Studies on the biosynthesis of thyroid hormone: reconstruction of a defined in vitro iodinating system

The pentose phosphate pathway in skeletal muscle under patho-physiological conditions. A combined histochemical and biochemical study

Over the last 30 years, research into the neuromuscular apparatus, has expanded greatly. Multidisciplinary investigations have rapidly advanced our understanding both of diseases and of the basic neuromuscular mechanisms. The mode of pathological reaction of the neuromuscular apparatus is now quite well understood. The most notable aspect of the reaction of the injured neuromuscular apparatus is the remarkably stereotyped character of the resulting pathological changes as demonstrated by a wide variety of harmful causes, producing surprisingly similar effects. The findings of our combined histochemical and biochemical investigations presented in this monograph, are in complete harmony with the stereotyped character of the pathological changes. For example, it is particularly striking that many affected muscle fibres of patients with muscular dystrophies, congenital myopathies, inflammatory myopathies, metabolic myopathies, endocrine myopathies, or with diseases of the lower motor neuron, display an enhanced activity of both oxidative enzymes of the pentose phosphate pathway. Likewise, we found that experimental animals with disordered skeletal muscles, provoked by different types of agents or treatments, reveal the same marked rise in activity of GPDH and PGDH in the muscle fibres, with a positive correlation between the activity of both enzymes. Other findings of our investigations point to a positive correlation between the activity of GPDH and PGDH on the one hand and that of the non-oxidative enzymes of the pentose phosphate pathway, the enzymes TA, TK, RPI and RPE on the other hand. The rise in activity of PGDH and, in particular, of GPDH is regulated by two different mechanisms. The first represents a rapid control mechanism based on the stimulation of both oxidative enzymes of the pentose phosphate pathway by NADP+ and on their inhibition by NADPH. The other mechanism represents a long-term effect directed at the synthesis of the enzymes. It is this type of mechanism which is responsible for the rise in activity of GPDH and PGDH we observed. The findings obtained with the applied enzyme histochemical techniques clearly demonstrated that the rise in activity of both enzymes is not homogeneously distributed in the disordered skeletal muscles of man and experimental animals. For that reason, in order to obtain reliable quantitative information about enzyme activities in the muscle fibres themselves, the application of biochemical assays on a micro-scale was indispensable. The biochemical assay of enzyme activities was performed on histologically and histochemically selected dissected muscle specimens.(ABSTRACT TRUNCATED AT 400 WORDS)

Relation between thyroid peroxidase, H2O2 generating system and NADPH-dependent reductase activities in thyroid particulate fractions

In thyroid gland, iodination takes place on the apical plasma membrane and requires the presence of the thyroid peroxidase and H2O2 generating system. H2O2 generation and NBT (nitro blue tetrazolium) reductase activity (both of which are NADPH-dependent) as well as peroxidase activity were compared for their respective orientations in membrane vesicles. The possible role of NADPH-NBT reductase activity in H2O2 generation was also examined. Results favor the conclusion that thyroid peroxidase is oriented towards the luminal side of the vesicles, whereas the NADPH site of NADPH oxidase-dependent H2O2 generation is located on the external side of the same or of different vesicles. Furthermore, it is shown that different NADPH-NBT reductase activities are present on both the outer and inner surfaces of the membrane vesicles, and that none of these activities is able to produce either H2O2 or O-2. The idea that a multi-component complex is involved in H2O2 generation is discussed, and a model is proposed which takes into account the possible spatial separation of the thyroid peroxidase site from the NADPH site of this H2O2 generation system on the apical membrane of the thyrocyte.

Cytochemical localization of peroxidase and hydrogen-peroxide-producing NAD(P)H-oxidase in thyroid follicular cells of propylthiouracil-treated rats

The distribution of endogenous peroxidase and hydrogen-peroxide-producing NAD(P)H-oxidase, which are essential enzymes for the iodination of thyroglobulin, was cytochemically determined in the thyroid follicular cells of propylthiouracil (PTU)-treated rats. Peroxidase activity was determined using the diaminobenzidine technique. The presence of NAD(P)H-oxidase was determined using H2O2 generated by the enzyme; the reaction requires NAD(P)H as a substrate and cerous ions for the formation of an electron-dense precipitate. Peroxidase activity was found in the developed rough endoplasmic reticulum (rER) and Golgi apparatus, but it was also associated with the apical plasma membrane; NAD(P)H-oxidase activity was localized on the apical plasma membrane. The presence of both enzymes on the apical plasma membrane implies that the iodination of thyroglobulin occurs at the apical surface of the follicular cell in the TSH-stimulated state which follows PTU treatment.

