Showing papers in "Current Topics in Cellular Regulation in 1981"

The Insulin-Like Growth Factors (IGF) of Human Serum: Chemical and Biological Characterization and Aspects of Their Possible Physiological Role

Abstract: Publisher Summary This chapter discusses the chemical and biological characterization, and properties of the insulin-like growth factors (IGF). The chapter also discusses their possible physiological role. Blood is a large reservoir of insulin-like activity. Insulin represents only a small portion of this activity. IGF I and II are polypeptides with molecular weights of 7649 and 7471. They contain 70 and 67 amino acid residues, respectively, each in a single polypeptide chain with three intra-chain disulfide bridges. Each of the two polypeptides contains two regions—domains—in which parts of the sequences can be aligned to identical parts in human pro-insulin: 38% of the sequence of residues 42 to 70 in IGF I and 44% of the sequence of residues 41 to 67 in IGF II (termed A-domains), are identical to the insulin A-chain. As in pro-insulin, the A- and B-domains are connected by a C-domain. However, in contrast to the C-peptide of pro-insulin, which consists of 35 amino acids, the C-regions of IGF I and II are only 12 and 8 residues long, respectively.

Does vanadium play a role in cellular regulation

Abstract: Publisher Summary This chapter highlights physiological and metabolic effects on enzyme activities. Vanadium, a group V element, belongs to the first transition series and can form compounds mainly in valence states 3+, 4+, and 5+, both anionic and cationic species. Several criteria of essentiality of an element are satisfied by the properties of vanadium such as low molecular weight, excellent catalytic activity, appropriate atomic structure, its position as a transition metal, ability to form chelates potentially with biologically active compounds, ubiquity in the geosphere and possibly in the biosphere, homeostatic regulation by controlled accumulation and rapid excretion, deficiency in animals and plants showing characteristic symptoms, and low toxicity on oral intake. At least three significant enzymes of the plasma membrane are affected by vanadate—Na, K-ATPase is inhibited in nanomolar concentrations and adenylate cyclase and NADH oxidase are activated in micro-molar to milli-molar concentrations. These enzymes are interrelated. ATP is the substrate for the first two, and both substrates are nucleotides. Vanadate, affecting the transition state of phosphate, seems to have an affinity for the binding sites on these enzymes.

Multimodulation of enzyme activity

Abstract: Publisher Summary This chapter discusses the multimodulation of enzyme activity. Multimodulation of enzyme activity arises from the accumulation in a given enzyme of several regulatory mechanisms, whether of different types—cooperative, allosteric, interconversion, or of the same type—multiple allosteric effects, or of any combination of these. Modulation of enzyme activity was considered initially as synonymous with feedback inhibition, typically by the end product. With time, some enzymes appeared to have more than one mechanism for modulation of activity. Eventually, plurality increased for certain enzymes from more than one to many modulation mechanisms, including both noncovalent and covalent types. Phosphorylase provided the first example for many aspects of metabolic regulation at the enzyme level. It shows that marked co-operativity for the substrate P i , is inhibited by glucose 6-phosphate (Glc 6-P) and can be activated by either noncovalent (adenosine monophosphate —AMP) or covalent (phosphorylation) modulation. The two activation mechanisms make excellent physiological sense—AMP serves as energy-need signal and the phosphorylation ultimately reflects hormonal control.

Regulation of ribonucleotide reductase.

Abstract: Publisher Summary This chapter discusses the regulation of ribonucleotide reductase. Ribonucleotide reductase catalyzes the first unique step of DNA synthesis by converting the four ribonucleotides to the corresponding deoxyribonucleotides. Deoxyribonucleotides are highly specialized metabolites playing only limited roles apart from their function as DNA precursors. A rate-limiting function of ribonucleotide reduction in DNA replication is observed in many systems. All ribonucleotide reductases catalyze the replacement by hydrogen of the OH group at the 2' position of the ribose moiety of a ribonucleotide. During purification, ribonucleotide reductase is separated from its natural hydrogen donor substrate; the purified enzymes require certain dithiols such as dihydrolipoate or dithiothreitol. The presumed rate-limiting function of ribonucleotide reductase in DNA synthesis has been utilized for designing inhibitors of the enzyme with chemotherapeutic potential. Drugs affecting the reaction mechanism of the enzymes and inhibitors operating via the allosteric mechanism have been described in the chapter.

