POLLUTANTS DANGEROUS FOR ECOSYSTEMS

Актуальные публикации по вопросам экологии и природопользования.

NEW ЭКОЛОГИЯ


ЭКОЛОГИЯ: новые материалы (2024)

Меню для авторов

ЭКОЛОГИЯ: экспорт материалов
Скачать бесплатно! Научная работа на тему POLLUTANTS DANGEROUS FOR ECOSYSTEMS. Аудитория: ученые, педагоги, деятели науки, работники образования, студенты (18-50). Minsk, Belarus. Research paper. Agreement.

Полезные ссылки

BIBLIOTEKA.BY Беларусь - аэрофотосъемка HIT.BY! Звёздная жизнь


Автор(ы):
Публикатор:

Опубликовано в библиотеке: 2021-08-30
Источник: Science in Russia, №3, 2010, C.22-27

by Sergei KOTELEVTSEV, Dr. Sc. (Biol.), leading research fellow, Laboratory of Biomembrane Physical Chemistry, Biological Faculty of Lomonosov Moscow State University

 

The development of world industry and agriculture is inevitably fraught with side affects for biosphere. Despite protective measures, thousands of chemical compounds, heretofore unknown to animals and plants, are released into the environment. These are the so-called xenobiotics- substances alien for natural organisms, not involved in plastic and energy metabolism in their cells. Then why is their accumulation dangerous?

 
стр. 22

 

There are virtually no ecosystems in the world that do not contain pollutants, including the most dan-gerous. Today it is a usual thing to detect pesticides intended for plant pest control in fatty tissues of whales or to find polychlorinated biphenyls (used in industry and highly toxic even in low concentrations) in breast milk of women living on the islands in the Pacific-thou-sands of kilometers away from the regions of their use. How to detect such compounds and predict their after-effects on living organisms?

 

It goes without saying that this cannot be done with-out chemico-analytical methods: they help not only elu-cidate the "name" of the toxicant and its concentration, but also reveal the source of its penetration into the environment and hence serve as a basis of an ecological and toxicological analysis. Other sciences-genetics, molecular biology-are also indispensable to solve the problem. All these approaches have been actively devel-oped at the leading research centers of the world, including Lomonosov Moscow State University.

 

Together with colleagues from the Biological Faculty of the University, I have been working for 25 years at the Black and Baltic Seas, Lake Baikal, the Volga river, in the Altai Mountains, in the disaster area at Chernobyl, in Norway, Finland, and USA. The effects of ecotoxi-cants on biological objects of different levels-from bac-teria and unicellular algae to higher plants, fish, birds, mammals, and humans-were evaluated by biotesting and bioindication methods (we shall speak about them later). Samples of water, bottom deposits, plant and animal tissues were collected and the activity of enzymes detoxifying xenobiotics were determined at our Laboratory of Biomembrane Physical Chemistry. At the same time, concentrations of superdangerous pollutants were evaluated in the samples at the Laboratory headed by Albert Lebedev, Dr. Sc. (Chem.), Chemical Faculty of the Moscow State University. The concentrations of mutagens* and carcinogens in the samples were mea-sured at the Department of Genetics of the Biological Faculty in collaboration with Vadim Glazer, Cand. Sc. (Biol.). This is our next topic.

 

INCONSPICUOUS BUT PERFIDIOUS

 

Mutagens are most destructive for ecosystems, as their effect can manifest itself in subsequent genera-tions. It is proven that 90 percent of them are car-cinogenic, cause malignant tumors in animals and hu-mans. Such effects are intrinsic to some heavy metals (for example, nickel and chromium) and numerous organic compounds (polycyclic aromatic carbohy-drates, polychlorinated biphenyls, chlorophenols, some amines, etc.).

 

However, mutagenic compounds can have no acute toxicity, but, accumulating in the body, they exhibit pro-longed effects with superdangerous consequences. This group includes chemicals, directly reacting with DNA, and other ones, requiring preliminary metabolic changes or activation in the cell for revealing genotoxic-

 

 

* Mutagens are physical and chemical factors causing hereditary changes (mutations).-Ed.

 
стр. 23

 

 

Accumulation of toxicants along alimentary routes and induction of detoxication enzymes.

 

ity. The former include epoxy compounds, aromatic N-oxides, nitrosamides, etc., while the latter are aromatic carbohydrates, aromatic amines, and nitrogen-contain-ing compounds. Mutagenic compounds often form dur-ing oil refining.

