Preferential alkylation of mitochondrial deoxyribonucleic acid by N-methyl-N-nitrosourea

A novel glutathione transferase (13-13) isolated from the matrix of rat liver mitochondria having structural similarity to class theta enzymes

A rat liver mitochondrial-matrix fraction was prepared and shown to have 1-chloro-2,4-dinitrobenzene(CDNB)-metabolizing glutathione transferase (GST) activity. Further fractionation by sequential gel filtration, isoelectric focusing or chromatofocusing and hydroxyapatite chromatography yielded three GSTs of pI 9.3, 8.9 and 7.5, none of which bound to a GSH-agarose affinity matrix. Most of the activity was associated with the pI-9.3 form, which was selected for further study. Its activity was tested with the following potential substrates in addition to CDNB: 1,2-dichloro-4-nitrobenzene, p-nitrobenzyl chloride, trans-4-phenylbut-3-en-2-one, 1,2-epoxy-3-(p-nitrophenoxy)propane, ethacrynic acid, menaphthyl sulphate, cumene hydroperoxide, linoleic acid hydroperoxide and 4-hydroxynon-2-enal. Appreciable activity was obtained only with CDNB and ethacrynic acid (82 and 26 mumol/min per mg of protein respectively). The apparent Km for GSH, using 1 mM-CDNB, was 1.9 mM. The enzyme is a dimer of subunit Mr 26,500. It has a free N-terminus, which has enabled the first 33 amino acids to be sequenced. This portion of primary structure has a sequence in common with members of the Theta class of GSTs (eg. 36% identity with subunit 12) and also a sequence which might function as a mitochondrial import signal. It is novel and has been named 'GST 13-13'.

An integrated theory of aging as the result of mitochondrial-DNA mutation in differentiated cells

We maintain that aging of humans and animals derives from a mutation or inactivation (probably followed by endonuclease digestion) of the mitochondrial genome of differentiated cells. This extranuclear somatic mutation hypothesis of aging is based on the finding that mitochondrial DNA (mtDNA) synthesis takes place at the inner mitochondrial membrane near the sites of formation of highly reactive oxygen species and their products, such as lipoperoxides and malonaldehyde. The mtDNA may be unable to counteract the damage inflicted by those by-products of respiration because, in contrast to the nuclear genome, it lacks histone protection and scission repair. Since the mitochondrial genome controls the synthesis of several hydrophobic proteins of the inner mitochondrial membrane, the postulated mutation, inactivation or loss of mtDNA will prevent the replication of the organelles. Thus deprived of the ability to regenerate their mitochondrial populations, the cells will sustain an irreversible decline in their bioenergetic ability, with concomitant senescent loss of physiological performance and eventual death. The above hypothesis is integrated with the concepts of Minot, Pearl and others in order to offer a more comprehensive view of aging.

Mutations of mitochondrial DNA and human death

In the skeletal muscle of patients with mitochondrial myopathies (Kearns-Sayre syndrome and chronic progressive external ophthalmoplegia) and in the heart and skeletal muscle of healthy persons cells lacking cytochrome c oxidase are found. The respiratory-defective cells have the following features in common: onset of the defect at juvenile or adult age; progressive character of the defect with increasing age; and focal pattern of respiratory-deficient cells (fibers). A statistic mutation of mtDNA in affected cells is suggested to cause the defect of mitochondrial function. It is postulated that the continuous accumulation of respiratory-deficient cells, mainly in the human heart with increasing age, will finally limit the life-span of each human individual.

Mitochondrial mutations may increase oxidative stress: implications for carcinogenesis and aging?

The sensitivity of mitochondrial DNA to damage by mutagens predisposes mitochondria to injury on exposure of cells to genotoxins or oxidative stress. Damage to the mitochondrial genome causing mutations or loss of mitochondrial gene products, or to some nuclear genes encoding mitochondrial membrane proteins, may accelerate release of reactive species of oxygen. Such aberrant mitochondria may contribute to cellular aging and promotion of cancer.