Glycogen synthase and glycogen synthase kinases

Abstract: Publisher Summary This chapter focuses on properties and other biochemical and physiological processes related to glycogen synthase and glycogen synthase kinases. The reversible covalent modification of proteins appears to be one of the fundamental mechanisms that have evolved to regulate the inherent properties of enzyme molecules. Tracing the regulatory links between the extracellular stimulus and the modification of target enzymes in the cell is one of the major goals of research in this area. The hormone-generated signal or second messenger is cyclic adenosine monophosphate (AMP), and it is decoded by the cyclic AMP-dependent protein kinase and translated into the phosphorylation of substrates for this kinase. Through the interactions of different phosphorylations to determine enzymic properties, especially kinetic properties, multiple phosphorylations could integrate diverse extracellular signals to generate enzyme with properties precisely tailored to the prevailing physiological needs.

Regulation of Liver 3-Hydroxy-3-methylglutaryl-CoA Reductase

Abstract: Publisher Summary This chapter presents the results of studies on regulation of liver 3-hydroxy-3-methylglutaryl-CoA reductase. The role of cholesterol in the regulation of hepatic cholesterol biosynthesis has been investigated by several laboratories and the site of feedback inhibition by cholesterol shown to be 3-hydroxy-3-methylglutaryl (HMG)–CoA reductase. The identification of a feedback mechanism in cultured peripheral cells, including fibroblasts from human skin, suggested that cholesterol synthesis in non-hepatic cells was also subject to feedback control by cholesterol. In other studies the enzymic activity of hepatic HMG–CoA reductase decreases rapidly in rats fed cholesterol and in cultured fibroblasts incubated with either low-density lipoproteins or sterols.

Kinetic models of metabolism in intact cells, tissues, and organisms.

Abstract: Publisher Summary This chapter discusses the kinetic models of metabolism in intact cells, tissues, and organisms. Models can provide conceptual insights and help one think and ask questions in realistic, dynamic terms, related to the steady-state conditions of the living cell. Kinetic models can reveal circumstances that are initially not intuitively obvious or even acceptable. They make specific predictions to be tested experimentally, suggest experimental approaches otherwise difficult to imagine, and aid in the analysis of phenomena not easily investigated experimentally, such as intracellular compartmentation. Because dynamic models simulate metabolism in living systems, they serve as a framework in which to judge the relevance of in vitro data to metabolism in vivo, and in which to determine those elements that are probably critical variables, that is, rate-limiting or controlling essential reactions in an intact organism. Thus, in effect, the construction and analysis of kinetic models represent a powerful analytical technique. A model is a hypothesis formulated in quantitative detail. If the outcome of an experiment is consistent with the model, then the model's validity becomes more probable.

Mono(ADP-Ribosyl)transferases and Their Effects on Cellular Metabolism

Abstract: Publisher Summary This chapter discusses the inhibition of protein synthesis and activation of adenylate cyclase. Diphtheria toxin and Pseudomonas exotoxin A inhibit protein synthesis in susceptible cells and in cell-free systems by catalyzing the ADP-ribosylation of a single amino acid in elongation factor II (EF-II), which then becomes inactive in protein synthesis. These toxins apparently do not ADP-ribosylate other proteins, presumably because they lack the post-translationally modified histidine that serves as the ADP-ribose acceptor in EF-II. Choleragen has been a valuable tool in the work that has resulted in the isolation and characterization of the guanyl nucleotide-binding component (G/F) of the cyclase. In addition, demonstration of the enzymic activity of choleragen leads directly to the question whether animal cells employ ADP-ribosylation in a controlled way to regulate adenylate cyclase activity.

In vivo functioning of the Na+, K+-activated ATPase.

Abstract: Publisher Summary This chapter discusses the in vivo functioning of the Na + , K + -activated adenosine-triphosphatase (ATPase). Movement of Na + and K + across the plasmalemmal membrane is largely carried out by active energy-dependent mechanisms. Maintaining the Na + gradient is particularly important in view of the central role commonly played by this ion in fueling the uptake of sugars and amino acids, in control of voltage-dependent permeability, and in exchange for Ca 2+ . Regulation of Na + , in conjunction with K + , is central to control of cell volume, cell proliferation, and transcellular ion movement, to the maintenance of transmembrane electrical potential, and to the controlled release of neurotransmitters. In nervous tissue, this regulation is of singular importance because: (1) of the frequent partial dissipation of basal transmembrane concentration gradients by postsynaptic and action potentials and, (2) the accompanying ion shifts occurring within the relatively restricted extracellular volume may lead to significant concentration changes.

Cyclic AMP-dependent and cyclic GMP-dependent protein kinases of nervous tissue.