 

One of the characteristic features of mutagens and carcinogens is a capacity to produce a biological effect even in very low concentrations. This fact hampers their analysis in tissues. On the other hand, it is impossible to identify by chemical methods what kind of substances exhibit carcinogenic and mutagenic effects (though recent studies headed by Serikbai Abilev, Dr. Sc. (Biol.), at Vavilov Institute of General Genetics in Moscow, are carried out to develop mathematical methods, which will make it possible to predict such potential effects by the structure of the compound). The final answer can give only biological test systems, which acquire greater and greater importance and are used more and more often. However, if Ames' test (we shall deal with it below) is used in hundreds of laboratories in the USA, in Russia we have less than five such labs.

 

Like any xenobiotic, a mutagen undergoes many transformations in the organism. They can be tentative-ly divided into several stages. In the cell cytoplasm it faces many enzymatic systems, which can destroy its molecule, as a result of which it will lose its dangerous characteristics. However, there often arises a situation, when metabolites of the given xenobiotic are more mutagenic than the initial substance. This process is called metabolic activation (see details below).

 

Interactions between cell chromosomes and the muta-gen result in potential changes in the chromosomes- they are either realized in actual mutations or are "cured" by enzymatic systems, which look after the con-stancy of DNA structure and "repair" any of its disor-ders, emerging spontaneously or under the effect of external factors.

 

THE DETOXICATION SYSTEM OF XENOBIOTICS

 

Living organisms developed biochemical mechanisms against destructive effects of xenobiotics (including mutagens and carcinogens) in the course of evolution. Alien substances undergo biotransformations in mem-brane structures of the cellular endoplasmatic reticu-lum*. The monoxygenase enzymes stimulate the oxygen molecule, then one of its atoms is introduced into the oxidized substance molecule, while the other is used for the formation of a water molecule. The most important of these enzymes is cytochrome P-450. More than a hundred of its isoforms are known today. Let us note that Academician Alexander Archakov**, director of Orekhovich Institute of Biomedical Chemistry, Russian Academy of Medical Sciences (Moscow), made a great contribution to the studies of monoxygenases.

 

 

* Endoplasmatic reticulum is a system of tubules and vesicles, limited by membranes, in the cytoplasm. It is involved in metabolic processes, pro-viding the transport of substances from the environment to the cyto-plasm and between intracellular structures.-Ed.

** See: A. Archakov, "Time of Proteomics", Science in Russia, No. 3, 2009.-Ed.

 
стр. 24

 

 

Percentage of samples, selected on the coastline of Hurnoe Island, exhibiting the mutagenic effect: 1-algae, polychetes; 2-mollusks, crustaceans; 3-cod muscles; 4-seagull muscles; 5-seagull eggs; 6-tissues of seagull fledglings.

 

The endoplasmatic reticulum membranes, containing cytochrome P-450, are rather easily isolated from tissue homogenates by differential centrifugation. The resul-tant preparation is called the microsomal fraction, and it is in this fraction that the concentrations of the enzyme's isoforms are measured by spectral and immunological methods. The results of such analysis indirectly indicate the extent of toxins' penetration: the higher the detected concentration of cytochrome, the more active was the monoxygenase system, neutralizing xenobiotic effects.

 

However, in addition to detoxication, monoxygenase oxidation involves the metabolic activation of muta-gens. About ten isomers, part of which acquire muta-genic properties, are formed from benz(a)pyrene (this substance cannot bind up with DNA independently) in the cytochrome P-450 system after the interaction with enzymes in the cells.

 

One of the main characteristics of the system under consideration, particularly well-developed in liver cells, is its remarkable polyspecificity, in other words, capaci-ty to rapidly react to penetration of any new enemy into the organism. Despite the efforts of modern industry manufacturing thousands of compounds, today we practically do not know hydrophobic chemicals, whose presence in the endoplasmatic reticulum membranes would not lead to their metabolic transformation and a many-fold increase of the activity of monoxygenases processing these xenobiotics.

 

Induction (intense stimulation of the synthesis of spe-cific isoforms of cytochrome P-450) starts with the xenobiotic reaction with specific receptors on the plas-matic membrane. Then the signal is transferred to the nucleus and special genes, responsible for the synthesis of the respective enzyme, are triggered. As this process is genetically controlled, the isoforms differ from each other by an amino acid sequence. Some of them are formed irrespective of the conditions the cell is exposed to, others are synthesized in trace amounts and reach a certain level in the presence of endogenous (forming in the cell) or exogenous (getting from the outside) induc-tor substrates.

