Hepatic iodothyronine 5'-deiodinase. The role of selenium

Selenium (Se) deficiency decreased by 8-fold the activity of type 1 iodothyronine 5'-deiodinase (ID-I) in hepatic microsomal fractions from rats. Solubilized hepatic microsomes from rats injected with 75Se-labelled Na2SeO3 4 days before killing were found by chromatography on agarose gels to contain a 75Se-containing fraction with ID-I activity. PAGE of this fraction under reducing conditions, followed by autoradiography, revealed a single 75Se-containing protein (Mr 27,400 +/- 300). This protein could also be labelled with 125I-bromoacetyl reverse tri-iodothyronine, an affinity label for ID-I. The results suggest that hepatic ID-I is a selenoprotein or has an Se-containing subunit essential for activity.

Non-isotopic, two-step free thyroxine immunoassay: a better measure of free thyroxine than analogue radioimmunoassay

Plasma or serum free thyroxine (T4) was measured by a novel non-isotopic, two-step immunoassay in 373 consecutive patients attending a thyroid clinic, in whom thyroid status was categorized according to clinical findings, supported by routine thyroid function tests. The 95% confidence limit of free T4 in the euthyroid patients (n = 112) was 7-20 pmol/L. Free T4 concentrations within the reference range were found in six of 40 patients with primary hypothyroidism and nine of 182 patients with overt thyrotoxicosis, six of whom had T3 toxicosis. Serum or plasma free T4 measured by the two-step method showed improved diagnostic specificity over an analogue RIA in selected groups of euthyroid patients in whom abnormal binding of analogue T4 can affect the validity of the result. Free T4 results found by analogue RIA and the two-step method in 58 patients who were receiving thyroxine replacement therapy were similar. The between-assay precision of the two-step method was poor ranging from a coefficient of variation of 9.7% to 19.3% over a free T4 concentration range of 5.0 to 46.0 pmol/L. We conclude that the two-step methodology offers diagnostic advantages for a laboratory which receives specimens from such patients for exclusion of thyroid disease but that improved assay precision is required before it could be used in a routine situation.

The effects of selenium depletion and repletion on the metabolism of thyroid hormones in the rat

Rats were fed selenium-deficient (less than 0.005 mg selenium/kg) or selenium-supplemented diets (0.1 mg selenium/kg, as Na2SeO2) for up to five wks from weaning to assess the effects of developing selenium deficiency on the metabolism of thyroid hormones. Within two wks 3:5,3'-triiodothyronine (T3) production from thyroxine (T4) in liver homogenates from selenium-deficient rats was significantly lower compared with the activity in liver homogenates from selenium-supplemented rats. This decreased activity was probably responsible, in part, for the higher T4 and lower T3 concentrations in plasma from the selenium-deficient rats after 3, 4, and 5 weeks of experiment. Repletion of selenium-deficient rats with single intra-peritoneal injections of 200 micrograms selenium/kg body wt. (as Na2SeO3) 5 days before sampling reversed the effects of the deficiency on thyroid hormone metabolism and significantly increased liver and plasma glutathione peroxidase activities. However a dose of 10 micrograms selenium/kg body wt given to rats of similar low selenium status had no effect on thyroid hormone metabolism or glutathione peroxidase activity but did reverse the increase in hepatic glutathione S-transferase activity characteristic of severe selenium deficiency. Imbalances in thyroid hormone metabolism are an early consequence of selenium deficiency and are probably not related to changes in hepatic xenobiotic metabolizing enzymes associated with severe deficiency.

Intravenous N-acetylcysteine, hepatotoxicity and plasma glutathione S-transferase in patients with paracetamol overdosage

The concentration of glutathione S-transferase B1 (GST B1) subunits was measured in sequential plasma samples taken at frequent intervals for 48 h from ten patients with severe paracetamol poisoning who were treated with intravenous N-acetylcysteine. No significant increase in plasma GST B1 concentration was observed over the study period and with 4 h of starting treatment with N-acetylcysteine there were significant decreases in plasma GST B1 concentrations. None of the patients subsequently developed significant liver damage. At the dose used for the treatment of paracetamol poisoning, N-acetylcysteine has no hepatotoxic effects.

