TOXIC EFFECT OF HEAVY METALS
BY
ZAINAB FAROK JIBRIL
MARCH, 2018
ABSTRACT
Heavy
metal are generally referred to as those metal which posses a specific density
of more 5gkm2 and adversely affect the environment and living organisms.
The most commonly found heavy metal in waste water include arsenic, cadmium,
chromium e.t.c. which risks for human health and environment and they also
enter surrounding by natural means and through human activities. This research was
conducted to investigate the toxic effect of heavy metals. Arsenic is none of
the most important heavy metals causing disquiet from both ecological and
individual health. Lead is a highly toxic metal whose widespread use has caused
extensive environmental contamination and health problems in many parts of the
word. Mercury adversely affects the marine environment. Cadmium is the deventh toxic
heavy metal as per ATSDR ranking. It is a by-product of zinc production which humans
or animals may get exposed to at work or in the environment. Once this metal gets
absorbed by humans, it will accumulate inside the body through life.
INTRODUCTION
Metal are substances
with high electrical conductivity, malleability and luster, which voluntarily lose
their electrons to form cations, metals are found naturally in the earth’s crust
and their composition vary among different localities, resulting in spatial
variations of surrounding concentrations. The metal distribution in the
atmosphere is monitored by the properties of the given metal land various environment
factors (Khlifi and Hamza, 2010). The main objective of this review is to provide
insight into the sources of heavy metals and their harmful effect on the environment
and living organisms. Heavy metals are generally referred density of more than 5gkm3
and adversely affect the environment and living organisms (Jarup, 2003). These metals
are quintessential to maintain various biochemical and physiological functions
in living organisms when in very low concentrations, however they become
noxious when they exceed certain there should concentrations. Although it is acknowledged
that heavy metals have many adverse health effects and last for a long period of
time, heavy metal exposure contimues and is increasing in manu parts of the world.
Heavy metals are significant environmental pollutants and their toxicity is a problem
of increasing significance for ecological, involuntary, nutritional and
environmental reasons (Jaishankar et al., 2013; Nagajyoti et al., 2010). The
most commonly found heavy metals in waste water include arsenic, cadium,
chromium, copper, lead, nickel and zink, all of which cause risks for human
health and the environment (Lambert et al., 2000). Heavy metals enter the surroundings
by natural weathering of the earth’s crush mining, industrial effluents, urban runoff,
sewage discharge, insect or disease control agents applied to crops and many others
(Moraise et al., 2012). Figure 1 show the global production and consumption of
selected toxic metal during 1850-1990 (adapted from Nriagu, 1996).
The global production and consumption
of selected toxic metals during 1850-1990 (adapted from Nriagu, 1996).
Although these metal have crucial biological
functions in plants and animals, sometimes their chemical coordination and oxidation-reduction
properties have given them an additional benefit so that they can escape
control mechanisms such as homeostasis, transport, compart metallization and binding
to required cell constituents. These metals bind with protein sites which are
made for them by displacing original metals from their natural binding sites
causing malfunctioning of cells and ultimately toxicity. Previous research has
found that oxidative deterioration of biological macromolecular is primarily due
to binding of heavy metals to the DNA and nuclear protein (flora et al., 2008).
Heavy metals and
their toxicity mechanisms
Arsenic
Arsenic is one of
the most important heavy metal causing disquiet from both the ecological and
individual health stand points [hughes et al 1988]. It has a semimetallic
property, is prominently toxic and carcinogenic and is extensively available in
the form of oxides or sulfides or as a salt of iron, sodium, calcium, copper,
etc. (singh et al., 2007). Asentic is the twentieth most abundant element on
earth and its inorganic forms such as arsenite and arsenate compounds are lethdl
to the environment and living creatures. Human may encounter arsenic by natural
means, industrial sources of from unintended sources. Drinking water may be containes
by use of arsenical pesticides, natural miniral deposits or in appropriate disposal
of arsenical chemicals. Deliberate consumption of arsenic in case of suicidal
attempts or accidental consumption by children may also result in cases of
acute poisoning (Manzumder, 2008; saha et al., 1999). Arsenic is a protophydryl
group of cells causing mal functioning the sulphydryl group of cells causing mal
functioning of cell respiration, enzymes and mitosis (Garden and Quastel, 1948).