NADPH-dependent H2O2 generation and peroxidase activity in thyroid particular fraction

A NADPH-dependent H2O2 generating system associated with a thyroid particular fraction is described. H2O2 is measured by two different methods: iodination of NADPH itself when the system is supplemented with lactoperoxidase and [125I]iodide, and by the scopoletin method. It is shown that: H2O2 generation is inhibited by catalase and is dependent on NADPH or particulate protein concentration; radical scavengers of OH and of singlet oxygen have no effect while superoxide dismutase has only a marginal effect; disruption of the particular fraction by phospholipase A2 or digitonin treatment completely abolished H2O2 generation activity while thyroid peroxidase activity appears, suggesting different sites for the two activities in the membrane vesicles.

Iodination of thyroglobulin. An intracellular or extracellular process?

The location in the thyroid follicle of the iodination of thyroglobulin has been a matter of debate for several decades. This problem is not a question of mere academic interest. Knowledge of the locus--or loci--of iodination is necessary for a full understanding of the mechanisms involved in thyroid-hormone synthesis and release. In the discussion about this problem 3 fundamentally different views have been--and still are--advocated. The first view implies that the site of iodination is the follicle lumen, the second that iodination is an intracellular process restricted to some organelle(s) in the follicle cells and the third that iodination occurs at the interface between the follicle cells and the follicle lumen. Below I will survey the major observations on which these different opinions are based and discuss the validity of the interpretations.

Mammalian endogenous peroxidases as cellular markers and as biosynthetic endpoints of hormone-mediated activity: viewpoint from cytochemistry

Aldehyde-resistant, diaminobenzidine-stained endogenous peroxidases form ideal markers for the biochemical endpoints of hormone stimulation and differentiation of certain mammalian cells and tissues. The lactoperoxidase (LPO)-type of endogenous peroxidases are synthesized by the acinar cells of the salivary, Harderian, lacrimal and mammary glands and are present in their secretions. These LPO-type enzymes, that are inhibited by cyanide and aminotriazole, appear to operate extracellularly as bactericidal agents in milk and in other biological fluids. In the mammary gland, lactoperoxidase is a consistent marker enzyme for differentiated acinar cells engaged in lactogenesis. Myeloperoxidase (MPO)-type endogenous peroxidases are prominent markers for the GERL endomembrane system and differentiated lysosomes in certain cells of the reticuloendothelial system and phagocytes. MPO is prominent within eosinophils, peritoneal macrophages and in Kupffer cells. The MPO-type endogenous peroxidases function primarily within lysosomes as bactericidal agents. Thyroid peroxidase (TPO) is relegated to the cisternae of the granular endoplasmic reticulum and Golgi apparatus, to apical cytoplasmic vesicles and to the luminar cell membrane surface of acinar cells. The enzyme is probably activated at release and functions both in the organification reaction (T leads to To) and in the biosynthesis of thyroxine. Thyroid stimulating hormone (TSH) appears to play a key role in the regulation of TPO levels and activity in the thyroid gland. Certain tissues displaying growth-dependency on estrogen (i.e., uterus, cervix, vagina and the DMBA-induced rat mammary tumor) synthesize and secrete endogenous peroxidase into their lumina. These enzymes serve as important marker proteins of estrogen action, in that they occur distal to the binding of estrogen to its receptor protein. Estrogen antagonists, particularly CI-628 (Parke-Davis) and Nafoxidine (Upjohn) that appear to function through the estrogen receptor mechanism, also induce synthesis of the reproductive tract endogenous peroxidase but inhibit growth of these tissues. Progesterone antagonizes the synthesis of the reproductive tract peroxidases and inhibits growth of the tissues as well, in part, through the reduction of the cytosol estrogen receptor protein. Endogenous peroxidase activity appears to represent a reliable marker for rodent breast cancer tissues displaying dependency for estrogen and is of potential interest as a diagnostic marker protein in human breast cancer. Rat uterine peroxidase (UP) has been investigated by microelectrophoretic techniques. The molecular weight of UP has been determined in the range of 100,000 by using polyacrylamide gradient gels in the absence and presence of nonionic and anionic detergents. The isoelectric point of UP is located between pH 4.5 and 5.9. Employing the two-dimensional combination of isoelectric focusing and gel gradient electrophoresis, UP was separated into two subunits, one having a molecular weight of 70,000, the other less than 20,000.