The changes in hepatic enzyme expression caused by selenium deficiency and hypothyroidism in rats are produced by independent mechanisms

Selenium (Se) deficiency for 5 weeks in rats produced changes in the activity of a number of hepatic, renal and plasma enzymes. In animals whose food intake was restricted to 75% of normal for 2 weeks, Se deficiency produced significant increases in the activity of hepatic cytosolic 'malic' enzyme and mitochondrial alpha-glycerophosphate dehydrogenase (GPD), two enzymes that are particular sensitive to the thyroid-hormone concentrations in tissue. Propylthiouracil-induced hypothyroidism produced significant decreases in 'malic' enzyme and GPD activities. The effect of hypothyroidism on the activity of 'malic' enzyme, GPD and other enzymes studied in liver and plasma was often opposite to that seen in Se deficiency. Glutathione S-transferase (GST) activity was increased by both Se deficiency and hypothyroidism, but in hypothyroid animals further significant increases in GST were produced by Se deficiency. These data suggest that the changes in enzyme expression observed in Se deficiency are not caused by decreased tissue exposure to thyroid hormones.

Measurement of glutathione S-transferase pi isoenzyme in plasma, a possible marker for adenocarcinoma of the lung

Using a specific radioimmunoassay, we measured glutathione S-transferase (GST; EC 2.5.1.18) pi isoenzyme in the plasma of 29 patients diagnosed as having carcinoma of the bronchus. Concentrations of the enzyme were significantly higher (P less than 0.01) than those measured in a control group of 16 patients with respiratory disorders other than malignancy. Although, overall, only 19 (66%) of patients with cancer of the bronchus had increased concentrations of GST pi in plasma, eight of the nine (89%) patients diagnosed as having adenocarcinoma had above-normal concentrations of GST pi. To obviate spuriously high results for GST pi concentrations caused by platelet release, we assayed blood samples collected in "Thrombotect" tubes, which contain inhibitors of platelet activation.

Glutathione S-transferase and glutathione peroxidase expression in normal and tumour human tissues

Glutathione S-transferases play a central role in drug detoxification and have been implicated in the sensitivity of tumour cells to anticancer drugs. In this study, glutathione S-transferase (GST) isozyme expression in normal and tumour tissue from human lung, colon, stomach, breast, kidney and liver tissue has been quantified using sensitive and subunit specific radioimmunoassays (RIA), together with Western blot analysis and measurement of substrate metabolism. Glutathione S-transferase pi was the predominant GST in the majority of the tumours examined. The concentration of this enzyme was increased significantly in tumour tissue relative to normal lung, colon, and stomach tissue. A strong correlation was observed (r = 0.77, P less than 0.01) between GST activity and GST pi levels in those tumour samples. The concentrations of the alpha class GST, the predominant isoenzymes in normal stomach, kidney and liver, decreased dramatically in tumour tissue from these organs. Western blot analysis revealed the presence of novel polypeptides that cross-reacted with antisera raised against alpha and mu class GST. Our data demonstrates that although GST pi is the predominant GST isoenzyme in many tumours, significant levels of the other GST subunits are also present and collectively can represent a significant proportion of the GST content. Therefore the properties of all the GST isoenzymes need consideration when assessing the role of these proteins in drug resistance. Selenium-dependent glutathione peroxidase, an enzyme activity also implicated in the mode of action of certain antitumour agents, was also studied and shown to be the predominant glutathione-dependent peroxidase in all tumours except the hepatoma.

