Purification and properties of DNA polymerase-beta from guinea pig liver

Chemical, Physical, and Genetic Factors Interfering with DNA Repair-a Review

Because of its function as transmitter of genetic information, DNA is the most important macromolecule in need of protection from attack by chemical and physical agents, but mechanisms have evolved for repairing such damage to DNA. The presence of the adaptive response and other cellular repair systems (excision, post-replication, SOS, etc.) diminishes the toxicologic effects of low doses of toxic or muta-genic substances. Whether or not these genotoxic effects can be reduced to undetectable levels is not certain. Nonetheless, this repair-mediated diminution of damage due to chemicals constitutes one of the arguments in favor of existence of “safe” threshold levels of chemical exposure (Schendel, 1981).

In turn, the repair process itself may be affected by chemical and physical agents. To determine the mode of action of a specific compound on the process of DNA repair becomes complex when all factors are taken into consideration. There are agents which interfere with DNA repair but they are also as active or more active in suppressing replicative DNA synthesis, as well as RNA and protein synthesis. The interference with repair may arise from other major processes such as alteration of energy metabolism and effects on precursor pathways and/or enzymatic cofactors. Whether or not an agent can specifically inhibit DNA repair enzymes has not been answered. The point must be made, however, that this type of interference with essential protective mechanisms is taking place and it may change anticipated outcomes of chemical or physical exposures. The magnitude of this effect due to the exposure of people to so many chemicals should be recognized and studied for their degree of interference with all the processes of DNA repair.

Identification of a beta-like DNA polymerase activity in bovine heart mitochondria

A new DNA polymerase activity, distinct from DNA polymerase gamma, has been identified in bovine heart mitochondria. First detected among proteins isolated in a complex with mitochondrial DNA, the DNA polymerase activity has been partially purified 47,000-fold. Enzyme activity separates from DNA polymerase gamma on several chromatographic columns and appears to copurify with a 38 +/- 2-kDa polypeptide. Unlike DNA polymerase gamma, this enzyme is relatively resistant to inhibition by N-ethylmaleimide and dideoxynucleotides, has moderately low monovalent and high divalent cation requirements, and possesses 20-fold-higher apparent K(m) values for deoxynucleotides. The enzyme polymerizes deoxynucleotides onto a primed template DNA in a relatively nonprocessive fashion and lacks a detectable 3' to 5' exonuclease activity. Many of these characteristics resemble a beta-like mitochondrial DNA polymerase previously identified in, and considered unique to, trypanosomes. We propose that the bovine and trypanosomal enzymes are related and represent a new class of ubiquitous mitochondrial DNA polymerases.

Opening the blood-brain barrier to zinc

Sheep with guide tubes implanted over the brain lateral ventricles, in order to facilitate episodic sampling of cerebrospinal fluid (CSF), were used to determine the effects of increasing cranial blood osmolality or electroconvulsive shock (ECS) on the permeability of the blood-brain barrier (BBB) to zinc. Zinc acetate solution (1 mg Zn/ml) was infused intravenously (i.v.) at 1.0 ml/min for 30 min and then continuously at 0.125 ml/min. This infusion increased plasma total zinc concentration (pZn) approximately 10-fold without altering CSF zinc concentration (CSFZn). After 1.5-3.5 h, 4 M NaCl was infused at 5-10 ml/min for 10 min into one carotid artery with the other carotid artery occluded, or the animals were anaesthetized and given an ECS (140 V, 2 s). Paired samples of blood and CSF were collected before and after these treatments. Results were: (i) CSFZn was approximately one tenth of pZn; (ii) zinc administered i.v. was almost completely excluded from the CSF; (iii) increased cranial blood osmolality or ECS increased CSFZn in all experiments, but the time course and extent of the rise were variable. CSFZn reached the concentrations of zinc in normal sheep plasma in some experiments; (iv) CSFZn subsequently fell towards the low values of zinc in normal CSF; (v) the animals suffered no evident ill-effects from either procedure. The procedures may, therefore, be used for reversible opening of the BBB to particles such as zinc in conscious or anaesthetized sheep with no troublesome sequelae.

Non-disruptive detection of DNA polymerases in nondenaturing polyacrylamide gels

A non-disruptive method is described with which DNA polymerases can be detected in homogeneous preparations and unfractionated cell extracts after electrophoresis in nondenaturing polyacrylamide gradient gels. The technique involves diffusion of DNA polymerase activity into an overlay assay agarose gel, the synthesis of radioactive DNA, removal of excess substrates and autoradiography. Cell extracts from a variety of organisms were studied using this method. The activity from Escherichia coli crude extracts migrated in a position corresponding to a higher molecular mass than did purified preparations of DNA polymerase I. DNA polymerases of higher organisms generally migrated in positions corresponding to 400--900 kDa, in some cases, close to 200 kDA.