Mechanism of arsenie
toxicity
In arsenic biotrans formation,
harmful in organic arsenic biotra, algae, fungi and humans to give monomethylarsonic
acid (MMA) and dimethy larsimi acid (DMA). In this biotransformation process, these
inorganic arsenic species (IAS) are converted enzymetically to methy lated
arsenicals which are the end metabolites and the biomarker of chronic arsenic
exposure.
iAs (V) Ã iAs (iii) Ã MMA (V) Ã DMA (V)
biomethylation is a detoxification
process and end products are methylated inorganic arsenic such as MMA (V) and
DMA (V) is not excreted and remains inside
the cell as an intermediate and remains inside the cell as an intermediate product.
Monomethylarsenic acid
(MMA iii), an inter mediate product is found to be highly toxic compared to
other arsenicals, potentially accountable for arsenic induced carcinogenesis (singh
et al., 2007).
LEAD
Lead is a hihly toxic metal whose widespread
use has cause extensive environmental contamination and health problems in many
parts of the world. Lead is a bright silvery metal, slightly blush in a dry atmosphere.
It begins to tarnish on contact with air, thereby forming a complex mixture of
compounds, depending on the given conditions. Figure 2 shows various sources of
led pollution in the environment (Sharma and ubey 2005). The sources of lead exposure
include mainly industrial processes, food and smoking, drinking water and
demostic sources. The sources of lead were gasoline and house paint which has been
extended to lead bullets, plumbing pitcher, storage batteries, toys, and
faucets (Thurmer et al., 2002). In the US, more than 100 to 200, 000 tons of
lead per year is being released from vehicle
exhausts some is taken up by plants, fixation to soil and flow into water bodies,
hence human exposure of lead in the general population is either due to food or
drinking water (Goyer, 1990). Lead is an extremely toxic heavy metal that disturbs
various plant physiological processes and unlike other metals, such as Zinc, copper
and manganese, ir does not play any biological functions. A plant with high lead
concentration fastens the production of reactive oxygen species (ROS), causing tipid
memberance damage that ultimately leads to damage of chlorophyll and photosynthetic
processes and suppresses the overall growth of the plant (Najeeb et al., 2014).
Some research revealed that lead is capable of inhibiting the growth of tea plant
by reducing biomass and debases the tea quality by changing the quality of its components
(young sheng et al., 2011). Even at low concentrations, lead treatment was
found to cause huge instability in ion uptatke by plants, which in turn leads to
significant metabolic changes in photosynthetic capacity and ultimately in a
strong inhibition of plant growth (Mostafa et al., 2012).
Various pollution in
the environmental (Adapted from sharama and Dubey, 2015).
Mechanisms of lead
toxicity
Lead metal causes toxicity in living
cells by following ionic mechanism and that oxidatioive stress. Many researchers
have shown that oxidative stress in living cells is caused by the imbalance between
the production of free radicals and the generation of antioxidants to detoxify the
reactive intermidiates of to repair the resulting damage. Figure 3 shows the
attack of heavy metals on a cell and the balance between ROS production and the
subsequent defense presented by antioxidants. Antioxidants as e.g glatathing, present
in the cell protect it from free redicals such as H2O2
under the influence of
lead, however, of antioxidabts decreases. Since glutathione exists both in reduced
(GSH) and oxidized (G S S G) state, the reduced form of glutathione gives Its reducing equivalents (h++e-) from its
thiol groups of cystein to ROS in order to make them stable. In the presence of
the enzyme glutathrome peroxidase, reduced glutathione readily binds with
another molecule of glutathione after donating the election and forms
glutathione disulfide (GSSG). The reduced form (GSH) of glutathione accounts
for 90% of the total glutathione content and the oxidezed form (GSSG) accounts
for 10% under normal additionas. Yet under the condition of oxidative stress,
the concentration of GSSG ezcellds the concentration of GSH. Another biomarker
for oxidative stress in lipid peroxidation, since the free radicals collects
electron from lipid molecules present inside the cell membrane, which
eventually cause lipid peroxidation (wadhwa et al., 2012; flora et al., 2012).
