Thymidine kinase. A novel affinity chromatography of the enzyme and its regulation by phosphorylation in Physarum polycephalum

N3-substituted thymidine bioconjugates for cancer therapy and imaging

The compound class of 3-carboranyl thymidine analogues (3CTAs) are boron delivery agents for boron neutron capture therapy (BNCT), a binary treatment modality for cancer. Presumably, these compounds accumulate selectively in tumor cells via intracellular trapping, which is mediated by hTK1. Favorable in vivo biodistribution profiles of 3CTAs led to promising results in preclinical BNCT of rats with intracerebral brain tumors. This review presents an overview on the design, synthesis, and biological evaluation of first- and second-generation 3CTAs. Boronated nucleosides developed prior to 3CTAs for BNCT and non-boronated N3-substituted thymidine conjugates for other areas of cancer therapy and imaging are also described. In addition, basic features of carborane clusters, which are used as boron moieties in the design and synthesis of 3CTAs, and the biological and structural features of TK1-like enzymes, which are the molecular targets of 3CTAs, are discussed.

A monoclonal antibody specific for human thymidine kinase 1

Previous research has shown that thymidine kinase 1 (TK1), a nucleotide salvage pathway enzyme, is an accurate prognostic and diagnostic tumor marker. However, the current radioisotope assay for TK1 is cumbersome and has hampered the clinical application of this diagnostic technique in cancer management. To overcome the problems of the current radioisotope assay, we have produced monoclonal antibodies (MAbs) using purified TK1 from Raji cell extract. Production and confirmation of their specificity was confirmed using Western blot, immunohistochemical staining, TK1 activity inhibition assays, and enzyme-linked immunoadsorbent assay (ELISA) techniques. Thus, in the future, these antibodies may aid in the early detection of cancer and more accurate prognosis, as well as allowing for an increased ability to study the function of TK1 in basic cellular processes.

Synthesis of 5-(carboranylalkylmercapto)-2'-deoxyuridines and 3-(carboranylalkyl)thymidines and their evaluation as substrates for human thymidine kinases 1 and 2

Derivatives of thymidine containing o-carboranylalkyl groups at the N-3 position and derivatives of 2'-deoxyuridine containing o-carboranylalkylmercapto groups at the C-5 position were synthesized. The alkyl spacers consist of 4-8 methylene units. The synthesis of the former compounds required 3-4 reaction steps in up to 75% overall yield and that of the latter 9-10 reaction steps with significantly lower overall yield. Derivatives of thymidine substituted with carboranylalkyl substituents at the N-3 position and short spacers were phosphorylated by both recombinant and purified cytosolic thymidine kinase (TK1) to a relatively high degree. None of the tested 2'-deoxyuridine derivatives possessing carboranyl substituents at the C-5 position were phosphorylated by either recombinant or purified TK1. The amounts of phosphorylation product detected for some of the C-5-substituted nucleosides with recombinant mitochondrial thymidine kinase (TK2) were low but significant and decreased with increasing lengths of the alkyl spacer. The data obtained in this study do not seem to support the tether concept applied in the synthesis of the new C-5- and N-3-substituted carboranyl nucleosides intended to reduce possible steric interference in the binding of carboranyl nucleosides with deoxynucleoside kinases. Instead, it appeared that a closer proximity of the bulky carborane moiety to the nucleoside scaffold resulted in better substrate characteristics.

Life on the salvage path: the deoxynucleoside kinase of Lactobacillus acidophilus R-26

In Lactobacillus acidophilus R-26, the synthesis of DNA precursor deoxynucleotides occurs exclusively by salvage of deoxynucleosides, beginning with phosphorylation by four deoxynucleoside kinases. Subunits bearing three of these activities are uniquely organized into two heterodimers, deoxyadenosine/deoxycytidine kinase (dAK/dCK) and deoxyadenosine/deoxyguanosine kinase (dAK/dGK), which, along with a distinct deoxythymidine kinase (TK), catalyze the parallel first committed steps of dNTP biosynthesis. Whereas TK is common to most prokaryotes (and eukaryotes), the other three activities that are the emphasis of this review are quite unusual in bacteria. Each activity is regulated in cis by its homologous end-product (dNTP) which is understood to act as a multisubstrate inhibitor capable of binding to both nucleoside and phosphate subsites. Conversely, the inactive dAK subunit is progressively activated by 1) association with a dGK or dCK subunit and 2) the conformationally driven heterotropic affect of dGuo or dCyd bound to the opposing subunit. Limited proteolysis has proven to be a powerful probe of conformational states. Further indication of conformational or structural differences between dAK and dGK (or dCK) is that the former follows an ordered kinetic path, while dGK or dCK exhibits rapid-equilibrium random kinetics. The multi-substrate behavior of end-product binding provides a convenient new diagnostic tool for distinguishing kinetic mechanisms. Tandem dak-dgk genes have been cloned from Lactobacillus DNA and expressed in Escherichia coli as dAK/dGK, utilizing the associated promoter. Sequence alignments reveal 65% identity in their DNA and 61% in their derived amino acid sequences. Encoded N-terminal sequences are identical for the first 18 residues, and both subunits share conserved sequences in common with adenylate kinase and viral TK. A more unusual conserved element, which appears to play a role in the activation of dAK, resembles the G2 loop of p21 ras. Remarkably, no homologous gene(s) for the dAK/dCK pair could be found. Comparisons of amino acid sequences, isoelectric pHs and subunit masses strongly indicated that native dCK and dGK are identical in sequence, except at their extreme N-termini (M-IVL for dCK and -TVIVL for dGK), suggesting that processing of a common precursor occurs in Lactobacillus. Accordingly, deletion of codons 2 and 3 from dgk resulted in the expression of dAK/dCK in the E. coli host; its kinetic properties are indistinguishable from those of native dAK/dCK. Subcloning the dgk or engineered dck gene resulted in expression of active dGK or dCK homodimers, each with a virtually unchanged Km toward its primary deoxynucleoside. However, in common with human dCK, dCK (or dGK) homodimer exhibits secondary activities with much larger Kms towards dAdo and dGuo (or dCyd). dCTP (or dGTP) is the best inhibitor of all three activities of the respective homodimer. Fully active heterodimers can be reconstituted simply by mixing a homodimer with independently expressed (inactive) dAK.