Measurement of glutathione S-transferase pi isoenzyme in plasma, a possible marker for adenocarcinoma of the lung

Using a specific radioimmunoassay, we measured glutathione S-transferase (GST; EC 2.5.1.18) pi isoenzyme in the plasma of 29 patients diagnosed as having carcinoma of the bronchus. Concentrations of the enzyme were significantly higher (P less than 0.01) than those measured in a control group of 16 patients with respiratory disorders other than malignancy. Although, overall, only 19 (66%) of patients with cancer of the bronchus had increased concentrations of GST pi in plasma, eight of the nine (89%) patients diagnosed as having adenocarcinoma had above-normal concentrations of GST pi. To obviate spuriously high results for GST pi concentrations caused by platelet release, we assayed blood samples collected in "Thrombotect" tubes, which contain inhibitors of platelet activation.

Glutathione S-transferase isoenzymes in human bronchoalveolar lavage: a possible early marker for the detection of lung cancer

The levels of glutathione S-transferase (GST) isoenzymes, GST pi, B1, B2 and mu were measured, by radioimmunoassay, in human bronchoalveolar lavage fluid from a series of patients presenting with neoplastic (n = 12) and non-neoplastic lung diseases (n = 10). Lavage fluid was obtained from the suspected abnormal area of lung and a presumed normal area of lung at the time of bronchoscopy. Concentrations of GST B1 and GST B2 were found to be significantly raised (P less than 0.02) in the lavage fluid obtained from the suspected abnormal areas of lung compared with the presumed normal area of lung, in patients later diagnosed as having cancer of the bronchus. The findings of the radioimmunoassay, of greater levels of GST B1 and B2 than GST pi in lavage fluid, were confirmed by a one-step purification of GST from lung lavage, using affinity chromatography, followed by their identification using SDS-polyacrylamide gel. We conclude that measurement of GST B1 or GST B2 in lung lavage fluid could be a useful aid in the diagnosis of lung malignancy.

The development expression of alpha-, mu- and pi-class glutathione S-transferases in human liver

The developmental expression of the alpha, mu and pi class glutathione S-transferases has been defined in human liver using radioimmunoassay and immunohistochemistry. Expression of alpha and mu class isoenzymes increased significantly at birth, while that of the pi isoenzyme declined during the first trimester. Mu-class isoenzymes (GST1 1, GST1 2, GST1 2-1) were expressed in hepatocytes but not in other liver cell types.

Expression of glutathione S-transferase B1, B2, Mu and Pi in breast cancers and their relationship to oestrogen receptor status

The concentrations of glutathione S-transferase (GST) B1 and B2 (Alpha), Pi and Mu have been measured by radioimmunoassay in cytosols from 28 oestrogen receptor (ER) rich an 30 ER-poor breast tumours. GST B1, B2 and Pi was detected in all 58 breast tumour cytosols whilst GST Mu was found in only 28. Of the GSTs, Pi was expressed most strongly in all cytosols and the concentration was significantly higher in ER-poor tumour cytosols than in ER-rich tumours (P less than 0.01). As with GST Pi, the highest levels of GST B1 and GST B2 were found in ER-poor tumour cytosols; the levels of GST B1 and GST B2 were positively correlated (r = 0.66, P less than 0.001). No quantitative or qualitative association was found between ER status and GST Mu which was expressed in 46% of ER-rich and 50% of ER-poor tumour cytosols. No relationship could be found between GST expression and age, menopausal status, lymph node involvement or tumour T stage in the subgroup of patients in whom this information was available. These data suggest that a common mechanism is responsible for GST induction in ER-poor tumours and that the nulled Mu phenotype has no increased susceptibility to developing breast cancer.