 

We can assert that the activity of the cytochrome P-450 isoforms in liver cells of present-day humans is sig-nificantly higher than in our grandfathers and grand-mothers, as they did not have to face modern drugs and chemical industry products abundantly released into the environment today.

 

Let us note that the activity of monoxygenase oxida-tion is higher in the tissues of humans and warm-blood-ed animals, lower in fish tissues, and very low in plants. In addition, we can speak about certain species speci-ficity of this process. Thus, the activity of hepatocyte (liver cells) monoxygenases in birds decreases in the fol-lowing series: quail > windhover > dove > heron > sea-gull. The lowest level of cytochrome P-450 (in compari-son with all other fishes) was detected in endemic gob-ies in Lake Baikal. It is more difficult for these organ-isms to control the toxins penetrating from the environ-ment, they are less resistant to their destructive effects, and accumulate them in their tissues. Measurement of cytochrome P-450, specifically, of some of its isoforms, is important for humans: detection of differences in the saturation rate of this enzyme can increase the risk of cancer and other diseases in humans exposed to toxins at work, as the monoxygenase oxidation results in not only detoxication, but also in metabolic activation of mutagens.

 

ACCUMULATION OF TOXICANTS ALONG ALIMENTARY ROUTES

 

Relationships between various animal species, partic-ularly in simple ecosystems, can be adequately described by trophic (alimentary) chains and "energy pyramids" built on this base. For example, in the tundra the lichen serves as food for reindeer, while they, in their turn, fall prey to wolves. Another series of sequences is characteristic of the taiga, for example, plant-hare-lynx. One of the simplest and well studied sequences is known for the ocean: phytoplankton-krill (small crus-taceans)-whale. A different order is characteristic of

 
стр. 25

 

 

Birds of Central Africa, whose tissues contain maximum quantity of xenobiotics.

 

Lake Baikal: phytoplankton-epischura (small crus-taceans)-sand hoppers (Baikal shrimps)-endemic gob-ies (mainly Baikal oil-fish)-whitefish-ringed seal. Any pyramid is, as a rule, crowned by carnivorous birds and mammals.

 

Pollutants get into living organisms with food and through exodermis from water and air. However, Nature has created a protective mechanism for them: virtually all organisms have enzymatic systems, which metabo-lize, bind, and eliminate the xenobiotics from biological tissues. It is remarkable that the higher is the organiza-tion of this or that living object, the more dynamic is detoxication and metabolic activation of pollutants and the more active is production of enzymes binding heavy metals. But liposoluble, chemically stable compounds are accumulated along the alimentary tract, and, as a rule, their concentrations increase from one "floor" of the pyramid to the other: they are much higher in tissues of organisms at its top than in the environment and in biological objects at its base. Moreover, the stable bal-ance of the ecosystem can be disturbed under conditions of excessive toxicant pressure. Besides, at first suffer the structures that can remove xenobiotics from tissues,

 

then physiological functions, reproduction, and eventu-ally death or critical reduction of the population becomes inevitable.

 

BACTERIAL TEST SYSTEMS

 

Studies of the mutagenic effects of environmental pol-lutants on animals is a long and difficult process. That is why tests on bacteria are so widely used. The classical objects of genetics of microorganisms, such as Bacillus subtilis, Escherihia coli, Salmonella typhymurium are most often used for this purpose. The bacteria rapidly multiply, which allows to get several generations within several hours. In addition, modern gene engineering has created a series of sensitive microorganism strains for detection of mutagenic effects of pollutants even in very low con-centrations. Among the advantages of the method is its low price and possibility of recording mutations of differ-ent types. However, these biotests have also some defects. The main of them is that the bacteria have no enzymes realizing the metabolic activation of xenobiotics. It is impossible to detect promutagens in these systems, as bacterial cells have no enzymatic systems transforming promutagens into mutagens.

 
стр. 26

 

Due to the use of analogs of such systems-membrane structures from mammalian or fish liver, containing active cytochrome P-450 isoforms-microsomes*-we managed to overcome the difficulties.

 

Such approach underlies the test developed in 1974 by Bruce Ames, an American scientist. It is widely used even today. Microsomal oxidation, triggered by this method, in fact realizes the same processes which occur in animal liver. Special bacterial strains have been creat-ed for such test systems: they allow to register one or several DNA mutations, manifesting themselves in sub-stitutions of nitrous bases or in the genetic code reading frame shift, in other words, "unplanned" inclusion or, on the contrary, loss of a nucleotide.

