CHARACTERIZATION OF A PHOSPHODIESTERASE FROM LAMB BRAIN

Single-strand-specific nucleases

Single-strand-specific nucleases are multifunctional enzymes and widespread in distribution. Their ability to act selectively on single-stranded nucleic acids and single-stranded regions in double-stranded nucleic acids has led to their extensive application as probes for the structural determination of nucleic acids. Intracellularly, they have been implicated in recombination, repair and replication, whereas extracellular enzymes have a role in nutrition. Although more than 30 single-strand-specific nucleases from various sources have been isolated till now, only a few enzymes (S1 nuclease from Aspergillus oryzae, P1 nuclease from Penicillium citrinum and nucleases from Alteromonas espejiana, Neurospora crassa, Ustilago maydis and mung bean) have been characterized to a significant extent. Recently, some of these enzymes have been cloned, their crystal structures solved and their interactions with different substrates have been established. The detection, purification, characteristics, structure-function correlations, biological role and applications of single-strand-specific nucleases are reviewed.

IMMUNOCHEMICAL IDENTIFICATION OF PARATHYROID HORMONE IN NON-PARATHYROID NEOPLASMS ASSOCIATED WITH HYPERCALCEMIA

1. Immunochemical cross-reactivity, but not identity, has been demonstrated between bovine parathyroid hormone and an antigen in biologically active extracts of human parathyroid tissue by quantitative C' fixation and C' fixation inhibition. 2. An antigen that fixes C' with rabbit antibody to bovine parathyroid hormone has been found in urea extracts of six human non-parathyroid neoplasms associated with a hypercalcemic syndrome mimicking primary hyperparathyroidism. Comparable extracts of control tissues and other tumors were serologically negative. 3. It is concluded that the tumor antigen is parathyroid hormone or a very closely related protein, and that its production by these neoplasms was the cause of the hypercalcemic syndrome in these six patients.

Genomic damage and its repair in young and aging brain

A brief review of the available information concerning age-related genomic (DNA) damage and its repair, with special reference to brain tissue, is presented. The usefulness of examining the validity of DNA-damage and repair hypothesis of aging in a postmitotic cell like neuron is emphasized. The limited number of reports that exist on brain seem to overwhelmingly support the accumulation of DNA damage with age. However, results regarding the age-dependent decline in DNA-repair capacity are conflicting and divided. The possible reasons for these discrepancies are discussed in light of the gathering evidence, including some human genetic disorders, to indicate how complex is the DNA-repair system in higher animals. It is suggested that assessment of repair potential of neurons with respect to a specific damage in a specific gene might yield more definitive answers about the DNA-repair process and its role in aging.

DNA repair mechanisms in neurological diseases: facts and hypotheses

DNA repair mechanisms usually consist of a complex network of enzymatic reactions catalyzed by a large family of mutually interacting gene products. Thus deficiency, alteration or low levels of a single enzyme and/or of auxiliary proteins might impair a repair process. There are several indications suggesting that some enzymes involved both in DNA replication and repair are less abundant if not completely absent in stationary and non replicating cells. Postmitotic brain cell does not replicate its genome and has lower levels of several DNA repair enzymes. This could impair the DNA repair capacity and render the nervous system prone to the accumulation of DNA lesions. Some human diseases clearly characterized by a DNA repair deficiency, such as xeroderma pigmentosum, ataxia-telangiectasia and Cockayne syndrome, show neurodegeneration as one of the main clinical and pathological features. On the other hand there is evidence that some diseases characterized by primary neuronal degeneration (such as amyotrophic lateral sclerosis and Alzheimer disease) may have alterations in the DNA repair systems as well. DNA repair thus appears important to maintain the functional integrity of the nervous system and an accumulation of DNA damages in neurons as a result of impaired DNA repair mechanisms may lead to neuronal degenerations.

DNA damage and repair in brain: relationship to aging

The usefulness of conducting DNA damage and repair studies in a postmitotic tissue like brain is emphasized. We review studies that use brain as a tissue to test the validity of the DNA damage and repair hypothesis of aging. As far as the accumulation of age dependent DNA damage is concerned, the data appear to overwhelmingly support the hypothesis. However, attempts to demonstrate a decline in DNA repair capacity as a function of age are conflicting and equally divided. Possible reasons for this discrepancy are discussed. It is suggested that assessment of the repair capacity of neurons with respect to a specific type of damage in a specific gene might yield more definitive answers regarding the role of DNA repair potential in the aging process and as a longevity assurance system.

Binding of monoclonal anti-native DNA autoantibodies to DNA of varying size and conformation

A microchemical assay for phosphorus was applied to the measurement of DNA in immune complexes formed with monoclonal or serum anti-DNA autoantibodies and DNA of varying size and conformation. Two monoclonal antibodies were produced by hybridomas derived from spleen cells of autoimmune MRL-lpr/lpr mice and were purified from culture fluid by affinity chromatography on columns of goat anti-mouse Ig-Sepharose. Double-helical DNA fragments were prepared by brief digestion of calf thymus DNA with micrococcal and S1 nucleases and fractionation on Sepharose 4B; their double-stranded structures was confirmed by measurement of thermal denaturation. Immune complexes were formed with monoclonal or serum antibodies and native DNA or DNA fragments or denatured DNA; the complexes were precipitated with goat anti-mouse IgG and washed, and DNA phosphorus content of the precipitates was measured. With one monoclonal autoantibody (H241), there were discontinuous increases in the amount of DNA that could be bound (and decreases in the antigen concn required for half-maximal binding) as the DNA size increased. There were especially marked increases in binding efficiency as fragment size increased from an average of 100 (range 85-105) to an average of 150 (range 105-170) base pairs, and again between 450 (range 360-620) and 600 (range 425-825) base pairs. A second monoclonal antibody (H143) did not show significant variation in binding with DNA fragments larger than 300 base pairs. With smaller fragments, the amount of DNA bound by H143 was reduced, but the DNA concn required for half-maximal binding was not. Affinities of these monoclonal antibodies were within the spectrum of human systemic lupus erythematosus serum IgG anti-DNA autoantibodies. The dependence of binding on mol. wt is important in the evaluation of these monoclonal antibodies as biochemical reagents and as potential participants in formation of immune complexes in vivo.

