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At least 19 recordsLinked to original sources
PubMed1979

Reaction of liver alcohol dehydrogenase with halogenoacids. Fate of the iodide anion released by carboxymethylation and enzymic catalysis of iodide solvolysis.

The fate of the iodide liberated during carboxymethylation of Cys-46 in horse liver alcohol dehydrogenase has been determined with 125I-labeled iodoacetate. The [125I]iodoacetic acid was prepared from mesyloxyacetic acid and sodium [125I]iodide. When carboxymethylation of the enzyme is carried out in solution or in the crystalline state, no iodide is bound to the protein. The rate of iodide during the reaction of iodoacetate, determined with an iodide-specific electrode, has been found to be biphasic: the fast phase corresponds to the carboxymethylation and the slow phase to iodide liberation due to the presence of protein. With 3-iodopropionate (2.5 mM), no inactivation was detected, but in the presence of the enzyme, 10 equivalents of iodide were liberated per subunit in 1 hr. NADH does not inhibit this reaction. The electron density attributed to an iodide bound to the zinc atom of the crystalline enzyme is reinterpreted in view of these results as due to an imidazole bound to the active-site zinc. In the carboxymethylation, the reactivity of bromoacetate is higher than that of iodoacetate.

Alcohol Oxidoreductases↗
PubMed1976

Use of methyl iodide for probing the polarity of the immediate environment of --SH groups in thiolenzymes. Reaction of methyl iodide with thiosubtilisin.

A new approach is proposed for probing the polarity of the immediate environment of -SH groups in thiolenzymes, based on the alkylation of the -SH group with methyl iodide, a relatively small and non-polar molecule. Rate and activation parameters (delta H*, delta S*) for the reaction of the enzyme are compared to those of glutathione, a simple -SH compound alkylated in aqueous medium. The enzyme and model compound are also reacted with iodoacetamide, a polar counterpart of the non-polar methyl iodide. The above method was applied to thiolsubtilisin, an artificial thiolenzyme. 1. The ratio of the rates of alkylation of thiolsubtilisin and glutathione is about 20 times as high with methyl iodide as with iodoacetamide. 2. delta H* and delta S* for enzyme alkylation, as compared to those for glutathione, are remarkably lower with methyl iodide whereas they are slightly higher with iodoacetamide. 3. delta H* and delta S* for alkylation of thiolsubtilisin with methyl iodide are similar to those found with glutathione in 40% dioxane/water mixture. 4. The activation enthalpy and entropy values for the reaction of thiolsubtilisin with D-2-bromo-n-valeramide are lower than those for glutathione reaction. Consequently, in this respect, D-2-bromo-n-valeramide is similar to methyl iodide rather than to iodoacetamide. It is concluded that the -SH group of thiolsubtilisin is located in an environment less polar than water. The concentration of methyl iodide in this non-polar layer is higher than in the bulk solution, which results in an enhanced reaction rate.

Alkylation↗
PubMed1992

Polarized efflux of iodide in porcine thyrocytes occurs via a cAMP-regulated iodide channel in the apical plasma membrane.

The intracellular regulation of thyrotropin-stimulated iodide efflux was studied in polarized porcine thyrocytes grown as a continuous, tight monolayer in bicameral culture chambers. From a previous study using this system we know that thyrotropin rapidly increases iodide efflux in the apical but not basal direction of the polarized epithelium. [125I]-iodide efflux in apical direction was stimulated by thyrotropin in a concentration-dependent manner (1-10 U/l), whereas efflux in basal direction was unchanged at any thyrotropin dose. Thyrotropin-induced elevation of intracellular cAMP showed a corresponding concentration dependence. The selective stimulation of apical efflux by thyrotropin was evident also when re-uptake of iodide released in basal direction was blocked by perchlorate. The effect of thyrotropin on apical efflux was mimicked by 8-bromo-cAMP and forskolin, whereas agents known to activate the Ca2+/phosphatidylinositol cascade (epidermal growth factor) and protein kinase C (phorbol ester) or increase cytosolic [Ca2+] (A23187) were inactive. We conclude that the selective stimulation by thyrotropin of apical iodide efflux, corresponding to efflux in luminal direction in intact follicles, occurs via cAMP-regulated iodide channels present in the apical domain of the plasma membrane.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1975

Alteration in tissue and serum concentrations of TSH, iodide, T4 and T3 induced by various dietary iodide levels.

Isotopic equilibrium and radioimmunoassay methods were used to evaluate the effects of increases in iodide intake on tissue and serum concentrations of thyroid hormones. Within the range of iodide levels used total iodine in peripheral tissues and serum increase directly with iodide intake but this change is mainly due to an increase in inorganic iodide. It is concluded that increases in tissue thyroid hormone concentrations occur within a relatively narrow range of iodide intake and maximal concentration occurs at an iodide intake of 3-10 mug/day.