Plasma glutathione S-transferase and F protein are more sensitive than alanine aminotransferase as markers of paracetamol (acetaminophen)-induced liver damage

Concentrations of glutathione S-transferase (GST; glutathione transferase; EC 2.5.1.18) B1 subunits, F protein, and the activity of alanine aminotransferase (ALT; EC 2.6.1.2) were measured in sequential plasma samples taken from nine patients with self-administered paracetamol (acetaminophen) poisoning. GST exceeded the reference interval in all patients at the time of admission, and F protein was increased in seven. In contrast, abnormal activities of ALT in plasma were found in only one of the nine on admission, a patient admitted 12 h after poisoning. Subsequent to admission nine, eight, and five patients, respectively, had abnormal concentrations of GST, F protein, and ALT. When expressed as multiples of the upper reference limit, the highest values for GST measured in each patient always far exceeded the greatest abnormalities in ALT; this was true for F protein in only five patients. Patients in whom the concentration of GST exceeded 10 micrograms/L on admission subsequently went on to develop moderate or severe liver damage, despite treatment with N-acetylcysteine. F protein and ALT measurements on admission were not as efficient as GST at predicting the clinical outcome of the patients. We conclude that GST and F protein offer clear advantages over ALT for detecting minor degrees of acute liver dysfunction, particularly when only centrilobular damage may be involved.

Plasma glutathione S-transferase and F protein are more sensitive than alanine aminotransferase as markers of paracetamol (acetaminophen)-induced liver damage

Concentrations of glutathione S-transferase (GST; glutathione transferase; EC 2.5.1.18) B1 subunits, F protein, and the activity of alanine aminotransferase (ALT; EC 2.6.1.2) were measured in sequential plasma samples taken from nine patients with self-administered paracetamol (acetaminophen) poisoning. GST exceeded the reference interval in all patients at the time of admission, and F protein was increased in seven. In contrast, abnormal activities of ALT in plasma were found in only one of the nine on admission, a patient admitted 12 h after poisoning. Subsequent to admission nine, eight, and five patients, respectively, had abnormal concentrations of GST, F protein, and ALT. When expressed as multiples of the upper reference limit, the highest values for GST measured in each patient always far exceeded the greatest abnormalities in ALT; this was true for F protein in only five patients. Patients in whom the concentration of GST exceeded 10 micrograms/L on admission subsequently went on to develop moderate or severe liver damage, despite treatment with N-acetylcysteine. F protein and ALT measurements on admission were not as efficient as GST at predicting the clinical outcome of the patients. We conclude that GST and F protein offer clear advantages over ALT for detecting minor degrees of acute liver dysfunction, particularly when only centrilobular damage may be involved.

Preoperative cimetidine does not prevent subclinical halothane hepatotoxicity in man

To assess the influence of pretreatment with cimetidine on changes in hepatocellular integrity after halothane anaesthesia, 53 patients were allocated randomly to receive either cimetidine 1600 mg orally or placebo tablets before anaesthesia. Plasma concentrations of glutathione S-transferase (GST) were measured as an index of hepatic damage. Data from 45 patients were available for analysis. Plasma GST concentration increased significantly 3 h after induction of anaesthesia in both groups (P less than 0.01, both groups) and at 6 h in the cimetidine group (P less than 0.05). Pretreatment with cimetidine did not influence the magnitude of increase in GST concentration. There was no difference between the groups in the frequency of abnormal GST concentrations at any time. Cimetidine does not appear to prevent release of GST from the liver by halothane anaesthesia in man.

Glutathione S-transferase in human bile

Glutathione S-transferase (GST) isoenzymes have been measured by specific radioimmunoassay in human bile samples. GST Mu was found in 50% of samples while GST Pi, GST B1 and GST B2 were present in all samples; GST Pi constituted the major isoenzyme identified. The findings of the radioimmunoassay were confirmed by a one-step purification of GST from bile, using affinity chromatography, followed by their identification using sodium dodecyl sulphate-polyacrylamide gel (SDS-PAGE). Inhibition studies showed that, at the concentrations of bile salts found in bile, GST Pi would have little or no enzymic activity. It is proposed that GST Pi acts as a carrier protein of toxic, non-substrate, ligands to remove as yet unidentified substances from biliary epithelial cells and prevent their reabsorption.