Biosynthesis of thyroid hormone: basic and clinical aspects

Thyroid hormone formation requires the coincident presence of peroxidase, H2O2, iodide, and acceptor protein at one anatomic locus in the cell. The peroxidase enzyme appears to be a protoporphyrin lX containing heme protein, with binding sites for both iodide and tyrosine. It is probable that both iodide and tyrosine are oxidized to free radical forms which unite to form iodotyrosine. The peroxidase is also involved through an uncertain mechanism in iodotyrosine coupling and probably in oxidation of sulfhydryl bonds in thyroglobulin. H2O2 may be supplied by microsomal NADPH-cytochrome c reductase or NADH-cytochrome b5 reductase. Other possible intracellular H2OI generating systems include monoamine oxidase and xanthine oxidase. The usual acceptor for iodide is thyroglobulin, which is currently believed to be iodinated within apical secretory vesicles at the cell border just prior to liberation into the colloid, or possibly after liberation into the colloid. Other soluble an insoluble proteins are also iodinated within the gland. The peroxidase is present in numerous cellular structures, but iodination activity occurs primarily, if not only, at the apical cell border. The controls of iodination are imperfectly known. Thyrotrophin modulation of iodide uptake, H2O2 generation, thyroglobulin synthesis, and peroxidase enzyme level obviously are the main regulations. Many of these actions are thought to involve mediation of adenyl cyclase and subsequent activation of intracellular phosphokinases. Antithyroid drugs of the thiocarbamide group are competitive inhibitors of iodination under some circumstances, but if much iodide is present, they react with the oxidized iodine intermediate and are irreversibly inactivated themselves. Clinical problems involving defective peroxidase function are among the most frequent hereditary defects of thyroid hormone formation. Recognized abnormalities include deficient peroxidase, abnormality in binding of the peroxidase apoprotein to its prosthetic group, and other less well-identified abnormalities in peroxidase structure and function. Peroxidase is typically elevated in thyroid tissue from patients with hyperthyroidism sometimes deficient in cold thyroid nodules, and frequently diminished in tissue from patients with Hashimoto's thyroiditis.

Deficient cytochrome b5 reductase activity in nontoxic goiter with iodide organification defect

A 37-yr-old woman with nontoxic goiter is presented. The thyroid 131I uptake at 3 and 24 hr were, respectively, 77.1% and 81.4% dose. Thiocyanate discharged 65.5% of the accumulated 131I in 30 min. In vitro organification of iodine in the thyroid homogenate from the patient was impaired and it was restored to normal by the addition of H2O2, glucose, and glucose oxidase system, FAD, or reduced cytochrome b5. Riboflavin, FMN, oxidized cytochrome b5, oxidized or reduced cytochrome c, NAD(H), and NADP(H) were ineffective in the reaction. The microsomal NADH-cytochrome b5 reductase activity was definitely low in the patient's thyroid. It was augmented to a normal level by incubation of the microsomes with FAD for 30 min or more. The activities of thyroid peroxidase, G6-PD, 6-PGD, catalase, protease, and NADPH-cytochrome c reductase were within normal limits. The major thyroid protein was normal thyroglobulin which could be readily iodinated in the presence of H2O2 and horse radish peroxidase. These findings suggest the correlation of an iodide organification defect with a cytochrome b5 reductase deficiency. Administration of high doses of FAD led to the restoration of thyroidal iodide organification mechanism associated with an increased thyroid hormone production and to a marked decrease of the goiter. Riboflavin was given without effect even at a high dosage level. Consequently, it seems likely that the deficient cytochrome b5 reductase activity in this patient is due to a defect in the biosynthesis of FAD, the coenzyme of the reductase, from riboflavin.