Animals↗
PubMed1992

1,25-Dihydroxycholecalciferol attenuates thyrotropin stimulated iodide accumulation in rat thyroid follicular FRTL-5 cells by reducing iodide porter number.

We have recently shown that rat thyroid follicular FRTL-5 cells have functional receptors for 1,25-dihydroxycholecalciferol (1,25- (OH)2D3) and that 1,25-(OH)2D3 attenuates the thyrotropin (TSH) induced iodide uptake. Here we show that the dibutyrylcyclic AMP induced iodide uptake was significantly reduced by 1,25-(OH)2D3, indicating that 1,25-(OH)2D3 affects the cAMP signal pathway beyond cAMP generation. The Vmax of the iodide porter was significantly reduced in 1,25-(OH)2D3 treated cells as compared to cells treated with TSH alone, indicating that 1,25-(OH)2D3 reduces the effective number of iodide porters in FRTL-5 cells.

Animals↗
PubMed1977

Cultured thyroid cell adenosine 3',5'-cyclic monophosphate response to thyrotropin: loss and restoration of sensitivity to iodide inhibition.

Unlike in all other thyroid preparations, exposure of dog thyroid cells in long-term monolayer culture to iodide (10(-7) to 10(-3) M for up to 19 h did not blunt the subsequent adenosine 3', 5'-cyclic monophosphate (cAMP) response to thyrotropin (TSH) stimulation. This lack of effect of iodide was observed even when confluent thyroid cells were "follicularized" by the action of TSH in the culture medium. Preincubation of these cells in thyroxine (T4) and triiodothyronine (T3) was similarly without effect on the subsequent cAMP response to TSH. Study of thyroid cells during the early phase of primary culture demonstrated that inhibition by iodide (10(-4) M) of the cAMP response to TSH occurred after 7 h but was lost after 48 h of cell culture. This inhibitory effect of iodide was prevented by the inclusion of methimazole in the preincubation medium. As with iodide-insensitive cells, T4 and T3 were without effect on the cAMP response to TSH in iodide-sensitive thyroid cells. Exposure of iodide-insensitive thyroid cells to iodide-containing medium obtained after 2 h of incubation with dog thyroid slices, as well as to medium enriched with the 100,000 g supernatant fraction of homogenates prepared from these thyroid slices, did not restore the inhibitory action of iodide. However, iodide-sensitivity of the cAMP response to TSH was restored by preincubation of iodide-insensitive cells in 10(-4) M iodide plus an H2O2-generating system (glucose-glucose oxidase). These data suggest that T4 and T3 are not organic iodine inhibitors of the thyroid cAMP response to TSH. In addition, they provide evidence against the existence of a soluble, freely diffusible, organic iodine inhibitor of thyroid adenylate cyclase. The loss of sensitivity to iodide inhibition of adenylate cyclase that occurs in thyroid cells shortly after initiation of primary culture appears to be related to a defect in the cellular organification mechanism, possibly the H2O2-generating system.

Adenylyl Cyclases↗
PubMed1976

Inhibition of thyroglobulin biosynthesis and degradation by excess iodide. Synergism with lithium.

Lithium and excess iodide inhibit the release of thyroid hormone from preformed stores. We thus tested the hypothesis that this was due to an inhibition of thyroglobulin breakdown. Rats were pre-treated with propyl-thiouracil (PTU) for 3 weeks in order to deplete their thyroids of thyroglobulin. While the PTU was continued, lithium chloride (0.25 mEq./100 g weight) or potassium iodide (3 mg per rat) were injected every 12 h for d days. Thereafter the thyroglobulin content in thyroid gland homogenates was measured. PTU pre-treatment lowered the thyroglobulin content from 4.21 to 0.22 mg/100 mg gland. Lithium caused a marked re-accumulation of thyroglobulin to 0.60 mg/100 mg within 3 days. While iodide alone had only a borderline effect, it markedly potentiated the action of lithium and a combination of the two drugs increased the thyroglobulin content to 1.04 mg/100 mg. Thyroxine was injected into similarly pre-treated animals to suppress secretion of thyrotrophic hormone. This markedly inhibited the proteolysis of thyroglobulin and 1.3 mg/100 mg gland accumulated after 3 days. Excess iodide, given in addition to thyroxine, decreased the amount of thyroglobulin accumulated to 0.75 mg/100 mg gland. To study whether this could be explained by an inhibitory action of iodide on thyroglobulin biosynthesis, thyroid glands from animals treated with excess iodide were incubated in vitro in the presence of 0.2 mM iodide for 3 h. Iodide decreased the incorporation of radioactive leucine into total thyroidal protein and into thyroglobulin by 25 and 35% respectively. Iodide did not inhibit protein synthesis in the kidney, liver or muscle tissue. Thus, large doses of iodide selectively inhibit thyroglobulin biosynthesis.