Comparative biochemistry of the guinea-pig: a partial checklist

A great deal is known about guinea-pig biochemistry, but the information is scattered and difficult to assemble. The guinea-pig also possesses a number of unusual biochemical features which add to its interest. For these reasons we have compiled a list of biochemical characteristics of the guinea-pig, organized in a series of tables, with brief discussions of some of the entries.

Identification of mammalian DNA repair factors using a reconstituted subcellular system. Partial characterization and subcellular location of a DNA repair-stimulating protein in hamster cells

By reconstituting lysolecithin-permeabilized hamster cells with endogenous proteins, a protein(s) which stimulated bleomycin-induced DNA repair synthesis was identified. The repair protein was inactivated by proteinase K and had an apparent molecular weight of 12 000-15 000 D. The following enzymatic activities were not detected in the partially purified DNA repair protein: general endonuclease, apurinic endonuclease, exonuclease, DNA polymerase or DNA polymerase beta-stimulating activity. The subcellular location of the DNA repair-stimulating activity was investigated by cytochalasin B enucleation; approx. 80% of the activity was associated with karyoplasts, suggesting a nuclear location. Neither the activity nor subcellular location of the repair protein fluctuated appreciably during the cell cycle, consistent with a physiological role in DNA repair. Although the function of the DNA repair protein is not yet known, this approach should be useful in identifying and characterizing mammalian DNA repair proteins.

Hepatitis-B virus-associated deoxyribonucleic acid polymerase: a partial characterization by the use of chemical agents

The very limited coding capacity of the HBV-DNA led us to study the nature of the HBV (Dane particle) -- associated DNA polymerase. The HBV-associated DNA polymerase met in many respects the characteristics of the repair enzyme of the host: the DNA polymerase beta. It operates under high salt conditions, and exhibits similar salt effects with NaCl, KCL, and PO4(3-). It is insensitive to sulfhydryl group blockers, such as p-hydroxymercuribenzoate and N-ethylmaleimide, is resistant to phosphonoacetic acid, and not inhibited by 5 mol/l urea. It requires a divalent cation (Mg2+) for activity, the Mg2+ concentration revealing optimal activity is somewhat higher than that described for most DNA polymerases beta. The HBV-associated DNA polymerase differs also from most DNA polymerase beta in its sensitivity ot ddTTP and its optimal pH. The fact that DNA polymerase beta of different origin vary considerably in their response to chemical agents and that the DNA polymerase beta from human liver has not been studied allows no definite conclusion as to the nature of the Dane particle-associated DNA polymerase.

Rat liver DNA polymerase-beta: thermal inactivation of RNA- and DNA-dependent activities

Purified DNA polymerase-beta from rat liver was exposed to thermal inactivation and the remaining activities were then measured either with a hybrid template such as poly(A).(dT)12-18 (R-activity) or with a DNA template such as poly(dA).(dT)12-18 (D-activity). Time course of inhibition of R- and D-activities were identical. Neither activity was protected when the thermal treatment was performed in the presence of the template or dNTPs.

Distinction between mouse DNA polymerases alpha and beta by tryptic peptide mapping

Results presented here and in a previous paper (Tanabe et al. (1979) Biochemistry 18, 3401--3406) indicate that mouse beta-polymerase is a single polypeptide with an apparent molecular weight of 40,000. This polypeptide has now been analyzed by tryptic peptide mapping. Comparison of the results with identical analysis of mouse alpha-polymerase reveals that the tryptic peptides derived from the two enzymes are different. These results indicate that beta-polymerase is neither a subunit of alpha-polymerase nor a proteolytic degradation product of alpha-polymerase.

DNA polymerase beta from brain neurons is a repair enzyme

DNA polymerase beta was isolated from rat cortex neurons and characterised. Its properties were strikingly similar to those of other mammalian beta-polymerases. In adult rats, this was the major DNA polymerase occurring in neuronal nuclei, which contained no alpha-polymerase, 99.2% beta-polymerase and only 0.8% gamma-polymerase. Isolated neuronal nuclei of this developmental stage were shown to perform ultraviolet-induced repair DNA synthesis in vitro. Since beta-polymerase was virtually the exclusive DNA polymerase in these nuclei it was concluded that the beta enzyme was responsible for the observed DNA repair. This was further substantiated by demonstrating a virtually complete suppression of DNA repair in irradiated nuclei by 2',3'-dideoxyribosylthymine 5'-triphosphate (d2TTP), a potent beta-polymerase inhibitor. However, the presence of minute amounts of gamma-polymerase in neuronal nuclei and its susceptibility to d2TTP did not allow one to rule out an ancillary role of DNA polymerase gamma in DNA repair. In view of the similarity of the neuronal DNA polymerase beta with all other mammalian beta-polymerases it may be speculated that the ability to perform repair DNA synthesis is not unique to the neuronal enzyme but is a general function of all beta-polymerases.