Showing papers in "Vitamins and Hormones Series in 1971"

The action of thyrotropin on thyroid metabolism.

Abstract: Publisher Summary This chapter discusses the action of thyrotropin on thyroid metabolism. Growth and function of the thyroid are mainly controlled by the level of thyrotropin (TSH) concentration in the plasma. The principal element of this control, the action of TSH on its target, the thyroid follicular cell, has been investigated intensively during recent years. TSH activates rapidly the steps of thyroidal iodine metabolism—that is, the formation of thyroid hormones from iodide and tyrosines and the secretion of these hormones, and induces, after a delay, the growth of thyroid tissue. Among the various systems available for the study of the interaction of a tropic hormone with its target cell, the thyroid and TSH present several advantages and disadvantages. The specialized metabolism of the follicular cell, the metabolism of iodine, is easily studied with the various radioisotopes of iodine. Thyroid tissue is very resistant to adverse conditions, which explains the great stability and the great variety of in vitro thyroid systems.

The Excretion of Steroid Hormone Metabolites in Bile and Feces

Abstract: Publisher Summary This chapter discusses the excretion of steroid hormone metabolites in bile and feces. The presence of steroid hormone activity in bile was first demonstrated nearly fifty years ago. Progress in the analysis of bile is slow compared to that of urine because of the technical problems associated with the qualitative and quantitative analysis of the small amounts of steroids excreted in bile and feces. The availability of labeled steroid hormones greatly facilitated investigations. More recently, the use of gas-liquid chromatography, particularly when associated with mass spectrometry, has facilitated research in this field, and in the last few years a vast amount of data has been accumulated. An understanding of the significance of biliary excretion of steroid hormone metabolites requires some knowledge about the structure of the biliary system and of current ideas on the formation and secretion of bile. The primary units of the biliary system are the bile canaliculi. They are formed as very fine tubules between adjacent pairs of parenchymal cells, each cell contributing one half of the canaliculus. The canaliculi can be seen only by electron microscopy. The membrane of that part of the cell which forms the canaliculus has numerous microvilli which project into the lumen of the canaliculus.

Quinones and Nicotinamide Nucleotides Associated with Electron Transfer

Abstract: Publisher Summary This chapter concentrates on the principles of function of nicotinamide nucleotides (NAN) and of menaquinone and ubiquinone in terminal substrate oxidation In mitochondria, NAN and quinones have been considered as members of the respiratory chain In particular for the NAN, this association is too limited because by the pool function the NAN is linked to the majority of cellular redox reactions In animal cells one may concentrate on the mitochondrial NAN system, whereas in protists and plants, the extramitochondrial NAD also is directly linked to the respiratory chain In prekaryotic cells, this compartmentation of NAN does not exist In this chapter, particularly interesting principles of function for the quinones are revealed In view of these various associations, the function of the coenzymes is considered both in terminal oxidation and in the substrate–substrate hydrogen transfer The molar relations between coenzymes, respiratory carriers, and dehydrogenases in mitochondria are elaborated Topochemical organization of the hydrogen-transferring coenzymes and enzymes in the mitochondrial membrane is described The redox state of nicotinamide nucleotides in mitochondria is also discussed

The Biosynthesis of Vitamins E and K and Related Compounds

Abstract: Publisher Summary This chapter discusses the biosynthesis of vitamins E and K and related compounds. The past five years have been active and rewarding ones for those studying the biosynthesis of the vitamins E and K, because in this short time the outlines and many of the details of the pathways involved in their formation have been elucidated. The rapid progress made owes much to the ready availability of a wide range of radioactive isotopes, to the marked improvements in the techniques of lipid fractionation and purification by chromatographic methods, and to parallel studies on the biosynthesis of biogenetically related compounds. The two groups of vitamins have many biogenetic features in common; however, the obvious difference is that the synthesis of the vitamins K requires the formation of a naphthoquinone ring, whereas that of the vitamins E requires the formation of a benzene nucleus.

Roles of vitamin B 12 and folic acid in methionine synthesis.