Measurements of glutathione S-transferase B1 in plasma after birth asphyxia: an early indication of hepatocellular damage

Concentrations of glutathione S-transferase (glutathione transferase; EC 2.5.1.18) B1 and B2 subunits (B1 and B2) and activity of alanine aminotransferase (ALT; EC 2.6.1.2) were measured in sequential plasma samples taken from 14 infants with birth asphyxia. Within 6 h of asphyxia, abnormal concentrations of B1 were found in 11 infants, whereas only seven infants showed abnormal ALT activities at this time. In plasma sampled 24 h after birth, values for ALT were abnormal in 10, whereas values for B1 were abnormal in six. Abnormal concentrations of B2 were found in relatively few of these infants, apparently because this monomer is poorly expressed in liver samples obtained up to 41 weeks after conception. We conclude that measurement of B1 may provide a useful index of hepatic impairment in birth-asphyxiated infants.

Effect of nicardipine infusion on the release of glutathione S-transferase following halothane anaesthesia

To assess the possible protective effect of calcium channel blockade on hepatic function after halothane anaesthesia, 80 patients were allocated randomly to receive an i.v. infusion of either nicardipine or normal saline. Plasma concentration of glutathione S-transferase B1 subunits (GST) was measured as a sensitive index of hepatic damage. Data from 53 patients were analysed. Plasma GST concentration increased significantly at 3 and 6 h after induction of anaesthesia in the placebo group (P less than 0.01), and at 3 h (P less than 0.01) and 6 h (P less than 0.05) in the nicardipine group. The administration of nicardipine resulted in a greater increase in plasma GST concentrations at 3 h than did placebo (P less than 0.05), mainly because of a greater increase in males than in females. The increase in GST at 3 h was related inversely to plasma concentration of nicardipine both at the end of the exponential infusion (P less than 0.01) and at 2 h after induction (P less than 0.05), when males had lower plasma nicardipine concentrations than females (P less than 0.05). Calcium channel blockade with nicardipine in the dose administered was not shown to reduce liver dysfunction after halothane anaesthesia.

Inhibition of type I and type II iodothyronine deiodinase activity in rat liver, kidney and brain produced by selenium deficiency

Selenium deficiency for periods of 5 or 6 weeks in rats produced an inhibition of tri-iodothyronine (T3) production from added thyroxine (T4) in brain, liver and kidney homogenate. This inhibition was reflected in plasma T4 and T3 concentrations, which were respectively increased and decreased in selenium-deficient animals. Although plasma T4 levels increased in selenium-deficient animals, this did not produce the normal feedback inhibition on thyrotropin release from the pituitary. Selenium deficiency was confirmed in the animals by decreased selenium-dependent glutathione peroxidase (Se-GSH-Px) activity in all of these tissues. Administration of selenium, as a single intraperitoneal injection of 200 micrograms of selenium (as Na2SeO3)/kg body weight completely reversed the effects of selenium deficiency on thyroid-hormone metabolism and partly restored the activity of Se-GSH-Px. Selenium administration at 10 micrograms/kg body weight had no significant effect on thyroid-hormone metabolism or on Se-GSH-Px activity in any of the tissues studied. The characteristic changes in plasma thyroid-hormone levels that occurred in selenium deficiency appeared not to be due to non-specific stress factors, since food restriction to 75% of normal intake or vitamin E deficiency produced no significant changes in plasma T4 or T3 concentration. These data are consistent with the view that the Type I and Type II iodothyronine deiodinase enzymes are seleno-enzymes or require selenium-containing cofactors for activity.