At very high concentration ROS may cause structural damage to cells, proteins,
nucleis acid, membrance and lipids, resulting in a stressed situation at
cellular level (Mathew et al., 2011
Figure 3
The attack of heavy metals on a cell and the balance between ROS
production and the subsequent defense presented by antioxidants.
The ionic mechanism of lead toxicity occurs mainly due to the
ability of lead metal ions to replace other bivalent cations like ca2+,
mg2+ fe2+ and monovalent cations Na2+, which
ultimately disturbs the biological metabolism of the cell. The ionic mechanism
of lead toxicity causes significant changes in various biological processes
such as cell adhension, intra and inter-cellular signaling, protein folding,
maturation, apoptosis, ionic transportation, enzyme regulation, and release of
neurotransmitters. Lead can substitute calcium even in picomolar concentration affecting
protein kinase C, which regulates neural excitation and memory storage (flora
et al., 2012).
Mercury
The metallic mercury is a naturally occurring metal which is a
shiny silver-white, odorless liquid and becomes colorless and odorless gas when
heated. Mercury is a very toxic and exceedingly bio accumulative. Its presence
adversely affects the marine environment and hence many studies are directed
towards the distribution of mercury pollution include anthropogenic activities
such as agriculture, municipal waste water discharges, mining, incineration,
and discharges of industrial waste water (chen et al. 2012).
Mercury exists mainly in three forms: metallic elements, inorganic
salts and organic compounds, each of which possesses different toxicity are
present widdly in water resources such as lakes, rivers and oceans where they
are taken up by the microorganisms and get transformed into methyl mercury
within the microorganism, eventually undergoing bio magnification casing
significant disturbance to aquatic lives. Consumption of this contaminated
equatic animal is the major route of human exposure to methyl mercury
(transande et al., 2005). Mercury is extensively used in thermometers,
barometers, pyrometers, hydrometers, mercury are lamps, fluorescent lamps and
as a catalyst. It is also being used in pulp and paper industries as a
component of batteries and in dental.
Figure 4
The global usage of
mercury for various applications (total in 2005: 3,760 metric tons). Mechanism
of mercury toxicity
Mercury is well
known as a hazardous metal and its toxicity is a common cause of acute heavy
metal poisoning with cases of 3,596 in 1997 by the American Association, of
poison control centers. Methyl mercury is a neurotoxic compound which is
responsible for micro tubule destruction, mitochondrial damage, lipid
peroxidation and accumulation of neurotoxic molecules such as serotonin,
aspartate and glutamate (Patrick 2002). The total amount of mercury emission
into the environment has been assessed at 2,200 metric tons annually (ferrara
et al., 2000). It is estimated that 8 to 10% of American women have merury
levels that would induce neuro logical disorders in any child they gave birth
to, according to the both the environment protection agency and national academy
of science (haley, 2006). Animal which are exposed to toxic mercury have shown
adverse neurological and behavioral changes. Rabbits when exposed to 28.8mg/m3
mercury vapor for 1 to 13 weeks have shown vague pathological changes, marked
cellular degeneration and brain necrosis (Ashe et al. , 1953).