Abstract: Publisher Summary This chapter analyzes the role of vitamin B 12 and folic acid in methionine synthesis. In Escherichia coli , there are two types of enzymes capable of catalyzing methyl group transfer from N 5 -methyltetrahydrofolic acid ( N 5 -methyl-H 4 -folate) to homocysteine. One of these enzymes is a cobamide-containing protein that requires catalytic levels of S‑ adenosylmethionine (AMe), and a reducing system. N 5 -Methyl-H 4 -folates containing one or more L-glutamate moieties can function as methyl group substrates for the B 12 enzyme. Unlike the B 12 protein, the bacterial noncobamide enzyme requires Mg 2+ and is stimulated by inorganic phosphate. It utilizes only poly- L -glutamate forms of N 5 -methyl-H 4 -folate to synthesize methionine. Another distinguishing feature between these two bacterial methyltransferases is the influence of oxygen, which causes marked inhibition of the cobamide-containing enzyme but has no effect on catalysis by the noncobamide protein. In this chapter, characteristics of the B 12 transmethylase enzyme are discussed. Studies with radioactive N 5 -methyltetrahydrofolate are presented. Studies on the role of S -adenosyl- L -methionine (AMe) are elaborated. Mechanism of the B 12 ‑dependent transmethylase reaction is also described.

The hypothalamic hypophysiotropic thyrotropin-releasing factor.

Abstract: Publisher Summary This chapter discusses the hypothalamic hypophysiotropic thyrotropin-releasing factor. To write a review on a subject that has occupied many years of one's life is almost an enjoyable proceeding if it can be done in some sort of a post facto manner that is when the major questions that constituted that subject have been satisfactorily answered. Even though no question in science is ever fully answered, there are some times in the evolution of a research program when one can stop for a while and take stock of what has been accomplished. Such a point was recently reached in the efforts of many investigators over the last 20 years at characterizing the “elusive hypothalamic hormones,” with the isolation, determination of the molecular structure, and total synthesis of one of these, the thyrotropin releasing factor (TRF). At the moment, there are no methods available, such as radioimmunoassay or double isotope dilution methods, which would allow direct measurement of TRF in biological fluids.

The progressive increase in estrogen production in human pregnancy: an appraisal of the factors responsible.

Abstract: Publisher Summary This chapter focuses on progressive increase in estrogen production in human pregnancy. Estrogens are synthesized in the placenta largely from androgen sulfates secreted by the fetal zone of the fetal adrenal. The level of supply of these precursors determines the quantity of estrogen produced. This chapter considers factors that modify the concentration of cortisol in the fetal circulation. These are discussed in terms of synthesis, metabolism, transport from the maternal system, and transport to the maternal system. It is concluded that the increasing production of estrogens in the second half of gestation arises from an increasing production of androgen sulfates by the fetal adrenal associated with progressive growth of and stimulation of steroid biosynthesis in this gland, which, in turn, are provoked by an elevated secretion of ACTH from the fetal pituitary. The secretion of ACTH in increased quantities is a response to an increased metabolic clearance rate of cortisol from the fetal plasma. A major factor concerned in the increased clearance of cortisol is the disparity in the number of available binding sites for cortisol in fetal and maternal plasma.

Structure and Catalytic Role of the Functional Groups of Aspartate Aminotransferase

Abstract: Publisher Summary This chapter describes structure and catalytic role of the functional groups of aspartate aminotransferase (AAT). AAT is a particularly interesting enzyme because its catalytic activity is duplicated, though with a much smaller efficiency, by its coenzyme moiety, even in the absence of the specific apoprotein. An impressive mass of information on the structure and function of AAT has been obtained in recent years. The general pattern of the chemical events occurring at the active site has been established; the reaction proceeds through a great number of discrete steps, each of which proceeds fast because it is catalyzed by a number of suitably disposed groups on the enzyme; conformational changes contribute to placing the various groups involved in the most appropriate position for the catalysis of each step. Many of the rate and equilibrium constants for the interconversion of the intermediates have been measured. In this chapter, concepts related to the assembly of the holoenzyme are discussed. The binding of the coenzyme to the protein is elaborated. The interaction between the holoenzyme and the substrate is explained. The quaternary structure of the enzyme is also described.

Regulation of calcium transport in bone by parathyroid hormone.

Abstract: Publisher Summary This chapter discusses regulation of calcium transport in bone by parathyroid hormone. It attempts to review current concepts concerning the mode of action of parathyroid hormone, and points out apparent similarities between in vivo effects of this hormone on a variety of metabolic functions and those produced by elevated circulating calcium concentrations. It also presents a model to account for parathyroid control of calcium fluxes in bone. In this chapter, it is proposed that by enhancing entry of calcium into the interior of the cell, parathyroid hormone produces two divergent effects on the overall metabolism and physiology of bone and other target tissues. The first effect is that by allowing increased intracellular ionic calcium levels, the hormone increases the amount of calcium transported across specific layers of target cells. This net movement of calcium is unidirectional or “polarized” and occurs against a concentration gradient via a “pump” mechanism. The second is that if the intracellular concentration of ionic calcium is appreciably increased, many enzymatically controlled processes within the cell may, in turn, be affected. This secondary function of parathyroid hormone can be termed “stimulation of calcium-influenced metabolic activities” and may occur in any cell that responds to the hormone by allowing increased entry of calcium.

