Regulation of uridine kinase quaternary structure. Dissociation by the inhibitor CTP

Substrate specificity and phosphorylation of antiviral and anticancer nucleoside analogues by human deoxyribonucleoside kinases and ribonucleoside kinases

Structural analogues of nucleosides, nucleoside analogues (NA), are used in the treatment of cancer and viral infections. Antiviral NAs inhibit replication of the viral genome, whereas anticancer NAs inhibit cellular DNA replication and repair. NAs are inactive prodrugs that are dependent on intracellular phosphorylation to their pharmacologically active triphosphate form. The deoxyribonucleoside kinases (dNK) and ribonucleoside kinases (rNK) catalyze the first phosphorylation step, converting deoxyribonucleosides and ribonucleosides to their corresponding monophosphate form. The dNKs have been studied intensively, whereas the rNKs have not been as thoroughly investigated. This overview is focused on the substrate specificity, tissue distribution, and subcellular location of the mammalian dNKs and rNKs and their role in the activation of NAs.

Nifedipine and nimodipine competitively inhibit uridine kinase and orotidine-phosphate decarboxylase: theoretical relevance to poor outcome in stroke

Nifedipine and nimodipine, dihydropyridine calcium channel blockers, are commonly used as antihypertensive and antianginal agents in patients at risk for stroke. At least one stroke trial suggests that patients receiving calcium channel blockers at the time of an acute stroke have worse outcomes than those receiving other or no antihypertensive medications. We hypothesize that the poor outcome may not be related to blood pressure changes but instead may be mediated by competitive inhibition of important enzymes of pyrimidine synthesis whose products are needed to repair nerve cell membranes after an acute stroke. Both drugs acted as competitive inhibitors of the only enzymes that are known to synthesize the nucleotide uridine-5'-phosphate: uridine kinase and orotidine-5'-phosphate decarboxylase. Nifedipine produced Ki values of 28 microM for uridine kinase and 105 microM for orotidine-5'-phosphate decarboxylase. Nimodipine produced Ki values of 20 microM for uridine kinase and 18 microM for orotidine-5'-phosphate decarboxylase. For uridine kinase, these inhibitors bound more tightly than the physiologic substrates uridine or cytidine. For the decarboxylase, the inhibitors bound less tightly than the normal physiologic substrate orotidine-5'-phosphate. Additional experiments are needed to determine whether the concentrations of nifedipine or nimodipine, and of cytidine, uridine, and orotidine-5'-phosphate in human brain, are such that this inhibition would affect stroke outcome.

Uridine kinase: altered enzyme with decreased affinities for uridine and CTP

Uridine kinase is the rate-limiting enzyme in the salvage pathway for uridine or cytidine of mammalian cells. Alignment of the uridine kinase sequence with other nucleoside and nucleotide kinases supports a common ancestor for all of these. Three polypeptide segments for the ATP site and three polypeptide segments for the acceptor nucleoside site have been identified. We report here the characterization of an altered form of the enzyme with a single amino acid change, Q146R, within or near the uridine-binding site. This single amino acid change leads to a 160-fold increase in Km for uridine (Km = 6.5 mM) and a decrease in kcat by more than 99%. This variant has normal affinity for ATP (Km = 130 microM), but shows substrate inhibition at ATP concentrations >3 mM. Mouse uridine kinase is normally an active tetramer that will dissociate to inactive monomers in response to CTP. In contrast, the altered protein is monomeric, but will associate to dimers and then to tetramers with increasing ATP. The Q146R enzyme has a 100-fold loss in affinity for the allosteric inhibitor CTP; this supports a model for CTP inhibition being caused by CTP binding backward at the catalytic site, as a bisubstrate analog.

Dissociation of enzyme oligomers: a mechanism for allosteric regulation

Most enzymes exist as oligomers or polymers, and a significant subset of these (perhaps 15% of all enzymes) can reversibly dissociate and reassociate in response to an effector ligand. Such a change in subunit assembly usually is accompanied by a change in enzyme activity, providing a mechanism for regulation. Two models are described for a physical mechanism, leading to a change in activity: (1) catalytic activity depends on subunit conformation, which is modulated by subunit dissociation; and (2) catalytic or regulatory sites are located at subunit interfaces and are disrupted by subunit dissociation. Examples of such enzymes show that both catalytic sites and regulatory sites occur at the junction of 2 subunits. In addition, for 9 enzymes, kinetic studies supported the existence of a separate regulatory site with significantly different affinity for the binding of either a substrate or a product of that enzyme. Over 40 dissociating enzymes are described from 3 major metabolic areas: carbohydrate metabolism, nucleotide metabolism, and amino acid metabolism. Important variables that influence enzyme dissociation include: enzyme concentration, ligand concentration, other cellular proteins, pH, and temperature. All these variables can be readily manipulated in vitro, but normally only the first two are physiological variables. Seven of these enzymes are most active as the dissociated monomer, the others as oligomers, emphasizing the importance of a regulated equilibrium between 2 or more conformational states. Experiments to test whether enzyme dissociation occurs in vivo showed this to be the case in 6 out of 7 studies, with 4 different enzymes.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

Enzymes of nucleotide metabolism: the significance of subunit size and polymer size for biological function and regulatory properties