 

And now let us speak about the results of some of our studies with the use of these methods in various regions of the world. During studies of the fishes of Lake Baikal in the 1980s, we discovered mutagenic compounds not too often. However, the tissues of the ringed seal and the eggs of birds nesting in the mouth of the Selenga river, falling into the lake, as a rule, contained many muta-gens: the chemical analysis confirmed the presence of considerable amounts of not only polycyclic, but also of polychlorinated carbohydrates, including DDT. It is noteworthy that the content of pollutants in adult ani-mals was higher than in young ones.

 

In the first half of the 1990s, we studied the activity of xenobiotic detoxication enzymatic systems as exempli-fied by breams in the Rybinsk reservoir on the Volga. The fish was caught in various areas, including those contaminated by drainage from the Cherepovets Metallurgical Plant (Vologda Region). The toxicity of water there was so high that the structure of fish liver membranes was impaired, which led to the reduction of the membrane-bound enzyme activity and hence, to limitation of the potentialities of xenobiotic control by living organisms.

 

One more observation-on Hurnoe Island in Northern Norway at the beginning of the 2000s. No genotoxic compounds (with the only exclusion) were detected in specimens of water and algae. No apprecia-ble accumulation of xenobiotics was revealed in the mollusks, though they are usually strong accumulators of ecotoxins-due to their specific lifestyle and metabo-lism. By contrast, tissue extracts from local sand hop-pers exhibited an indirect mutagenic effect. The direct effect was detected in the muscles of capelins, which were caught in the coastal waters of Hurnoe Island (Norway). A direct mutagenic effect (genetic code read-ing frame shift) was fixed in 10 out of 11 specimens of muscle tissues from adult seagulls. However, in six of these cases the metabolic activation reduced or even eliminated the detected effect. Thus, according to the results of our tests, no appreciable accumulation of genotoxins occurs at the initial links of the alimentary chain under consideration, but their concentration increases in the subsequent links of the chain at the expense of their accumulation.

 

Similar studies were carried out in the 1990s on the Saimaa Lake in Central Finland in collaboration with Prof. Pirjo Lindstrom-Seppa and Prof. Osmo Hanninen from the University of Kuopio. Trout (by groups of 10 1-year-old males) was placed into tanks in the immedi-ate vicinity of and at a distance of 500, 1,000, 5,000, and 10,000 m from sewage from the paper and pulp plant. A significant increase of monoxygenase activity in the fish liver was detected not only near, but even at a distance of 5 km from the source of contamination. On the basis of obtained data, we concluded that the water system at such distance from the sewage release place, was exposed to the negative effects of the plant. A scheme of biochemical monitoring has been suggested for the lakes of Finland.

 

A very high concentration of mutagenic compounds was detected in 2007 in tissues of birds and fishes inhab-iting the central part of the Niger river in Africa. This was presumably caused by sewage from the enterprise, manufacturing plastic, near the city of Bamako (capital of the Republic of Mali) and by the use of agricultural pesticides and herbicides. The samples for analysis were sent to our laboratory by Vital Truare, a former student of the University of Peoples' Friendship in Moscow, Cand. Sc. (Biol.), Professor of the University of Bama-ko. The studies were carried out within the framework of UNESCO Program.

 

In conclusion we should like to emphasize that the studies of the mechanisms of metabolism and accumu-lation of pollutants, particularly of those with mutagenic and carcinogenic properties, are essential not only for the environment protection, but also for improving pub-lic health. That is why it is so important to continue these studies in our country too.

 

 

* Microsomes are membranes of the cellular endoplasmatic reticulum, released from pre-homogenized liver. They are closed bubbles several microns in diameter, containing all enzymes of this reticulum.-Ed.


Новые статьи на library.by:
ЭКОЛОГИЯ:
Комментируем публикацию: POLLUTANTS DANGEROUS FOR ECOSYSTEMS

© Sergei KOTELEVTSEV () Источник: Science in Russia, №3, 2010, C.22-27

Искать похожие?

LIBRARY.BY+ЛибмонстрЯндексGoogle
подняться наверх ↑

ПАРТНЁРЫ БИБЛИОТЕКИ рекомендуем!

подняться наверх ↑

ОБРАТНО В РУБРИКУ?

ЭКОЛОГИЯ НА LIBRARY.BY

Уважаемый читатель! Подписывайтесь на LIBRARY.BY в VKновости, VKтрансляция и Одноклассниках, чтобы быстро узнавать о событиях онлайн библиотеки.