Protein-polypeptide hormones of the human placenta.

Abstract: Publisher Summary This chapter discusses the protein–polypeptide hormones of the human placenta. Human chorionic gonadotropin (HCG) was the first protein hormone isolated from the placenta. It is established that HCG is synthesized by the chorionic tissue. At the beginning of the 1960's, another hormone-human chorionic somatomammotropin, or HCS (originally designated as human placental lactogen), was isolated, and its synthesis by human chorionic tissue has been confirmed. The presence of other substances whose hormonal activities resemble those of the pituitary gland, like ACTH, TSH, MSH, and oxytocin, has been demonstrated in the human placenta; also, the presence of relaxin, acetylcholine, and other substances with uterotropic and pressor activities has been reported. The interstitial cell stimulating effect of HCG, both on the testes and the ovaries of several animal species as well as humans, has been repeatedly demonstrated. The gametokinetic effect of HCG on the ovaries is debatable. The weight increase of accessory male organs in immature rats has been described repeatedly. This effect of HCG is now used as a reliable and sensitive assay for its biological activity.

The Chemistry of Parathyroid Hormone and the Calcitonins

Abstract: Publisher Summary This chapter discusses the chemistry of parathyroid hormone and the calcitonins. In the last several years, there have been striking advances in knowledge of the chemistry of parathyroid hormone and calcitonin. The complete amino acid sequence of the major form of bovine and porcine parathyroid hormone has been determined, and, as well, the complete structure of porcine, human, ovine, bovine, and three different isohormonal forms of salmon calcitonin. These advances are potentially of considerable interest and importance to investigators and clinicians concerned with the physiology, mode of action, and clinical significance of parathyroid hormone and calcitonin. The significance of these chemical advances, particularly the availability of highly potent and completely homogeneous preparations of synthetic parathyroid hormone and calcitonin can best be appreciated in terms of a historical perspective. Intensive physiological and clinical investigations have established the importance of parathyroid hormone in normal calcium homeostasis and in certain diseases involving disorders of calcium and bone metabolism. The hormone acts to increase rates of bone resportion and to reduce the rate of urinary calcium loss, thereby maintaining serum calcium and preventing hypocalcemia.

Modified thiamine compounds

Abstract: Publisher Summary This chapter describes modified thiamine compounds. There has been a great increase in Japan in the sale of vitamin preparations containing thiamine, and also in the enrichment of staple foods, candies, and other food items with thiamine. This increase has encouraged the search for derivatives of thiamine that are odorless and more stable than its chloride-hydrochloride form. It has stimulated the successful development of modified thiamine compounds both for therapeutic and enrichment purposes. Modified thiamine compounds can be classified into two types: masked compounds which still retain thiamine activity in animals, and permanently altered compounds, which have no thiamine activity even though they are chemically convertible to thiamine to a greater or lesser extent. The masked thiamine compounds are interesting examples of successful changes of the thiamine molecule, and they may be superior to thiamine both in physical and in biochemical properties. Other vitamins than thiamine do not usually form this type of modified compound, these thiamine-active modified compounds may be divided into the disulfide and the S-acyl thiamine types.

The role of phosphopantetheine in the yeast fatty acid synthetase complex.

Abstract: Publisher Summary This chapter discusses the role of phosphopantetheine in the yeast fatty acid synthetase complex. The study of fatty acid biosynthesis in various organisms disclosed a completely new role of pantothenic acid in cellular metabolism. In experimental studies of soluble enzyme system from bacteria, the involvement of a new cofactor, protein-bound 4'-phosphopantetheine was discovered in bacterial fatty acid synthesis. Subsequently, fatty acid synthetases from plants, mammals, and birds as well as from yeast could equally be shown to contain this component. In Escherichia coli, 4′-phosphopantetheine is bound to a heat-stable acidic protein that has a molecular weight of 8847 and contains 77 amino acid residues. The results of studies relating to the isolation, structure, and function of this cofactor in the yeast fatty acid synthetase complex are presented in this chapter. Reaction scheme of fatty acid biosynthesis is explained. Substrate binding sites of the multienzyme complex are discussed. Isolation of the acyl carrier protein is also described.