Different hepatic responses to thyroxine replacement in spontaneous and 131I-induced primary hypothyroidism

The serum levels of a range of analytes known to change with thyroid status were measured in two groups of patients with primary hypothyroidism commencing T4 replacement therapy. One group (group 1; n = 9) had spontaneous hypothyroidism whilst in the second (group 2; n = 10), hypothyroidism had resulted from radioiodine therapy. The replacement dose was increased in 50 micrograms increments each month to 200 micrograms/day; this produced similar serum concentrations of thyroid hormones and TSH in the two groups at each dose. Dose-dependent increases in glutathione S-transferase (GST) were seen in both groups but changes in alanine aminotransferase (ALT) and gamma glutamyltransferase (GGT) activities occurred only in group 1 patients. Group 1 patients had significantly higher levels of GST than group 2 at the 150 micrograms (P less than 0.01) and the 200 micrograms (P less than 0.005) doses of T4, and they had higher activities of ALT (P less than 0.01) and GGT (P less than 0.02) at the 200 micrograms dose. Seven patients in group 1 had abnormalities in GST and four had high levels of ALT, whereas three patients from group 2 had high GST concentrations and all had ALT activities within reference limits. The concentrations of the other analytes measured in serum showed the same response to T4 in the two groups, particularly the concentrations of certain transport proteins whose serum concentrations depend on hepatic protein synthesis. These data suggest that patients with spontaneous primary hypothyroidism are more susceptible to hepatocellular damage than patients who have radioiodine-induced primary hypothyroidism when given oral doses of thyroxine greater than 150 micrograms/day.

Plasma hepatic glutathione S-transferase concentrations after insulin-induced hypoglycaemia in normal subjects and diabetic patients

Plasma glutathione S-transferase basic isoenzyme (GST B1) concentrations have been measured by specific radioimmunoassay in Type 1 diabetic patients and in normal subjects, before and after controlled insulin-induced hypoglycaemia, and in a further group of Type 1 diabetic patients in hypoglycaemic coma. The activities of alanine aminotransferase (ALT), aspartate amino-transferase (AST), and gamma-glutamyl transferase (gamma GT) were also measured. GST B1 concentrations were significantly increased 3 h after controlled insulin-induced hypoglycaemia, both in the diabetic patients (p less than 0.02) and in the normal group (p less than 0.05), but the magnitude of the rise did not differ between these two groups. Four of the 9 patients presenting in hypoglycaemic coma had a GST B1 concentration above the reference range. ALT, AST, and gamma GT activities did not rise following hypoglycaemia in any of the groups.

Purification of acidic glutathione S-transferases from human lung, placenta and erythrocyte and the development of a specific radioimmunoassay for their measurement

Human acidic glutathione S-transferases (GST) have been purified from placenta, lung and erythrocytes. The purification protocol resulted in a high yield of pure protein for each tissue, when compared to previous procedures. An apparent subunit Mr of 24,800 was calculated for each of the acidic GST and each enzymes had a pI of 4.75. No immunochemical differences were detected between the acidic GST isolated from the three tissues. A specific and sensitive radioimmunoassay suitable for the measurement of human acidic GST in plasma or tissues is described.

Restoration of normal thyrotrophin secretion reduces the abnormally high serum glutathione S-transferase levels found in patients receiving thyroxine replacement therapy

The peripheral tissue thyroid status of 12 patients receiving thyroxine replacement therapy was investigated both when pituitary secretion of TSH was suppressed and later, when on a lower dose of thyroxine that restored thyrotroph responsiveness. Heart rate and various analytes in serum known to be sensitive to thyroid status were measured in addition to TSH by immunoradiometric assay. Initially, the serum T4 concentration was raised in seven patients and free T4 raised in nine; all patients had normal T3 concentrations. Later, on the lower dose of thyroxine, most patients had concentrations of thyroid hormones within reference limits. Concentrations of the liver-specific form of glutathione S-transferase (GST) in serum decreased (P less than 0.01) after the reduction in thyroxine dose; abnormally high GST levels, found in eight patients when TSH was suppressed, returned to normal in six of these patients when normal basal and TRH-stimulated TSH concentrations had been restored. The response of the pituitary to excess thyroxine may be more representative of other tissues (e.g. the liver) than previously thought.