Interaction of drugs with extranuclear genetic elements and its consequences

Bacterial ancestry of mitochondria and plastids is now generally accepted. Both organelles contain their own DNA and transcription-translation apparatus of a prokaryotic type. Due to this fact these systems carry bacteria-like properties. Thus organellar DNA and ribosomes are essentially different from nuclear DNA and cytoplasmic ribosomes in physical as well as in functional respects. Due to the bacterial character of both types of organelles they are susceptible to various antibacterial chemicals. Inhibitors of bacterial protein synthesis inhibit mitochondrial (plastidial) biogenesis. Therefore the cellular content of mitochondria (plastids)-made proteins decreases during cytoplasmic turnover or cell division in the presence of these drugs. Such drug activity consequently leads to a reduced capacity for oxidative phosphorylation or photosynthesis. Organellar genomes are less stable and more sensitive to mutagenesis as compared to nuclear genome. It means also that genotoxic agents induce various disorders of mitochondrial (plastidial) functions. Impairments in the respiratory chain are associated with structural as well as functional abnormalities of mitochondria. These are clinically expressed mostly in tissues with a high demand for ATP: brain, heart, skeletal muscle, and retina. On the other hand, some antibacterial inhibitors of mitochondrial biogenesis (e.g., tetracyclines) inhibit selectively tumor cell proliferation. Therefore they may be considered for use in anticancer therapy. The article summarizes the response of mitochondria and plastids in various organisms to drugs and environmental xenobiotics. Various model organisms suitable for detection of xenobiotic effect on mitochondria (plastids) are presented as well as the possible consequences of such interaction.

Do mitochondrial DNA fragments promote cancer and aging?

Reactive oxygen species are important in carcinogenesis, diseases, and aging, probably through oxidative damage of DNA. Our understanding of this relationship at the molecular level is very sketchy. It has recently been found that in mitochondria oxidative DNA damage is particularly high and may not be repaired efficiently. I propose that oxidatively generated DNA fragments escape from mitochondria and become integrated into the nuclear genome. This may transform cells to a cancerous state. Time-dependent nuclear accumulation of mitochondrial DNA fragments may progressively change the nuclear information content and thereby cause aging. This proposal can be tested experimentally.

7-Methylguanine adducts in DNA are normally present at high levels and increase on aging: analysis by HPLC with electrochemical detection

The 7-methylguanine adduct in the DNA of rat liver is determined as an indicator of exposure to exogenous and endogenous methylating agents. A method for the analysis of 7-methylguanine adducts has been developed by combining the selectivity of separation of reversed-phase HPLC with the specificity and high sensitivity of electrochemical detection. The sensitivity of the method is about 10,000-fold that of optical methods and is sufficient to determine the endogenous background of DNA methylation. DNA from the liver of normal young rats (6 months old) contains 7-methylguanine at a level of 1 residue per 31,000 bases in mitochondrial DNA and 1 residue per 105,000 bases in nuclear DNA. These levels increase about 2.5-fold in old rats (24 months old). We attribute this strikingly high level of adducts to endogenous methylation, which could contribute to aging and cancer.

Normal oxidative damage to mitochondrial and nuclear DNA is extensive

Oxidative damage to DNA can be caused by excited oxygen species, which are produced by radiation or are by-products of aerobic metabolism. The oxidized base, 8-hydroxydeoxyguanosine (oh8dG), 1 of approximately 20 known radiation damage products, has been assayed in the DNA of rat liver. oh8dG is present at a level of 1 per 130,000 bases in nuclear DNA and 1 per 8000 bases in mtDNA. Mitochondria treated with various prooxidants have an increased level of oh8dG. The high level of oh8dG in mtDNA may be caused by the immense oxygen metabolism, relatively inefficient DNA repair, and the absence of histones in mitochondria. It may be responsible for the observed high mutation rate of mtDNA.