Thiamine Pyrophosphate-Catalyzed Enzymatic Decarboxylation of α-Oxo Acids

Abstract: Publisher Summary This chapter focuses on thiamine pyrophosphate-catalyzed enzymatic decarboxylation of α-oxo acids. The first enzymatic decarboxylation of an α-oxo acid was described as: a fermenting extract from brewer's yeast cleaves pyruvate into acetaldehyde and carbon dioxide. Later, it was found that besides the protein of the enzyme, a thermostable cofactor (“cocarboxylase”) is necessary for the enzymatic cleavage of pyruvate. For decarboxylation of other α-oxo acids to the corresponding aldehydes, for their oxidative decarboxylation leading to acyl coenzyme A derivatives, and for a number of other enzymatic reactions, the same coenzyme was found to be necessary. By comparison of thiamine catalysis with that of cyanide in the acyloin condensation, Breslow came to the conclusions that thiamine forms a 2-carbanion with a p K comparable to that of HCN; the upper limit found for the p K was 20. The exclusively kinetic measurements could not account for activation energy, and therefore the p K of thiamine is expected to be several units lower. In this chapter, mechanism of nonenzymatic thiamine (Thiazolium) catalysis are analyzed in detail. Isolation of 2-α-hydroxyalkyl derivatives of thiamine pyrophosphate from enzymatic incubation mixtures is described. Effects of substituents at the thiazolium ring are also discussed.

Influences of pyridoxine derivatives on the biosynthesis and stability of pyridoxal phosphate enzymes.

Abstract: Publisher Summary This chapter discusses the results obtained related to the involvement of pyridoxal phosphate and other pyridoxine derivatives on the biosynthesis and stability of pyridoxal phosphate-dependent enzymes. This survey of the available data has revealed that the regulation of the activities of pyridoxal phosphate enzymes even in so restricted a field as pyridoxal phosphate and other pyridoxine derivatives, is very complex. Remarkable differences have been observed, in responses to pyridoxine deficiency, depending on the enzyme under consideration. In general, the enzymes of the cell sap are far more sensitive to pyridoxine deprivation than the enzymes associated with the particulate fractions. The enzymes whose turnovers are high are more rapidly altered than the enzymes whose turnovers are low. This chapter reveals that pyridoxal phosphate and pyridoxine derivatives have interesting properties that afford some insight into the still rudimentary knowledge of the mechanism involved in the physiological biosynthesis and degradation of enzymes. Effects of pyridoxine deficiency are elaborated. Effects of pyridoxal phosphate and other pyridoxine derivatives on the stability of purified enzymes are also described.

Binding of pyridoxal phosphate to apoenzymes as studied by optical rotatory dispersion and circular dichroism.

Abstract: Publisher Summary This chapter discusses binding of pyridoxal phosphate to apoenzymes as studied by optical rotatory dispersion (ORD) and circular dichroism (CD). The enzyme-catalyzed reactions are, in general, stereospecific in contrast to the nonenzymatic reactions or reactions catalyzed by inorganic or simple organic catalysts, because the active centers of an enzyme are so organized as to bind and react with substrate in terms of stereochemical specificity. To understand the nature of the binding forces involved in the formation of the enzyme-substrate complex and also the nature of the specific loci responsible for the catalytic function, it is necessary to elucidate the mode of interaction between apoenzymes and coenzymes. This chapter briefly reviews the principle of ORD and CD. It discusses some of experimental results in regard to the binding of PLP and its analogs with bovine serum albumin (BSA) as a model system and also with biodegradative threonine deaminase. The chapter also briefly summarizes spectropolarimetric studies from several laboratories on other pyridoxal enzymes.

Model studies on flavin-dependent oxidoreduction.

Abstract: Publisher Summary This chapter discusses model studies on flavin-dependent oxidoreduction. Fully reduced flavin is known to be a bent molecule, though the inversion time of the “butterfly wing” conformations is far below the half time of hydrogen transfer. If, on the other hand, the flavin is fixed within a protein active site, the flipping of the leucoflavin conformers is largely restricted, and that one conformation is stabilized. Therefore, flavin-dependent substrate dehydrogenation might exhibit a stereospecificity problem like nicotinamide-dependent dehydrogenation while free flavin is oxidoreduced unspecifically. The flavin asymmetry is removed upon detachment of the coenzyme from the protein, while that of reduced nicotinamide coenzymes is retained. A given coenzyme analog might be a model for one function of the coenzyme, and a true “nonmodel” for another function of the same coenzyme. If, like in many flavoproteins, enzyme action comprises both functions, no suitable model for both functions at the same time may be possible at all, while there might be quite plausible models for each single function. In this chapter, an overview of model reactions of flavin-dependent substrate dehydrogenation is presented. Flavin “model” radicals are discussed. Flavin-dependent O 2 -activation is also described.