Ligands for the affinity chromatography of mammalian thymidine kinase. 1: Strategy, synthesis and evaluation

Selected thymidine derivatives were synthesized with various spacers and fixed as model compounds at position N-3', C-5, C-3' and C-5', respectively, to simulate the preparation of an affinity gel matrix. Compounds 3, 6, 7 and 9 were evaluated for their effect on pure human cytosolic thymidine kinase (TK). All four compounds showed competitive inhibition with respect to thymidine, with Ki-values between 80 and 1000 microM. In the same positions as the model compounds were bound to the spacers thymidine derivatives were coupled with different Sepharose gel matrices. These affinity matrices were tested for isolation of thymidine kinase out of placental enzyme material. Except for one matrix, more than 98% of the applied activity was retained by the affinity matrices tested. The strongest binding to the enzyme resulted from a fixation at C-5' of the thymidine molecule to the gel matrix.

Independent characterization of thymidine transport and subsequent metabolism in Hymenolepis diminuta--II. Purification and preliminary analysis of thymidine kinase

1. An affinity column for the purification of thymidine kinase (TK) from the cestode Hymenolepis diminuta is described. Using an epoxy-activated Sepharose 6B affinity column containing thymidine as a ligand, a 698-fold purification of thymidine kinase was obtained. 2. Thymidine kinase eluted from this affinity column was partially characterized as having an apparent Km value of 3.94 microM thymidine. This value is very similar to those observed in mammalian systems. 3. Thymidine kinase appears to be an extremely active and ubiquitous enzyme, whose primary function is to rapidly phosphorylate incoming thymidine and thus "trap" it for the cell's use, reducing efflux to a minimum. 4. The apparent Km for TK is two orders of magnitude lower than the Kt for thymidine transport. Thus, theories postulating that long-term (2 min) uptake kinetics for thymidine actually represent subsequent metabolism must look further along the thymidine phosphorylating pathway, beyond TK and its very active role.

A rapid two-step purification of rat liver fetal thymidine kinase

The fetal isoenzyme of thymidine kinase was purified to apparent homogeneity from cytosols of rat fetuses liver. A two-step purification including anion exchange chromatography and affinity chromatography was developed. The purified enzyme appears as oligomeric with a relative molecular weight of 71 kDa. In denaturing media its molecular weight was 24 kDa, and its pHi 8.3.

Regulation of mitosis onset and thymidine kinase activity during the cell cycle of Physarum polycephalum plasmodia: effect of fluorodeoxyuridine

The effects of fluorodeoxyuridine were investigated during three events of the cell cycle: S-phase, mitosis, and the cyclic synthesis of thymidine kinase in the synchronous plasmodium of the myxomycete Physarum. DNA synthesis was inhibited, and there was limited action on other macromolecular syntheses. When DNA synthesis was slowed down, onset of the following increase of thymidine kinase synthesis occurred at approximately the same time as in the control, but mitosis was blocked in a very early prophase stage and metaphase was never observed. These effects were suppressed when the action of fluorodeoxyuridine was prevented by the addition of thymidine to the medium. In agreement with the action of aphidicolin and hydroxyurea, these observations show that: 1) perturbation of the S-phase does not prevent the nuclei from entering a very early prophase stage, but it does prevent them from proceeding through metaphase; 2) blockage of DNA synthesis does not perturb the normal timing of the triggering of thymidine kinase synthesis; and 3) the signal that triggers the arrest of thymidine kinase synthesis is postmitotic and does not require extensive DNA synthesis. In contrast with hydroxyurea and aphidicolin, in the presence of fluorodeoxyuridine metaphase was not observed. Thus, the triggering of thymidine kinase synthesis is unambiguously dissociated from metaphase and postmitotic events. Because synthesis of thymidine kinase remains under the control of temperature shifts from 22 to 32 degrees C, a simple model of the cell cycle involving two regulatory pathways could account for the triggering of thymidine kinase synthesis, early prophase stage, and metaphase.

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.