3 Lead (Pb) Toxicity

Dr. Anju

TABLES OF CONTENTS

1. Learning Outcomes 

2. Introduction

3. Sources of Lead in the Environment

4. Exposure to Lead

   4.1 . Exposure to Lead through Inhalation

4.2 . Exposure to Lead through Ingestion

4.3 . Exposure to Lead via skin

4.4 . Occupational Environments

5. Lead Absorption, distribution and elimination/excretion

    5.1. Lead Absorption

5.2. Lead distribution

5.3. Lead elimination/excretion

6. Biomarkers of Lead

7. Toxic effects of Lead poisoning on human health

8. Mechanisms of Lead Toxicity

8.1. Interruption in activity of Enzymes

8.2. Ionic mechanism

8.3. Deactivation of antioxidants sulphadryl (-SH) pools

8.4. Free radical damage and oxidative stress

9. Lead Toxicity in Plants

10. References

1.Learning Outcomes

After going through this module, you shall be able to:

• To know about sources of lead (Pb) in the environment.
• To gain knowledge about lead absorption, distribution and excretion from the body.
• To understand about the Mechanism of lead toxicity.
• To know about toxic effects of lead.

2.Introduction

Lead is a metallic element from group IVA of the periodic table. It is a soft metal with little tensile strength, very malleable and flexible, and a poor conductor of electricity. The properties of lead are detailed in Table 1. Lead poisoning has been described in antiquity. Lead is an extremely toxic heavy metal that does not play any biological function.

3.Sources of Lead in the environment 

Besides natural weathering, the main anthropogenic sources of lead in the environment include Mining and Smelting of Pb ores, metal plating and finishing operations, additives in gasoline (organic lead compounds used as antiknocking agent) and pigments, industrial sources, paint pigments (white lead-Lead carbonate hydroxide [PbCO3]2Pb[OH2]), inks (used for printing newspaper), glazes, fertilizers, pesticides, leaded storage batteries (used in vehicles, industries and electricity backup systems), ammunition, X-ray shielding, crystal glass production, solders, alloys (brass and bronze), pesticides and fertilizers, plumbing pipes, as stabilizer in polyvinyl chloride (PVC) etc. During the age of Roman Empire, lead acetate was used to sweeten port wine.

In urban areas lead emissions originate from use of leaded petrol, over the last few decades, however, lead emissions in these areas have decreased markedly due to the introduction of unleaded petrol. Organo-lead compounds such as tetramethyl lead (Me4Pb) and tetraethyl lead (ET4Pb) are highly lipophilic with low water solubility. Both tetramethyl lead and tetraethyl lead can penetrate body and cell membranes and may also cross the blood–brain barrier in adults

4.Exposure to Lead

Lead is a multimedia pollutant, with various sources and media contributing to its exposure. Although, exposure to lead occurs mainly through air and food, the absorption through skin has also been reported.

4.1. Exposure to Lead through Inhalation

Lead exposure through inhalation occurs from following sources:

• In urban areas where leaded gasoline is still in use, or has only recently been phased out, mean air-lead levels are quite high.
• In the vicinity of Lead emitting industries e.g. lead smelters, lead acid battery recycling plants etc. Lead exposure occurs through inhalation as well as ingestion.
• Additionally, inhalation exposure occurs through cigarette smoking, both in active and passive smokers.

Airborne lead gets deposited on soil and water, thus reaching humans via the food chain.

4.2. Exposure to Lead through Ingestion

Dust (street/home/playground) has been recognized as a major source of lead exposure especially of young children because of their greater susceptibility to a given dose of toxin and the likelihood to ingest inadvertently significant quantities of dust through ‘‘Pica’’ (the mouthing of non-food objects) and repetitive hand or finger sucking. Dust around homes painted with leaded paint and around lead-emitting industries may contain very high lead.

In cities with old plumbing system, use of lead pipes are very common. In such areas lead is found in tapwater. Lead intake also occurs through foods. In diet, fruits, vegetables, cereals, bakery wares and beverages are the major source of lead. Fruits and vegetables contain lead as a result of deposition from air and uptake by roots. Lead acetate was previously used as a sweetener in wines. Wines may contain considerable lead concentrations as lead arsenate was used as a fungicide in vineyards and lead contamination from containers may also occur.

Another significant source of lead is lead-glazed or lead-painted pottery. Particularly, storage of acidic foods, such as fruit juice, pickles in lead-glazed ceramics may lead to a large lead intake.

4.3. Exposure to Lead via skin

Although, lead mainly enters the body via oral ingestion or inhalation, the absorption through skin has also been reported. Exposure may occur from use of cosmetics e.g. leaded eye powder (e.g. surma, kohl). Skin penetration of Pb may occur in occupationally lead-exposed workers.

4.4. Occupational Environments

Occupational exposure to inorganic lead occurs in mines, smelters, welding of lead painted metal, and in battery plants. High levels of air emissions may pollute areas in the vicinity of lead mines and smelters. Occupational exposure to lead may be categorized in high risk and moderated/low risk categories.