Interaction of the mutagenic metabolite of sodium azide, synthesized in vitro, with DNA of barley embryos

The in vitro synthesized sodium azide mutagenic metabolite (azidoalanine) produced single-strand breaks and proteinase K-sensitive sites in isolated, germinating barley embryos. In contrast with sodium azide, the efficiency of DNA damage induction was lower, and both types of DNA lesions were totally or partially repaired in the course of subsequent 24 h incubation of the embryos. The mutagenic azide metabolite did not inhibit DNA replication, while azide did so even at doses which are not highly mutagenic. The metabolite labelled with 14C at the amino acid residue was taken up with a similar efficiency both into barley embryos germinating for 2 days and into cells of Salmonella typhimurium TA100. The majority of the radioactivity was incorporated into proteins, less into RNA and a negligible amount into DNA.

Effect of adriamycin on heart mitochondrial DNA

The effect of Adriamycin on rat heart mitochondrial DNA was determined by initially labelling the DNA with [Me-14C]thymidine. Animals subsequently received intravenously either Adriamycin (10 mg/kg) or saline (1 ml/kg) (control group). Animals later received [Me-3H]thymidine and were killed at 6, 14, 24 and 36 h after this. The heart mitochondrial DNA was separated into the linear, non-supercoiled and supercoiled forms by ultracentrifugation on formamide/sucrose sedimentation gradients, and the amount of 14C and 3H incorporated into the various forms of the DNA was determined by scintillation counting. The data suggest that Adriamycin may cause breakage of the mitochondrial DNA helix, and that it slows the rate of mitochondrial DNA synthesis and the formation of complete DNA molecules. These experiments show that Adriamycin does interact with heart mitochondrial DNA, and may explain the cardiac dysfunction association with Adriamycin use.

Nucleic acid chemistry: resolution of supercoiled, intact relaxed, and nicked double-stranded DNAs by formamide-sucrose gradients

A technique which will separate superhelical, intact nonsuperhelical, and damaged mitochondrial DNA molecules by use of sedimentation gradients of formamide-sucrose is described. This technique can be used to detect damage to the mitochondrial DNA helix.

Mitochondrial regulation of cell surface components in relation to carcinogenesis

The recently discovered mitochondrial regulation of certain surface properties of yeast and mouse cells is explained in terms of the intragenomic conflict and interallelic competition that can arise within an organism from the cytoplasmic inheritance of mitochondrial genes. These considerations lend support to the view that an alteration of mitochondrial DNA is one of the steps in chemical carcinogenesis.

A comparative study of guanine N7-alkylation in mice in vivo by the organophosphorus insecticides trichlorphon, dimethoate, phosmet and bromophos

Following intraperitoneal administration to male mice (strain AB Jena/Halle) of 14C-methyl-labelled trichlorphon, dimethoate, phosmet and bromophos, 10-20 Ci/mol, in dosages of 0.06-0.55 mmol/kg, DNA from liver and kidneys was analyzed for 14C in N-7 methylguanine (7-MeG). The extents of methylation were in the range of 5-10 mumol 7-MeG/mol guanine for trichlorphon and dimethoate and of 0.2-0.4 for phosmet and bromophos, for high doses, respectively Excretion half-lives of 7-MeG were differing between trichlorphon (5 hrs, high dose, and 15-17 hrs, low dose) and dimethoate (23-160 hrs, high dose). The extents of methylation at 0-6 of guanine were estimated to be around 0.01 mumol 0-6 MeG/mol guanine for high doses of organophosphates of sufficient water solubility. Factors associated with the partition of organophosphates in mammalian systems are useful for estimating DNA attack by organophosphates in mammals in vivo.

Preferential attack of mitochondrial DNA by aflatoxin B1 during hepatocarcinogenesis

Administration of the hepatic carcinogen aflatoxin B1 to experimental animals results in covalent binding to liver mitochondrial DNA at concentrations three to four times higher than nuclear DNA. The concentration of carcinogen adducts in mitochondrial DNA remains unchanged even after 24 hours, possible because of lack of excision repair. Similarly, mitochondrial transcription and translation remain inhibited up to 24 hours suggesting long-term effects of aflatoxin B1 on the mitochondrial genetic system.

