain insight into the non-genomic mechanisms of the thyroid hormones, we have previously studied the interactions and transmembrane diffusion of theses hormones with the liposomes membrane from dimyristoyl-and dipalmitoylphosphatidylcholine in the liquid-crystalline and gel phase and egg yolk phosphatidylcholine/cholesterol in the liquid-ordered phase. 2 -4 The assumption that thyroid hormones easily penetrate plasma membranes was strengthened by Hillier 5 using liposomes prepared from egg-yolk phosphatidylcholine.…”