The 72 enzymes in nucleotide metabolism, from all sources, have a distribution of subunit sizes similar to those from other surveys: an average subunit Mr of 47,900, and a median size of 33,300. The same enzyme, from whatever source, usually has the same subunit size (there are exceptions); enzymes having a similar activity (e.g., kinases, deaminases) usually have a similar subunit size. Most simple enzymes in all EC classes (except class 6, ligases/synthetases) have subunit sizes of less than 30,000. Since structural domains defined in proteins tend to be in the Mr range of 5,000 to 30,000, it may be that most simple enzymes are formed as single domains. Multifunctional proteins and ligases have subunits generally much larger than Mr 40,000. Analyses of several well-characterized ligases suggest that they also have two or more distinct catalytic sites, and that ligases therefore are also multifunctional proteins, containing two or more domains. Cooperative kinetics and evidence for allosteric regulation are much more frequently associated with larger enzymes: such complex functions are associated with only 19% of enzymes having a subunit Mr less than or equal to 29,000, and with 86% of all enzymes having a subunit Mr greater than 50,000. In general, larger enzymes have more functions. Only 20% of these enzymes appear to be monomers; the rest are homopolymers and rarely are they heteropolymers. Evidence for the reversible dissociation of homopolymers has been found for 15% of the enzymes. Such changes in quaternary structure are usually mediated by appropriate physiological effectors, and this may serve as a mechanism for their regulation between active and less active forms. There is considerable structural organization of the various pathways: 19 enzymes are found in various multifunctional proteins, and 13 enzymes are found in different types of multienzyme complexes.

Uridine kinase: altered subunit size or enzyme expression as a function of cell type, growth stimulation, or mutagenesis

Using antibody prepared against pure uridine kinase from Ehrlich ascites cells, we have measured the expression of enzyme protein by the Western blot technique. Variations were observed in the Mr of the enzyme subunit for uridine kinase from different species: 32,000 (mouse Ehrlich ascites cells), 30,000 (normal human lymphocytes), 28,000 (mouse tissues), 27,500 (rat tissues). For different normal tissues from the same species, there was no significant variation in the subunit size. Transformed human and mouse cell lines, selected for a deficiency of uridine kinase activity in the presence of inhibitors activated by this enzyme, expressed two cross-reacting proteins, one with a normal (30,000) and one with a smaller (21,000) subunit molecular weight than was found in the parental cell line (human lymphoma), or only a smaller protein of Mr 25,000 (mouse lymphoma). Our results show that selection protocols using metabolite inhibitors do not always repress the expression of the enzyme but instead may lead to selection of those cells that have a mutation in the uridine kinase gene, resulting in the expression of an inactive enzyme. The expression of uridine kinase protein changes when cells are stimulated to divide. For both mouse fibroblasts and human lymphocytes, expression of uridine kinase protein as well as activity clearly increased after cells were stimulated to grow. In fibroblasts, increases are seen by 3 hr after stimulation, and plateau after 9 hr at a sevenfold increase. In lymphocytes, no change is seen until 12 hr after stimulation, and a plateau is not reached until 72 hr, with a total increase of approximately 50-fold. There has been considerable interest in the possibility of uridine kinase isozymes. Except for cells that have been mutagenized, the present results show that, as judged by subunit molecular weight, there appears to be only one enzyme form in normal and neoplastic cells or in cells in which uridine kinase activity is induced.

Postnatal ontogeny of uridine kinase in the cerebellum, hypothalamus, and cerebral cortex of the rat

Postnatal developmental patterns of uridine kinase were determined in crude subcellular fractions of the rat cerebellum, hypothalamus and cerebral cortex at ages 3 through 60 days. The highest specific activity and predominant distribution of enzyme was in the 105,000 g supernatant of the 3 brain regions. Enzyme activity in hypothalamus and cerebral cortex was maximum at 3 days and decreased with age; in cerebellum it increased through 13 days and decreased thereafter. Thus, the pattern of activity in hypothalamus and cerebral cortex paralleled changes in DNA and RNA synthesis through age 60 days; in cerebellum, it more closely approximated changes in DNA synthesis during early development. Changes in Km with aging suggest that the brain regions contain more than one form of enzyme. The highest particulate activity was in the microsomal fraction of the cerebellum and hypothalamus at all ages and in the cortex at 35 and 60 days. Relative specific activity for microsomal fractions of the brain regions at 60 days indicate a concentration of the enzyme which may be relevant in the maintenance of RNA activity in adult brain.

Quantitative determination of nucleosides and their phosphate esters--1. The acidic nucleosides, 3-deazauridine and 6-azauridine

A relatively rapid, high-resolution chromatographic procedure, using mini-columns of DEAE cellulose equilibrated with 10 mM sodium phosphate, pH 6.0, is described in sufficient detail to permit ready replication. This initial paper demonstrates the quantitative separation, using suction, of the acidic nucleosides, 3-deazauridine and 6-azauridine, from their phosphorylated derivatives. The chemically stable, tritium-labeled nucleosides are eluted from the mini-columns (capacity approximately equal to 1.8 ml) with 10 mM sodium phosphate, pH 6.0; subsequently, the nucleotides are eluted completely with 0.5 M HCl/0.5 M NaCl. Quantitation is based on liquid scintillation counting of aliquots of the eluates.