Manifestation of carcinogenesis as a stochastic process on the basis of an altered mitochondrial genome

Computer calculations are used to show the feasibility of a concept which explains the manifestation of a pathological cell function from a latent state by the phenomenon of extrachromosomal inheritance (through the mitochondrial genome) in mammalian cells. A hypothesis is submitted in which this principle is applied to the process of carcinogenesis. According to this concept, the manifestation of a tumor cell--after the initiation stage--entirely depends on stochastic events, i.e., random distribution of mitochondria during cell divisions, with an accumulation of the lesion in a few out of many cells. We feel that this concept comprises a better explanation of many characteristics and peculiarities of the phenomenon of carcinogenesis than do attempts which explain tumor formation as a phenomenon caused by mutation in a nuclear genome. A consideration of the principles presented automatically leads to a number of specific consequences with regard to carcinogenesis. Some of these consequences are discussed. They include: 1. the process of malignant transformation should not be irreversible for all the cells of a progeny; 2. the number of mitochondria in a cell type should be inversely correlated to tumor frequency; 3. the latent period should mainly be determined by the cell division rate and the "extent" of the initiating event; 4. susceptibility to carcinogenesis may be substantially higher if the number of mitochondria per cell line is increasing or decreasing, i.e., during the embryonic and fetal periods; 5. heterogeneous types of cells may arise from a single "initiated" cell, and 6. the process of malignant transformation should not necessarily be confined to one generation of the species. In addition, experimental approaches to support the submitted concept are suggested.

Covalent binding of polycyclic aromatic compounds to mitochondrial and nuclear DNA

Since the pioneering work of the Millers it has become clear that most chemical carcinogens require metabolism to reactive electrophiles and then exhibit their carcinogenic potential by reacting chemically with, and modifying, cellular macromolecules. At first modification of proteins was considered most likely to be of importance in carcinogenesis. Later, Brookes and Lawley demonstrated that the extent of binding of several polycyclic hydrocarbons to DNA, but not to RNA or protein isolated from the skin of mice treated topically with these compounds, correlated with their known carcinogenic potency to this tissue. Mammalian cells, particularly mouse embryo cells, treated with chemical carcinogens have often been used, and DNA has been involved almost exclusively from whole cells. However, mitochondria possess unique DNA which accounts for 0.1-1% of the total DNA present in mammalian cells, and three studies have shown that carcinogenic alkylating agents modify the michondrial DNA by a factor about five times greater than the nuclear DNA from the same cells. We demonstrate here that with six polycyclic aromatic compounds, all of which require metabolic activation and bind to DNA to a much smaller extent than direct than direct-acting alkylating agents, the binding to mitochondrial relative to DNA is dramatically increased by a factor of nearly 50 to over 500.

Mitochondrial DNA is a major cellular target for a dihydrodiol-epoxide derivative of benzo[a]pyrene

When mammalian cell cultures are exposed for 2 hours to (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, a mutagenic and carcinogenic derivative of benzo[a]pyrene, the extent of covalent modificationof mitochondrial DNA is 40 to 90 times greater than that of nuclear DNA. Evidence is presented that this reflects the lipophilic character of the derivative and the very high ratio of lipid to DNA in mitochondria. These results suggest that mitochondrial DNA may be an important cellular target of chemical carcinogens.