Animals↗
PubMed1976

Increased uptake of iodide by hormone-responsive compared to hormone-independent mammary tumors in GR mice.

The uptake of 125-iodide by transplanted hormone-responsive (HR) or hormone-independent (HI) mammary tumors, normal mammary tissue and skeletal muscle was compared in GR mice. The uptake of 125-iodide by HR mammary tumors in mice treated with progesterone and oestrone (P+O) was about 20 times greater than the uptake of 125-iodide by HI mammary tumors in mice not treated with P+O. This difference in uptake of 125-iodide by HR and HI mammary tumors could not be attributed to the difference in the hormonal status of the mice since uptake of 125-iodide was also low in HI tumors in mice receiving P+O treatment. The uptake of 125-iodide by HR mammary tumors was greatly reduced by the simultaneous injection of either an excess of non-radioactive iodide or of perchlorate. Uptake of 125-iodide by normal mammary tissue and skeletal muscle was similar was similar in all groups of tumor bearing mice, and was not influenced by the presence of an excess of non-radioactive iodide or of perchlorate. The possible clinical significance of these findings is discussed.

Adenocarcinoma↗
PubMed1977

Excess iodide and the accumulation of 125I by the thyroid, plasma and developing oocytes of the Japanese quail.

1. Developing oocytes of the Japanese quail accumulated 0-44 microgram of each 1 microgram of 125I-labelled iodide after intra-muscular injection of doses up to 500 microgram iodide as NaI but only 0-007 microgram after injection of more than this: the abrupt change in the rate of accumulation was attributed to saturation of the iodide transport mechanism. 2. The proportion of available iodide transferred into the oocytes appeared to be more dependent on the amount of iodide injected and the total weight of growing oocytes than on a requirement for either a store of iodide for the embryo or an iodide excretory pathway for the hen. 3. The thyroid was about four times more active in accumulating iodide than the oocyte. 4. The percentage of iodide accumulated by the plasma was the same at all dose rates.

Animals↗
PubMed1992

The iodide channel of the thyroid: a plasma membrane vesicle study.

The uptake of radioactive iodide or chloride by plasma membrane vesicles of bovine thyroid was studied by a rapid filtration technique. A Na(+)-I- cotransport was demonstrated. When this Na(+)-I- cotransport is inactive (i.e., at 4 degrees C and in the absence of Na+), an uptake of iodide above chemical equilibrium could be induced, driven by the membrane potential. The latter was set up by allowing potassium to diffuse into the membrane vesicles in the presence of valinomycin and of an inward K+ gradient. This potential difference (positive inside) induced the uptake of iodide (or other anion present). The data support the existence of two anionic channels. The first one, observed at low near-physiological iodide concentration (micromolar range), which exhibits a high permeability and specificity for iodide (hence called the iodide channel), has a Km of 70 microM. The other one appears similar to the epithelial anion channel as described by Landry et al. (J. Gen. Physiol. 90: 779-798, 1987); it is still about fourfold more permeable to iodide than to chloride and presents a Km of 33 mM. Under physiological conditions the latter channel would mediate chloride transport, and the iodide channel, which is proposed to be restricted to the apical plasma membrane domain of the thyrocyte, transports iodide from the cytosol to the colloid space.

Animals↗
PubMed1979

Inhibition by iodide of the cholinergic stimulation of prostaglandin synthesis in dog thyroid.

Iodide is shown to inhibit the cholinergic stimulation of prostaglandin E2 (PGE2) and F2 alpha (PGF2 alpha) synthesis in dog thyroid slices. The inibitory effect of iodide was already detectable at 1 micron and reached its maximal level at 10 microns. At this concentration, iodide inhibited the carbamylcholine-stimulated release of PGE2 and PGF2 alpha by 59% and 73%, respectively (eight experiments). The effect of iodide was neither immediate nor rapidly reversed after a change of incubation medium. The unstimulated release of PGE2 and PGF2 alpha and the stimulation by epinephrine and ionophore A23187 were not modified by iodide in concentrations up to 0.1 mM. The effect of iodide was suppressed in the presence of methimazole (0.1 mM) but not NaClO4 (2 mM). Iodid (0.1 mM) did not inhibit the stimulation by carbamylcholine of PGF2 alpha and PGE2 release by rat pancreas in vitro. These data demonstrate a new action of iodide on the thyroid and establish a link among iodide metabolism, PG synthesis, and cholinergic action in the dog thyroid.

Animals↗