Table 2. Occupational exposures to lead

5.Lead Absorption, distribution and elimination/excretion

5.1. Lead Absorption

Lead is absorbed through respiratory tract, gastrointestinal tract and skin. Adults take up only 10–15% of lead in food, whereas children may absorb up to 50% of Pb via the gastrointestinal tract.

Gastrointestinal absorption of lead is affected by nutritional status. Milk and Vitamin D increase the gastrointestinal absorption of lead.

5.2. Lead distribution

Lead is one of the systemic poisons, once absorbed and comes into the circulation, it is distributed throughout the body, causing serious health effects. Most of the blood lead (B-Pb) is present in the erythrocytes (red cells), leaving only a fraction of less than 1% in plasma. Lead’s high affinity for the protein δ-aminolevulinic acid dehydratase (ALAD, equivalent to porphobilinogen synthase, PBGS. ALAD is an enzyme present in all cells, including the erythrocytes. It is the second enzyme in the heme pathway, promoting the asymmetrical addition of two molecules of δ -aminolevulinic acid (ALA) to form the monopyrrole porphobilinogen.

Absorbed lead from the blood plasma is distributed to other organs. Among the soft tissues, the liver and the kidney attain the highest concentrations. Lead does, to some extent, crosses the blood– brain barrier. In adults, inorganic lead does not penetrate the blood–brain barrier, whereas this barrier is less developed in children. The degree of passage of lead through blood-brain barrier into the nervous system is higher in infants/children than in adults. Thus, children are especially susceptible to lead exposure and subsequent brain damage due to high gastrointestinal uptake and the permeable blood–brain barrier.

A large fraction of the absorbed lead (approximately 90%) is incorporated into the calcified tissue such as skeleton/bones and teeth. Bones act as reservoir for Pb and a two way process of active influx and efflux of Pb to and from the bone and bloodstream has been recognised. Bones are no longer considered as sink of Pb in the human body.

5.3. Lead Elimination/Excretion

Lead is excreted from the body mainly through the urine and feces. Lead is also excreted through bile and pancreatic juices into the feces. To some extent, lead is also excreted in saliva and sweat, but is of little importance.

6.Biomarkers of Lead

Heme Synthesis

Lead inhibits several enzymes in the heme synthesis, primarily δ-aminolevulinic acid dehydrogenase (ALAD), and coproporhyrinogen oxidase. The effect of lead on heme synthesis causes accumulation/increase of ALA (δ-aminolevulinic acid) and coproporphyrins in serum/plasma and are also excreted into urine. ALA is neurotoxic.

Nucleotide Metabolism

P5N is present in the cytosol of erythrocytes. It catalyses, as a step in degradation of ribosomal RNA, hydrolytic dephosphorylation of pyrimidine-5′-monophosphates. Lead inhibits P5N (pyrimidine 5′-nucleotidase) enzyme activity, which leads to accumulation of pyrimidine nucleotides in erythrocytes (red cells), shortening the life span of cells.

Lead also interferes with calcium-dependent processes (i.e., such as related to neuronal signaling and intracellular signal transduction

7.Toxic effects of Lead poisoning on human health

The symptoms of acute lead poisoning are headache, irritability, abdominal pain and various symptoms related to the nervous system. The characteristics of lead encephalopathy are sleeplessness, restlessness and in severe cases, the affected person may suffer from acute psychosis, confusion and reduced consciousness. Children may be affected by behavioural disturbances, learning and concentration difficulties. Long term exposure to lead may lead to memory deterioration, prolonged reaction time and reduced ability to understand and anaemia (as lead poisoning causes disturbance of haemoglobin synthesis).

8.Mechanisms of Lead toxicity

Lead toxicity is multifaceted. Various  mechanisms of Pb toxicity are:

•  Interruption in activity of enzymes
•  Ionic mechanism
•  Deactivation of antioxidants sulphadryl (-SH) pools
•  Free radical damage and oxidative stress

8.1. Interruption in activity of enzymes

Two specific enzymes, δ-aminolevulinic acid dehydrogenase (ALAD) and glutathione reductase (GR) are inhibited by lead. ALAD is primarily involved in heme synthesis. Inhibition of ALAD by lead increases levels of the substrate δ -aminolevulinic acid (ALA) which is known to stimulate the formation of ROS.

Glutathione reductase (GR) is an enzyme responsible for recycling of oxidized glutathione (glutathione disulfide – GSSG) to reduced glutathione (GSH) and lead has been shown to interfere with this cycle resulting in depressed GSH levels. Under normal conditions, GSH (reduced glutathione) and GSSG (oxidized glutathione) are present in the ratio 90:10. Under the condition of oxidative stress the GSSG concentration exceeds the concentration of GSH.

8.2. Ionic mechanism

The divalent Pb2+ ion is similar in many ways to Ca2+ ion. The ionic mechanism of lead toxicity occurs mainly due to the ability of lead metal ions (Pb2+) to mimic with bivalent cations like Ca2+, Mg2+, Fe2+ in many biological systems, ultimately disturbing the biological metabolism of the cell. Lead also interferes with calcium-dependent processes (i.e., such as related to neuronal signaling and intracellular signal transduction). Significant changes in various biological processes such as cell adhesion, protein folding, intra- and inter-cellular signaling, maturation, apoptosis, enzyme regulation, ionic transportation, and release of neurotransmitters are caused by the ionic mechanism of Pb toxicity. Due to chemical similarity between calcium (Ca) and lead (Pb), lead can substitute calcium which may account for the fact that 90% or more of the total body burden of Pb is found in skeleton.

