Toxicology of Nitrogen Compounds & Pesticides – Comprehensive Study Notes

Nitrates (III) and (V)

• Occurrence in the environment

  • Decomposition products of organic nitrogenous substances

  • Natural mineral salts – mainly nitrates(V)

  • Water contamination → leaching from geological deposits

  • Atmospheric air → nitrate aerosols

  • Origin of nitrates(III): reduction of nitrates(V) or oxidation of ammonia

• Main sources of human exposure

  • Natural animal manure/fertiliser use

  • Chemical fertilisers

  • $\text{Drinking water}$ – especially private-well water in rural areas

  • Occupational exposure in fertiliser, agricultural & food-processing plants

  • Consumption of meat and vegetables preserved with nitrates

  • Medicines overdosed (e.g. nitroglycerine)

• Metabolism & biotransformation

  • Rapid GI absorption (stomach, duodenum)

  • pH & microflora dependent transformations: nitrates(V) → nitrates(III), nitrogen oxides, hydroxylamine, ammonia

  • Nitrates(III) = precursors of carcinogenic $N$-nitroso compounds

  • Fast systemic transport via blood

  • $\approx90\%$ of an oral dose excreted in urine within $8\text{ h}$ (only nitrates(V) detectable)

  • Saliva can reveal nitrates(III) produced by oral bacterial reduction

• Mechanism of toxic action

  • Nitrates(V) per se non-toxic but reduced to nitrates(III) by bacterial nitrate-reductase

  • Nitrates(III) oxidise ferrous $\text{Fe}^{2+}$ in haemoglobin → ferric $\text{Fe}^{3+}$ → methemoglobin (MetHb)

  • Hypoxia becomes clinically evident when MetHb ≥ $20\%$

  • Peripheral vasodilation – direct relaxation of smooth muscle → hypotension

• Clinical picture: MetHb %–dependent

  • <20\% – asymptomatic

  • $20\%-40\%$ – weakness, headache, dizziness, cyanosis, tachycardia, tachypnoea

  • $50\%-60\%$ – CNS disorientation, drowsiness, dyspnoea, profound hypotension, LOC, shock

  • >60\% – deep coma, respiratory failure, death

• Other acute signs

  • Abdominal pain, GI irritation, facial flushing, dyspnoea, cardiovascular collapse

• Additional toxic effects of nitrates(III)

  • Haemolytic anaemia (bone-marrow toxicity, $\text{Vit B}_6$ inactivation)

  • Growth inhibition (↓ appetite, interference with $\text{Vit A}$ & protein synthesis)

  • Formation of nitrosamines – potent carcinogens/mutagens

• Newborn susceptibility

  • ↓ Gastric acidity → greater bacterial reduction of nitrates(V)

  • Foetal haemoglobin (≈$60\%-80\%$ of total) converts to MetHb more readily

  • Immature NADH-MetHb-reductase pathway

  • Fluid intake per kg higher than older infants

• Diagnostics & treatment of acute nitrate poisoning

  • Measure blood MetHb %

  • Urinary nitrates(V)

  • Antidote: intravenous methylene blue

  • Adjunct: high-dose vitamin C (ascorbate → reductant)

• Prevention / risk-mitigation

  • Diet rich in antioxidants: $\text{Vit C, Vit E, }\beta\text{-carotene}$

  • High-fibre foods to slow nitrosation

N-Nitroso Compounds (Nitrosamines)

• Chemical identity

  • Derivatives of secondary dialkyl/diaryl/cyclic amines

  • Stable over wide $pH$ & temperature range

  • General structure $R<em>1\text{–CH}</em>2–N(\text{NO})–CH<em>2–R</em>2$ $\big(R<em>1,R</em>2 = \text{alkyl/aryl}\big)$

• Occurrence & formation

  • Present in cured/smoked meats, fish, beer, cheese, pickled foods

  • Generated during thermal processing, storage, or endogenously in GI tract

  • Major exogenous source: cigarette smoke

  • Precursors: primary/secondary/tertiary amines, amino acids, amides, indoles, phenols, ammonium salts, pesticides, and amine-containing drugs (penicillin, ephedrine, promethazine)

• Microbial nitrosation

  • Intestinal bacteria capable of nitrosation → genera: $\textit{Escherichia, Pseudomonas, Proteus, Klebsiella, Neisseria}$

  • Anaerobes $\textit{Clostridium, Bacteroides, Enterococcus faecalis}$ nitrosate diphenylamine; $E. coli\,A10$ synthesises NDMA

• Toxicological profile

  • Genotoxic, mutagenic, teratogenic, carcinogenic

  • Target organs: liver, colon, lung, pancreas, stomach, kidneys, bladder, oesophagus, tongue

  • Potency varies: shorter alkyl chains ↑ toxicity; cyclic species (e.g. N-nitrosomorpholine) moderate; NDMA most notorious

• Pathology

  • Most alkyl nitrosamines → severe hepatotoxicity (serous lacerative necrosis, haemorrhage)

  • Some cyclic species (NMOR, NPIP) → acute neurotoxicity without liver lesion

  • NDMA in animals → fatal liver damage, GI ulceration, mucous membrane/skin irritation

• Organ-specific carcinogenesis examples

  • NDMA – liver, kidneys, lungs

  • NDEA – liver, oesophagus

  • NDELA – liver, oral cavity

  • NBBOH – bladder

  • NEVA – oesophagus

• Inhibitors of endogenous nitrosamine formation

  • Ascorbic acid, ascorbyl palmitate, glutathione, tocopherol, gallic acid, sodium sulfate, thiol compounds