Abstract: Publisher Summary This chapter focuses on cyclic AMP (cAMP)-dependent and cyclic GMP (cGMP) -dependent protein kinases of nervous tissue. Information about cAMP-dependent and cGMP-dependent protein kinases of brain has increased considerably over the last few years. It is now fairly well established that two types of cAMP-dependent protein kinase and one type of cGMP-dependent protein kinase are the principal receptors of cAMP and cGMP in brain, as in other tissues. These protein kinases may occur in various tissues as soluble and as membrane-bound forms, and brain appears to be especially rich in membrane-bound cAMP-dependent protein kinases. cAMP-dependent and cGMP-dependent protein kinases are not the only types of protein kinases in the nervous system. A cyclic nucleotide-independent protein kinase, which was activated by a calcium-dependent protease, was found in bovine cerebellum and rat brain. Calcium also activates other protein kinases of brain by a calmodulin-dependent mechanism. Calcium and calmodulin can regulate the levels of cAMP and cGMP because of their ability to activate certain adenylate cyclases and cyclic nucleotide phosphodiesterases.

Regulation of the Histidine Operon: Translation-Controlled Transcription Termination (A Mechanism Common to Several Biosynthetic Operons)

Abstract: Publisher Summary This chapter discusses the regulation of the histidine operon. Regulation of the gene expression via attenuation represents a new kind of regulatory mechanism, well differentiated from that of the repressor-operator interaction. These two regulatory mechanisms can coexist, as in the trp system, or attenuation can be the only operon-specific regulatory mechanism, as appears to be the case for the histidine system. The most interesting feature of the attenuation mechanism is that it does not require any repressor or activator protein—it utilizes the protein synthesis machinery thus, linking the regulation of one single system to the growth of the cell. While histidine-specific regulation can be mostly explained by the attenuator mechanism, there are other observations that are not yet explained. The most interesting one may be the shift from a sequential to a simultaneous mode of derepression observed in different his mutants or in different metabolic conditions.

Glutamine metabolism in higher plants.

Abstract: Publisher Summary This chapter discusses metabolism and the role of glutamine in the flow of nitrogen. Under certain conditions, all the nitrogen assimilated by bacteria could pass through glutamine. The plant enzyme, in common with that from other sources, exhibits a number of activities that have been the basis of a number of different assays. In plants, as in other tissues, there are enzymes other than glutamine synthetase (GS) that may catalyze the transferase assay and thus, the assay is unlikely to be specific. The role of GS is highlighted by the build-up of glutamine at the expense of glutamate and other amino acids—as glutamate levels fall, presumably other free amino acids are transaminated to the available 2-oxoglutarate in an attempt to maintain the level of glutamate. Glutamate dehydrogenase (GDH) does not operate to assimilate NH 3 , even though the concentration of NH 3 is greatly increased and there is an excess of 2-oxoglutarate.

Phosphorylation Reactions That Influence the Activity of eIF-2

Abstract: Publisher Summary This chapter reviews phosphorylation reactions that influence the activity of eIF-2 Important sites for translational control in eukaryotic systems appear to be reactions involving peptide initiation factor 2 (eIF-2) Phosphorylation is measured routinely by analysis of the reaction mixture on polyacrylamide gels in sodium dodecyl sulfate followed by autoradiography Regulation of eIF-2 phosphorylation by phosphoprotein phosphatases may be an important control element The relatively nonspecific phosphatase activities isolated thus far from reticulocytes may themselves be controlled by low-molecular-weight, heat-stable activators and inhibitors Activators have been isolated that seem to impose a certain degree of substrate specificity on the reticulocyte phosphatase

Covalent Modification of Phosphofructokinase by Phosphorylation—Dephosphorylation*

Abstract: Publisher Summary This chapter reviews the process of covalent modification of phosphofructokinase by phosphorylation–dephosphorylation. The desensitizing factor had properties of a protein or peptide as it was destroyed by treatment with protease. The desensitization reaction was stimulated by a heat-stable fraction from yeast extract, which could be replaced by a combination of adenosine monophosphate (AMP) plus fructose 6-phosphate. The complete dependence of the desensitization reactions on fluoride makes it rather doubtful that they play a physiological role in the regulation of phosphofructokinase (PFK) activity in yeast or liver cells. Rather it underlines the importance of low-molecular-weight (LMW)-ligands in determining the stability and catalytic activity of this enzyme. Studies in which muscle or liver PFK was phosphorylated with the protein kinase catalytic subunit have not revealed any significant changes in the kinetic properties of the enzyme. Changes in the degree of PFK phosphorylation within the cell have been observed. Another, although completely hypothetical possibility would be that PFK phosphorylation is involved in the spatial arrangement of this enzyme within the cell.

Sterol Structure and Membrane Function

Abstract: (1983) Sterol, Structure and Membrane Function Critical Reviews in Biochemistry: Vol 14, No 1, pp 47-92