Arsenic toxicity and speciation in plants,
implications for human nutrition
Lecture of Seema Mishra in the VK Bioinorganic Chemistry & Biophysics of Plants 2012
Arsenic
Arsenic
2, 8, 18, 5
Arsenic: occurrence

As is the twentieth most abundant element in the Earth’s crust with an
average concentration of approximately 3 mg kg
kg−11.

More than 200 As-containing minerals exist; frequently As is associated
with S in minerals such as arsenopyrites (FeAsS), realgar (As4S4), and
orpiment (As2S3).
Arsenopyrites
Realgar
Orpiment
Arsenic: Applications

The Poison of Kings and the King of Poisons

Semiconductor industries

Strengthening alloys of copper and lead

Pesticides herbicides,
Pesticides,
herbicides insecticides

Wood preservatives

Feed additives

Medical uses

Military uses
Arsenic trioxide
Structures of the most common As compounds
Trimethyl
y Arsine
Sources of Arsenic Contamination

Through anthropogenic activities such as mining, smelting, phosphate
fertilizer, and the use of As-containing pesticides, herbicides, wood
preservatives, and feed additives.

Through natural processes
processes, such as weathering of rocks
rocks, volcanic
emissions and discharge from hot springs
Arsenic Contamination
Arsenic Contamination
Basic Facts of the Problem

The current limit of As in drinking water (WHO) is 10 μg/L.

WHO Provisional Maximum Tolerable Daily Intake (MTDI) limit of As is
2 μg
μg/Kg
g body
y weight.
g

Level of As in drinking in West Bengal above 50 μg/L are commonly
found Severely affected areas may contain as high as 2000 μg/L.
found.
μg/L

If rice grain with an As level of 0.1 μg/g (a typically normal As level) is
consumed at a rate of 420 g/day (representative of rice-based diet), then
ingestion of 0.7 μg/Kg (35% of MTDI) body weight would occur.

Far exceeding levels e.g. up to 3 μg/g are found. In addition, cooking rice
in contaminated water also adds to As consumption.
Arsenic Exposure
Toxicity to human
Acute effects

Gastrointestinal effects (Nausea,
(Nausea Diarrhea,
Diarrhea Abdominal pain)

Effects on central nervous system and cardiovascular system

Liver and kidney dysfunction

Anemia, leukopenia

Ingestion of 600 µg/kg body weight/day or inhalation of 25-50 ppm
arsine for a half
half- hour is lethal
Chronic effects

Gastrointestinal effects,

Skin lesions, hyperpigmentation,

Anemia, peripheral neuropathy, gangrene of the extremities, vascular
g
lesions,, and liver or kidneyy damage

Increased risk of Cancer : skin, bladder, liver, and lung cancer
Toxicity to human
Richard Wilson, Harvard University, and Dhaka Community Hospital
(www.soesju.org)
Arsenicosis patients from arsenic-affected areas
Why Rice ?
Pick your poison ?
Why Rice ?
Meharg (2004). TRENDS in plant science, 9, 415-417.
Arsenic Toxicity to Plants
Symptoms
Reduced germination,
Inhibited root growth
Inhibited shoot growth
Reduced chlorophylls
Low grain yield
To death
Castillo-Michel et al. (2007). Plant Physiol. Biochem. 45,
457-463
Arsenic Toxicity to Plants
Mechanism
Through uptake competition for essential nutrients
Through
oug subs
substitution
u o o
of p
phosphate
osp a e by iAs(V)
s( ) in e
enzyme
y e ca
catalyzed
a y ed
reactions
Byy binding
g of iAs(III)
( ) to sulfhydryl
y y g
group
p containing
g enzymes
y
ROS generation
Reduction of iAs(V) to iAs(III) using glutathione as reductant
Oxidation of iAs(III) to iAs (V) under physiological conditions
Arsenic Toxicity to Plants: Genotoxicity
Induction of micronuclei by heavy metal(loid)s
Tradescantia Stem
Root tip of Allium cepa
As3+> Pb2+> Cd2+> Zn2+> Cu2+
Root tip of Vicia faba
Trad MCN> Vicia root MCN> Allium root MCN
Steinkellner et al. (1998). Environmental and Molecular Mutagenesis 31, 183-191
Arsenic Toxicity to Plants: Genotoxicity
leaves
Root
Comet images of plant nuclei of Vicia faba under arsenate treatments
Lin et al. (2008). Environ Toxicol. Chem. 27, 413–419
Arsenic Toxicity to Plants: Proteomics
Control
A ; down regulated
B-D; up regulated
300 µM As(V)
250 µM As(III)
Ceratophyllum demersum L.
Ceratophyllum demersum
is rootless aquatic weed
shows
Rapid growth
and Worldwide distribution
andd can bbe Easily
E il harvested
h
t d
Arsenic Metabolism in non
non--hyperaccumulator plants
SO42-
SO42-
ATP
sulfurylase
SiR
APR
APS
Sulfide
Sulfite
Gly
GSH
SAT
Glu
Cys
EC
GS
CS
ECS
PCS
OAS
AsIII
?
AsIII
PCs
AsIII
GSH
As-Thiol
ABCC
AsIII,
As-Thiol
GSSG
AsV
AsV
AsIII
?
AsIII
Phytochelatins (PCs) are glutathione derived metal binding peptides with
general structure (γ -Glu-Cys)n Xaa, where n = 2–11 and Xaa is generally Gly.
As Hyperaccumulation in pteris vittata (Chinese Brake fern)
Accumulates up to 22,000 mg As /kg
/ (DW)
(
)
Up to 2.3-4.1% of biomass
As speciation analysis
HPLC
ICP-MS
ESI-MS
Phytochelatin synthase from C. demersum
WT
Transgenic
Transgenic
100 µM
WT
Transgenic
WT
300 µM
Transgenic
0 µM
WT
Transgenic
WT
100 µM
As (µg g-1 dw
w
Cd (µg g-1 dw
w
0 µM
WT
Cd Concentration (µM)
As(V) Concentration (µM)
Transgenic
200 µM
Arsenic: a building block of life
New Bacteria Makes DNA With Arsenic: NASA Life Discovery
The arsenic-rich
Lake, California
arsenic rich Mono Lake
A scanning electron micrograph of
the arsenic-based bacteria
A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus
Wolfe-Simon et al (2011), Science. 332, 1163-1166
Can arsenic bind to bacterial DNA ?
DNA structure with arsenic replacing phosphorus in the backbone
J Wang, J Gu and J Leszczynski, Chem. Commun., 2012, DOI: 10.1039/c2cc16600c