The brain remains
the target organ for mercury, yet it can impair any organ and lead to mal
functioning of nerves, kidneys and muscles. It can interrupt with intracellular
calcium homeo stasis. Mercury binds to freely available thiols as the stability
constants are high (Patrick, 2002). Mercury vapors can cause bronchitis, asthma
and temporary respiratory problems. Mercury plays a key role in damaging the
tertiary and quaternary protein structure and alfers the cellular function by
attaching to the selenohydryl and sulfhy dry groups which undergo reaction with
methyl mercury and hamper the cellular structure. It also intervenes with the
process of transcription and translation resulting in the disappearance of ribosomes
and eradication of endoplasmic reticulum and the activity of natural killer
cells. The cellular integrity is also affected causing free radical formation.
The basis for heavy metal chelation is that even though the mercury sulfhy dryl
bond is stable and divided to surrounding sulfhy dryl consisting ligands, it
also contributes free sulfhy dryl groups to promote metal mobility within the
ligands (bern hoft, 2011).
Cadmium
Cadmium is the
seventh most toxic heavy metal as per ATSDR ranking. It is a byproduct of zinc
production which humans or animals may get exposed to at work or in the
environment. Once this metal gets absorbed by humans, it will accumulate inside
the body throughout life. This metal was first used in World War 1 as a
substitute for tin and in paint industries as a pigment. In today’s scenario,
it is also being used in rechargeable batteries, for special alloys production
and also present in tobacco smoke. About three fourths of cadmium is used in
alkaline batteries as an electrode component, the remaining part is used in
coatings, pigment and plantings and as a plastic stabilizer. Human may get
exposed to this metal primarily by inhalation and ingestion and can suffer from
acute and chronic in toxications. Cadmium distributed in the environment will
remain in soils and sediment for several decades. Plants gradually take up
these metals which get accumulated in them and concentrate along the food
chain, reaching ultimately the human body. In the US, more than 500,000 workers
get exposed to toxic cadmium each year as per the agency for toxic substance
and diss=ease registry (Bernard, 2008, mutlu et l. , 2012). Researchers have
shown that in china the total area polluted by cadmium is more than 11,000
hectares and its annual amount of industrial waste of cadmium discharged into
the environment is assessed to be more than 680 tons. In japan and china,
environment cadmium exposure is comparatively higher than in any other country
(Han et al., 2009). Cadmium is predominanthly found in fruit and vegetables due
to its high rate of soil-to-plant aransfer (satarug et al., 2011). Cadmium is a
highly toxic non essential heavy metal that is well recognized for its adverse
influence on the enzymatic systems of cells, oxidative stress and for inducing
nutritional deficiency in plants (Irfan et al., 2013)
Mechanism of cadmium toxicity
The mechanism of
cadmium toxicity is not under stood clearly but their effects on cells are
known (patrrick, 2003). Cadmium concentration increases 3,000 fold when it
binds to cystein- rich protein such as me tallothionein. In the liver, the
cystein-matallothionein complex causes hepa to toxicity and then it circulates
to the kidney and gets acculated in the renal tissue causing nephro toxicity.
Cadmium has the capability to bind with cystein, glutamate, his tidine and
aspartate ligands and can lead to the deficiency of iron (castag netto et al.,
2002). Cadmium and Zinc have the same oxidation and hence can replace zinc
present in metallo thionein, there by inhibiting it from acting as a free
radical scavenger within the cell.
Conclusion
The toxic effects of
heavy metals affect the environment and living organisms (jarup, 2003). Heavy
metals are significant environmental pollutants and their toxicity is a
problems of increasing significant for ecological, evolutionary, nutritional
and environment reasons (jaishankar et al., 2013; nagajyti et al., 2010). Most
of the commonly heavy metals are found waste water include arsenic cadmium,
chromium, copper, lead, nickel and zinc, all of which cause risks for human
health and the environment (lambert et al., 2000) various sources at heavy
metals include soil erosion, natural weathering of the earth’s crust, mining,
industrial effluents, urbanrun off, sewage discharge, insect or disease control
agent applied to crops and many others (morals et al., 2012).
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