Antimitochondrial effects of thioacetamide and ethylenethiourea in human and yeast cell cultures

Cytological studies in the light microscope showed that thioacetamide (TAA) depressed the mitotic index in cultures of skin fibroblasts at the lowest concentrations used (100 μg/ml). At high concentration (1 mg/ml), TAA tended to cause aberration in nuclear morphology. Ethylenethiourea (ETU) had no effect on either mitotic index or nuclear morphology at 1 mg/ml. Fibroblast cultures treated with 1 mg/ml TAA and cultures grown in the presence of 2 mg/ml ETU were studied by electron microscopy. In some TAA-treated cells there was unfolding of the nuclear membrane and enlargement and granulation of the nucleolus, but these effects were not correlated. In all cells, TAA caused severe and characteristic damage to the majority of mitochondria, whether or not there were nuclear aberrations. The organelle showed extensive swelling of the cristae of the inner membrane and an increase in matrix density. Ultrastructure of other cell components appeared to be unaffected by this treatment. In ETU-treated cells some less severe swelling of inner mitochondrial membranes was seen and only in a minority of cells, whilst all other cell structures appeared normal. Similar membrane swelling and increase in matrix density was seen in isolated rat liver mitochondria after incubation with TAA, indicating a direct antimitochondrial effect of the carcinogen.When yeast cells were treated with TAA and ETU, primary antimitochondrial activity of these compounds was apparent from (1) inhibition of growth in non-fermentable medium, (2) selective blockage of mitochondrial protein synthesis and (3) induction of mitochondrial mutations. TAA was much more effective than ETU in all these respects.

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DNA dmaage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin M1, macromolecule-bound aflatoxin B1, diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DNA-binding assay are proposed. Possibilites and limitations in the use of DNA binding in chemical carcinogenesis are discussed extensively.

Toxic and mutagenic effects of carcinogens on the mitochondria of Saccharomyces cerevisiae

Nineteen haploid yeast (Saccharomyces cerevisiae) strains were used to assess the relative growth inhibitory potencies on fermentable vs. non-fermentable media of a collection of carcinogenic and non-carcinogenic chemicals. The majority of carcinogens were distinctly more potent on the non-fermentable (glycerol) medium, where mitochrondrial function is required for growth, than on the fermentable medium, where it is not. The anti-mitochondrial selectivity indicated by these growth tests was much slighter for the non-carcinogens. Similarly most carcinogens induced the cytoplasmic petite mutation whereas the non-carcinogens did not. Five carcinogens which were tested impaired the development of cytochromes aa3 and b in glucose cultures. Six carcinogens, when tested, inhibited growth on three fermentable sugars, the utilisation of which requires mitochondrial function. Out of five carcinogens which were examined, four suppressed the surface-dependent phenomenon of fluocculence in a flocculating strain of yeast, at concentrations primarily affecting the mitochondrial system; the fifth had a similar but less pronounced effect.

Studies on DNA repair in early spermatid stages of male mice after in vivo treatment with methyl-, ethyl-, propyl-, and isopropyl methanesulfonate

In vivo DNA repair occurring in early spermatid stages of the mouse has been studied with four mutagens that are chemical homologs: MMS, EMS, PMS and IMS. Using the well-studied sequence of events that occurs during spermatogenesis and spermiogenesis in the mouse, aatids was measured by the unscheduled incorporation of [3H]dT into these germ cells which were recovered from the caudal epididymides 16 days after chemical treatment. Purification of the caudal sperm DNA at this time verified that the [3H]dT was incorporated into the DNA. For each chemical mutagen a study was made on the level of DNA repair occurring in early spermatids as a function of the administered, in vivo dose. Within experimental errors, all four chemicals produced a linear increase in DNA repair in early spermatids with increasing dose. Only the highest dose of MMS (100 mg/kg) produced a greater repair response than expected for a linear curve. At equimolar doses the most effective chemical in inducing DNA repair was MMS, followed by EMS, IMS and PMS. When testicular injections of [3H]dT were given at the same time as the intraperitoneal injections of the mutagens, the amount of unscheduled incorporation of [3H]dT into the DNA of early spermatids was maximized. Since [3H]dT has been shown to be available for incorporation into germ-cell DNA for only approximately 1 h after injection, all four mutagens must reach the DNA of early spermatids and begin producing "repairable" lesions within 1 h after treatment. The amount of DNA repair occurring at later times after chemical treatment of early spermatids was studied by testicular injections of [3H]dT 1/2, 1, 2 and 3 days after chemical treatment. Repair was still occurring in the early spermatids at 3 days post-treatment; this repair is most likely a manifestation of the finite rate of the repair process rather than resulting from newly alkylated DNA. For MMS and EMS there was a rapid decrease in the level of DNA repair in the first 1/2 day following treatment. This was followed by a much slower, exponential decrease in the level of repair out to 3 days post-treatment. The curves suggest that the amount of repair is proportional to the number of repairable lesions still present in the DNA. For PMS and IMS the level of repair decreases rapidly in the first 1/2 day after treatment and thereafter remains relatively constant through 3 days post-treatment. With all four mutagens, DNA repair in early spermatids was detectable at doses 5 to 10 times lower than those required to observe other genetic end points such as dominant lethals, translocations and specific-locus mutations in any germ-cell stage. The sensitivity of detection of in vivo DNA repair in the germ cells of male mice makes such a system a useful adjunct to other genetic tests for studying chemical mutagenesis in mammals.

