Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

parts: the catabolism and 
anabolism. 
Anabolism (assimila‑
tion) – formation of 
new substances from 
low molecular weight 
compounds. 
Catabolism (dissimila‑
tion) – degradation of 
organic substances and 
the release of energy

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6. Action of toxic substances in the environment – basic concepts of ecotoxicology 
 101
process the energy is being used. The degradation processes of 
xenobiotics  are  mainly  associated  with  catabolic  processes.  At  the 
same time the degradation of xenobiotics takes place according to the 
same principles as the metabolism of nutrients and other naturally 
consumed substances. The transformations of toxic substances 
can lead to formation of metabolites both with a lower toxicity 
(detoxification) and a higher toxicity (bioactivation).
As a result of the metabolic transformation the substances with 
a wide variety of origins, properties and structure are modified so 
that they can be eliminated or accumulated. The discharge of the 
toxic  substances  from  the  body  affects  the  nature  of  their  effect, 
since, if the excretion takes place quickly, even at the high toxicity of 
substance the hazard of exposure can be significantly reduced. The 
substance excretion speed from the body is described as either the 
elimination half-life time τ1/2. Substance elimination half-life from 
the body is the time at which half of the substance amount absorbed 
by the organism is being excreted from it.
An  interesting  feature  of  the  toxic  substances’  detoxification 
system  is  that,  if  it  is  exposed  to  an  influence  of  a  small  amount 
of toxic substance over a long period, the system can be induced, 
respecti vely  –  its  ability  to  transform  a  particular  substance  may 
increase, thereby lowering the overall hazards of that particular 
toxic substance. However, on the other hand, there is also the 
possibility that if the system is induced to a non-toxic substance, a 
living organism can become more sensitive to the effects of other 
substances. 
As a result of metabolism, the toxicity of xenobiotics and adverse 
effects on a living organism usually decrease, but relatively many 
cases are known when precisely the metabolic transformation of the 
substances that have entered the organism increases their toxicity. 
Often, particularly the biological activity of the primary metabolites 
may be higher than that of the initial substance. Transformation 
of  many  carcinogens  (benzo[a]pyrene,  aflatoxin,  vinyl  chloride) 
resulting from the metabolic processes lead to the formation of 
metabolites  with  a  reactivity,  which  are  able  to  bind  to  DNA.  In 
other words, it is the metabolic processes wherein relatively inert 
molecules turn into their metabolites with a reactivity that cause 
damage to the biomolecules comprising the cells.
The metabolism of toxic substances stipulates that the organism 
or its tissue sensitivity to the effects of the toxic substance decreases 
as a result of exposure to the repeated dose. Such a phenomenon is 
called tolerance. 
The  metabolism  of  the  toxic  substances  significantly  affects 
the nature of the organism’s reaction if exposed to more than one 
substance at the same time.
Depending on the 
nature of the organism’s 
reaction it is possible to 
distinguish between four 
types of exposure:
1.  additive effect, in 
which case the total 
toxic effect is formed 
as the sum of each 
substance’s exposure 
effect («1 + 1 = 2»); 
2.  synergistic effect, 
in which case the 
effect of the toxic 
substances is increased 
(«1 + 1 > 2»);
3.  potentiation, in which 
case a non‑toxic 
substance increases 
the toxic substance’s 
effect («1 + 0 > 1»); 
4.  antagonism, in which 
case the effect of 
the combination of 
substances is less toxic 
than the individual 
effect of each substan‑
ce («1 + 1 < 2»).

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102 
ENVIRONMENT, POLLUTION, DEVELOPMENT:  THE CASE OF UZBEKISTAN
Toxic substances in the body are localized, depending on 
their  characteristics.  Fat-soluble  (lipophilic)  substances  first  of  all 
accumulate in adipose tissue. Examples of such substances are 
poly aro matic hydrocarbons, polychlorinated biphenyls, phenyl-and 
methyl-mercury, DDT. Since these substances usually are also quite 
stable, such depositing is particularly dangerous to a living organism.
An important role in binding the toxic substances is played by 
proteins, the first to mention here is the blood serum albumin, which 
normally binds metabolic products such as fatty acids, bilirubin 
and hormones, ensuring their transportation. If a toxic substance 
is bound to albumin, it loses its ability to form complexes with 
natural metabolites, and this in itself can lead to toxic effects. Of