Enzymology of DNA replication and repair in the brain

A number of enzymes thought to be involved in DNA replication have been identified in the brain. These include single-stranded DNA-binding proteins, topoisomerases I and II, DNA polymerase alpha, a protein that binds Ap4A and might be classified as a DNA polymerase alpha accessory protein, RNase H, DNA polymerase beta, DNA ligase, an endo- and an exonuclease of unknown function, DNA methyl transferase and poly(ADPR) synthase. In contrast, little is known about the enzymology of DNA repair in brain. The few enzymes identified comprise uracil-DNA glycosylase, DNA polymerase beta, DNA polymerase alpha (which in neurons is present only at immature stages), DNA ligase, poly(ADPR) synthase, and O6-alkylguanine-DNA alkyltransferase. In addition, an exonuclease acting on depurinated single-stranded DNA (tentatively listed here as 3'----5' exonuclease), an endonuclease of unknown function as well as ill-defined acid and alkaline deoxyribonucleases also occur in brain.

Exodeoxyribonuclease from rat brain specific for single-stranded DNA

An exodeoxynuclease with single-stranded DNA specificity has been isolated from rat brain and purified to electrophoretic homogeneity. Approximately 1100-fold purification with a yield of 16% has been achieved by chromatography on DNA-agarose, hydroxyapatite and Sephadex G-200. The enzyme has a pH optimum at 8.4, requires Mg2+ or Mn2+ as a cofactor and has a molecular weight of about 60000. It hydrolyzes homologous, heterologous, synthetic and depurinated substrates at the same rate liberating nucleoside 5'-monophosphates but does not attack ultraviolet-irradiated polydeoxyribonucleotides. This DNase is localized predominantly in neuronal cell nuclei and appears to be lacking in rat liver tissue.

Studies on sheep kidney nuclease. I. An improved purification method and some properties

An improved purification method of the sheep kidney nuclease (nuclease SK) specific for single-strans nucleic acid, which includes extraction with 0.85% NaCl, treatment with DEAE-cellulose, fractionation with polyethylene glycol, phospho-cellulose chromatography, CM-Sephadex chromatography and phospho-cellulose rechromatography is described. The nuclease was purified approx. 390-fold. Identity was established by comparison with known properties. Molecular weight was estimated to be 52 000-53 000 by gel filtration on Sephadex G-100. The properties of the purified enzyme agreed well those reported previously. The purified enzyme hydrolyzed heat-denatured calf thymus DNA, yeast RNA and no hydrolytic activity for native calf thymus DNA, A2'-pA, A3'-pA, ADP, ATP, 5'-AMP and cyclic AMP.

In vitro effect of antibodies to DNA on the template activity of DNA

Blood samples containing antibodies to DNA were obtained from patients with systemic lupus erythematosus (SLE) and rabbits immunized with denatured DNA complexed to methylated bovine serum albumin. The immunoglobulin fractions from these sources did not decrease the over-all template activity of singlestranded DNA with DNA polymerase or DNA-dependent RNA polymerase. In competition studies, both DNA polymerase and DNA-dependent RNA polymerase inhibited the binding of DNA antibodies to single-stranded DNA, as evidenced by inhibition of micro-complement fixation. These findings suggest that antibodies to DNA fail to decrease denatured DNA template activity because the enzymes which use a single-stranded DNA template can displace or block the antibodies from the denatured DNA as a result of greater binding affinity to the denatured DNA. The anti-DNA antibodies associated with SLE, therefore, may not be involved in the pathogenesis of the intracellular abnormalities associated with the disease.

Properties of a purified rat-liver nuclease

1. The pH optimum, ionic requirement and heat-stability of a purified liver nuclease have been examined with RNA and denatured DNA as substrates. 2. The enzyme attacked DNA and RNA in an endonucleolytic manner, forming oligonucleotides terminated by 5'-phosphate groups. No clear specificity was found with respect to the bases at the site of cleavage. 3. Comparison of the results obtained with RNA and denatured DNA as substrates suggests that the ribonuclease and deoxyribonuclease activities are associated with the same protein.

The purification from rat liver of a nuclease hydrolysing ribonucleic acid and deoxyribonucleic acid

1. The purification of a nuclease from rat-liver mitochondria is described. The mitochondria are rendered soluble by treatment with Triton X-100 and, after fractionation with ammonium sulphate and acetone, the active fraction is further purified by chromatography on DEAE-cellulose and Sephadex G-75 to give a purification of over 700-fold. 2. The purified enzyme was only very slightly contaminated with deoxyribonuclease II, phosphodiesterase and phosphomonoesterase. The individual activities of these enzymes did not exceed 0.1% of the activity of the liver nuclease. 3. The purified enzyme attacked RNA more rapidly than denatured DNA and hydrolysed native DNA more slowly than denatured DNA. 4. There is some evidence to suggest that the nucleolytic activity of the purified preparation towards native DNA, denatured DNA and RNA is associated with a single protein. 5. The enzyme is relatively labile but is stabilized in the presence of 20% (w/v) glycerol or 10mm-2-mercaptoethanol.