Methylation of rat liver mitochondrial deoxyribonucleic acid by chemical carcinogens and associated alterations in physical properties

The formation of 7-methylguanine in rat liver mitochondrial DNA following the administration of the powerful carcinogen, dimethylnitrosamine, and the weak carcinogen, methyl methanesulphonate was measured and compared to the alkylation of nuclear DNA by these agents. At all doses tested mitochondrial DNA was alkylated more extensively than nuclear DNA by dimethylnitrosamine but both types of cellular DNA were alkylated to about the same extent by methyl methanesulphonate. The physical structure of rat liver mitochondrial DNA isolated from animals treated with these agents was investigated by electrophoresis in agarose gels and by isopycnic centrifugation in CsCl gradients. These procedures carried out in the presence of ethidium bromide, an intercalating dye, separate closed circular forms of mitochondrial DNA from open circular molecules (containing a single-strand break) and linear molecules. Administration of dimethylnitrosamine produced a considerable decrease in the amount of mitochondrial DNA which could be isolated in the closed circular form and at higher doses of dimethylnitrosamine no closed circular mitochondrial DNA could be found. Methyl methanesulphonate was less effective at reducing the amount of closed circular mitochondrial DNA. One explantation of these results is that dimethylnitrosamine leads to strand breaks in mitochondrial DNA and the possible use of this system to investigate carcinogen-induced breaks in DNA is discussed.

The induction of phenylalanine ammonia lyase and phaseollin by 9-aminoacridine and other deoxyribonucleic Acid intercalating compounds

Bean pod tissue (Phaseolus vulgaris L. var. Top Crop) is induced to produce phaseollin when challenged with various microorganisms. The pods react in the same manner when challenged with 9-aminoacridine. This compound also caused an increase in concentrations of phenylalanine ammonia lyase, an enzyme of the phaseollin synthesizing pathway. Both the synthesis of phenylalanine ammonia lyase and phaseollin are subject to inhibition by actinomycin D, cycloheximide, or 6-methylpurine. The results suggest that both phaseollin production and increased phenylalanine ammonia lyase, when induced by 9-aminoacridine, require newly synthesized RNA and protein.The concentration of 9-aminoacridine optimal for synthesis of phaseollin and PAL (0.5 mg/ml) does not increase the rate of total protein synthesis. However, there is a differential effect of 9-aminoacridine on synthesis of certain protein fractions. Optimal concentrations of 9-aminoacridine induce phaseollin and phenylalanine ammonia lyase synthesis while reducing the net synthesis of RNA during the period of induction.The planar three-ring structure of 9-aminoacridine appears to be a desirable feature for phaseollin and phenylalanine ammonia lyase induction. Similar compounds, all DNA intercalators, having dimethylamino, diethylamino, amino, or 9-alkylamino substitutions of a three-ring acridine skeleton, are also inducers of phenylalanine ammonia lyase and phaseollin synthesis.It is suggested that 9-aminoacridine and other DNA intercalators function as inducers of phaseollin and phenylalanine ammonia lyase synthesis by reacting with the DNA template.