Flavin-radical-metal chelates.

Abstract: Publisher Summary This chapter presents an overview of flavin–radical–metal chelates. The existence of flavin-radical-metal chelates and of flavoquinonemetal charge-transfer chelates has been well established in the model systems and their properties have been determined. So far, however, there is no single case where any such chelate has been demonstrated in a flavin-metal containing protein. With metalloflavoproteins containing Mo and/or Fe, it has been observed in the native enzyme that only part of the flavin and the metal can under the conditions explored be recovered as species that give ESR signals. The possibility has been discussed that the missing portions of both species could be present as radical-metal chelates, which would become indetectable by ESR. This could possibly be demonstrated by the characteristic paramagnetic susceptibility of such a chelate. Such measurements would require great sensitivity and accuracy because several centers in the metalloflavoprotein must change their paramagnetism and each individual change depends critically on the redox state of the protein. In this chapter, an overview of chelate stability, electron spin resonance, and electron nuclear double resonance is presented. Light absorption is analyzed, and concepts of paramagnetic susceptibility are explained.

The binding of NAD + and NADH to glyceraldehydephosphate dehydrogenase.

Abstract: Publisher Summary This chapter discusses the binding of NAD+ and NADH to glyceraldehydephosphate dehydrogenase. Early studies demonstrated that the rabbit-muscle enzyme binds NAD+ much more firmly than other dehydrogenases, because the crystalline enzyme contains bound NAD+. However, it does not bind NAD+ as firmly as activated charcoal, which completely removes NAD+ from the enzyme. Binding of NAD+ to charcoal-treated enzyme causes the appearance of a broad absorption band with maximum at 360 nm, and the formation of the band is inhibited by iodoacetate or acetyl phosphate. The binding constants of NAD+ to each of the four subunits in the muscle enzymes have been determined by equilibrium dialysis, and using both ultracentrifugation and equilibrium dialysis. This chapter displays the results, given as a Scatchard plot, of the measurements with the lobster enzyme. The curved line, convex to the abscissa, shows that the binding constants become successively less as more NAP becomes bound to the enzyme—that is, there is negative cooperativity. The inset shows that a straight-line Scatchard plot is obtained when the binding of the fourth molecule is plotted on the assumption that the first three sites are completely occupied before NAD+ is bound to the fourth site.

Effect of conformation on the binding of flavins to flavoenzymes.

Abstract: Publisher Summary This chapter analyzes effect of conformation on the binding of flavins to flavoenzymes. The interaction of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) with specific proteins confers a great degree of versatility to these coenzymes as evidenced by the wide variety of reactions catalyzed by flavoproteins. An examination of the detailed tabulation of flavoproteins compiled by Palmer and Massey reveals the diversity of reactions catalyzed by these enzymes. A large number of flavoproteins effect the transfer of electrons between a donor and an acceptor; displaying considerable specificity for the former and much less for the latter. A smaller group of flavoenzymes serve as mixed function oxygenases. In addition, FAD and FMN are also members of multicomponent electron transfer systems such as those present in the metalloflavoproteins and respiratory chain-linked dehydrogenases. The functional diversity of these enzymes must to a large extent reside in the interactions between the flavocoenzyme and the protein matrices. In this chapter, basic concepts related to nature of binding are presented. Effect on flavin absorption spectrum is elaborated. Properties of apoflavoproteins are also described.

Historical Survey and Introductory Remarks

Abstract: Publisher Summary This chapter discusses the historical survey of enzymes. The “old yellow enzyme” was prepared by Warburg and Christian from brewer's yeast and reported in 1933 to consist mainly of polysaccharides and some proteins. The yellow color was attached to some high molecular carrier and exerted a catalytic oxidoreduction function, being reduced to the leuco form by a coenzyme-enzyme system from red blood cells and reoxidized by oxygen. The yellow color could be separated from the high molecular fraction after dialysis at slightly acid reaction and was found to be a colorless protein. The enzyme activity disappeared when the parts separated, but returned when they were recombined at neutral reaction. The yellow dye could not be riboflavin, which was much less active in restoring the activity than the natural dye, even though the absorption bands and fluorescence were identical.