8.3. Deactivation of antioxidants sulphadryl (-SH) pools

Pb is an electropositive metal and has high affinity for the sulfhydryl (-SH) groups. Pb reacts with the sulfhydryl group of the enzymes to form mercaptide, leading to enzyme inactivation.

                               2RSH + Pb2+ —————>    R-S-Pb-S-R + Pb2+

8.4. Free radical damage and oxidative stress

Transition metal homeostasis is maintained through tightly regulated mechanisms of uptake, storage and secretion. Exposure to lead can breakdown the metal ion homeostasis, which can lead to enhanced formation of reactive oxygen species, ROS (via the Fenton reaction, which produces hydroxyl radicals), and reactive nitrogen species (RNS), involved in causing oxidative damage to biological macromolecules such as proteins, lipids and DNA.

Imbalance between the generation of free radicals (ROS/RNS) and their detoxification by antioxidant defence systems (non-enzyme antioxidants such as glutathione (GSH), ascorbic acid, flavonoids, alpha-tocopherol, carotenoids, and enzymic antioxidant which include glutathione peroxidase, superoxide dismutases (SOD) and catalase) is termed oxidative stress. Free radical-induced damage by lead is accomplished two mechanisms. These two independent, although related mechanisms are: (i) direct formation of ROS including singlet oxygen, hydrogen peroxides and hydroperoxides and (ii) simultaneous depletion of the cellular antioxidant reserves.

8.4.1. Reactive Oxygen Species (ROS)

ROS are generated by various endogenous and exogenous sources. The main endogenous source of ROS is mitochondria, along with microsomes and peroxisomes (generate especially H2O2). Atmospheric pollutants, tobacco smoke, irradiation (gamma-ray, x-ray, UV irradiation), heavy metals and chemicals are some of the exogenous sources of ROS generation.

Reactive oxygen species (ROS) can be classified into two different groups:

• Free radicals also known as pro-oxidants – possess unpaired electrons in their outer orbitals: superoxide anion radical (O2.−), hydroxyl radical (OH∙), hydroperoxyl radical (HOO∙), alkoxy (RO∙) and peroxy radicals (ROO∙)
• Non-radical oxygen species –very reactive, also possess unpaired electrons and are capable of forming ROS radicals: organic hydroperoxide (ROOH), hydrogen peroxide (H2O2), ozone (O3) and trioxidan (HOOOH)

Under normal conditions, ROS are considered as redox regulators and fundamental keeper of cellular homeostasis, playing essential functions as signal molecules in several signaling pathways involved in cell differentiation, organogenesis, stress response and wound healing. Under oxidative stress, the free radical induced-toxicity at cellular level is expressed as lipid and protein peroxidation and damaged nucleic acids.

Fig. 1. Heavy metal induced oxidative stress (modified from Coricovac and Dehelean, 2015)

9.Lead Toxicity in Plants

Pb strongly inhibits the key enzyme of chlorophyll biosynthesis, δ-amino laevulinate dehydrogenase. In most cases, inhibition of enzyme activities is exerted through interaction of Pb with enzyme -SH groups. However, activities of some enzymes may be increased by Pb, e.g. Chlorophyllase activity, which leads to enhanced degradation of Chlorophyll. Pb reduces the uptake of nutrients viz. cations (K+, Ca2+, Mg2+, Mn2+, Zn2+, Cu 2+, Fe2+) and aniots (NO3-). Pb also promotes formation of Reactive Oxygen Species (ROS) in plants, leading to oxidative stress.

Lead toxicity symptoms in plants are: chlorosis, stunted growth, blackening of root system, inhibited photosynthesis, affects membrane structure and permeability, upsets mineral nutrition and water balance, decrease in hormonal status. Pb also inhibits germination of seeds and hence germination percent and retards seedling growth, root/shoot length and dry mass.

References

1. Sharma P., Dubey R.S. (2005) Lead toxicity in plants. Braz. J. Plant Physiol., 17(1): 35-52. Coricovac D.E., Dehelean C.A. (2015). Pathological aspects with global impact induced by toxicants at cellular level. Toxicology Studies – Cells, Drugs and Environment, Intech Open, pp. 3-21, http://dx.doi.org/10.5772/59945
2. Skerfving S., Bergdahl I.A. (2007). Lead. In Nordberg G.F., Fowler B.A., Nordberg M. (Eds.) Handbook on the Toxicology of Metals 3E, Academic Press, Inc. pp. 599-643.
3. Jomova  K.,  Valko  M.  (2011).  Advances  in  metal-induced  oxidative  stress  and  human  disease. Toxicology 283: 65–87.
4. Jaishankar M., Tseten T., Anbalagan N., Mathew B.B., Beeregowda K.N. (2014). Toxicity, mechanism and health effects of some heavy metals. Interdiscip Toxicol. 7(2): 60–72.