Pesticides – General Concepts

• Global burden

  • $\approx300\,000$ deaths yr⁻¹ from intentional pesticide self-poisoning (developing world)

  • Organophosphorus (OP) & carbamate agents cause majority of acute fatalities

• Definitions & purposes

  • Substances that repel, destroy, or control pests (insects, rodents, weeds, fungi) or regulate plant growth

• Toxicological categories

  • Acute poisoning: occupational, environmental, accidental, suicidal

  • Chronic poisoning: cumulative low-dose exposure; functional accumulation

  • Distant effects: genotoxicity, mutagenicity, teratogenicity

• Use-based subclasses

  • Zoocides: insecticides, rodenticides, molluscicides, nematocides, acaricides

  • Herbicides, fungicides, attractants, repellents

• LD50-based toxicological classification (rat, mg kg⁻¹ b.w.)

  • Class I (very toxic) <25 – symbol $T^{+}$

  • Class II (toxic) $25-200$ – $T$

  • Class III (harmful) $200-2000$

  • Class IV (practically harmless) >2000

• Chemical families

  • Organophosphorus, carbamates, pyrethroids, chlorophenoxyacetic acids, coumarins, organochlorines, bipyridyls, ureas, dithiocarbamates

Organophosphorus Compounds (OPs)

• Characteristics

  • Widely used household & agricultural insecticides; originally nerve-gas prototypes

  • Structure: central phosphate nucleus + diverse aliphatic side chains

  • Forms: crystalline solids / oily liquids, pungent odour; lethal dose $30-120\,\text{mg}$

  • Absorption routes: ingestion, inhalation, dermal

  • Examples: parathion, malathion, methyl-parathion, chlorpyrifos, diazinon

• Mechanism of action

  • Irreversible inhibition of acetylcholinesterase (AChE) & other cholinesterases → excess acetylcholine at muscarinic & nicotinic synapses

  • Result: continuous receptor overstimulation

• Acute toxicity manifestations

  • Mild (low dose): salivation, lacrimation, sweating, miosis, tachycardia, hypertension, bronchoconstriction

  • Moderate–severe: diarrhoea, urinary incontinence, bradycardia, muscle fasciculations → weakness → flaccid paralysis; bronchorrhoea, bronchospasm

  • CNS: anxiety, confusion, seizures, coma, respiratory depression

  • Complications: acute pancreatitis, arrhythmias, solvent aspiration pneumonitis

• Specific syndromes

  • Intermediate Syndrome (IMS): cranial-nerve muscle paralysis, occurs $24-96\text{ h}$ post-exposure due to persistent NMJ AChE inhibition; prevented by early treatment

  • OP-Induced Delayed Neuropathy (OPIDN): develops $1-3\text{ weeks}$ post-exposure; distal limb weakness → spastic paresis; may be irreversible

• Chronic exposure effects

  • Lower-limb weakness, sensory loss, depression, prolonged reaction time, haematological changes (↓ RBC, ↑ WBC)

  • Epidemiology: cognitive deficits, Parkinsonian symptoms, reproductive & immunological disturbances

• Diagnostics

  • Measure erythrocyte AChE activity: mild poisoning ↓ $50-80\%$; moderate ↓ $80-90\%$; severe ↓ >90\%

  • Plasma cholinesterase: toxic signs at $40-50\%$ inhibition; neuromuscular crises at $≈80\%$

  • Metabolite assays in blood/urine support exposure assessment

• Management

  • Airway, breathing, circulation; oxygenation

  • Atropine: adult $1-3\,\text{mg}$ IV (child $0.02\,\text{mg·kg}^{-1}$) doubling every $5\text{ min}$ until atropinisation (dry mucosa, mydriasis, tachycardia)

  • Oxime reactivators: obidoxime / pralidoxime mesylate $30\,\text{mg·kg}^{-1}$ over $30\text{ min}$ then $8-10\,\text{mg·kg}^{-1}\text{·h}^{-1}$ (target serum $4\,\mu\text{g·L}^{-1}$)

Carbamate Insecticides

• Properties

  • Household & agricultural use; crystalline, slightly water-soluble, unstable in alkali

  • Absorbed by GI, respiratory, dermal routes

  • Reversibly inhibit AChE by carbamylation – enzyme spontaneously regenerates within minutes–hours → shorter, milder toxicity vs OPs

  • Poor blood–brain-barrier penetration

  • Common agents: carbaryl, carbofuran, propoxur, aldicarb

• Acute presentation

  • Headache, dizziness, nausea, vomiting, abdominal pain, mental confusion

  • Hypersecretory state: sweating, lacrimation, salivation; miosis

  • Variable bradycardia/tachycardia, BP fluctuations; bronchorrhoea

  • Muscle tremor, convulsions; possible parenchymal organ injury & allergic dermatoses

• Diagnosis

  • Clinically & via cholinesterase activity (initially indistinguishable from OP)

  • Rapid symptom regression suggests carbamate poisoning

• Treatment

  • Atropine only; oximes unnecessary (enzyme already regenerates)

Pyrethroid Esters

• Origin & types

  • Natural pyrethrum from $\textit{Chrysanthemum cineriaefolium}$ flowers; used for centuries

  • Synthetic analogues (allethrin, etc.) ↑ photostability & potency

  • Fat-soluble, chemically unstable (natural degrade in light; synthetics more stable)

  • Minimal systemic absorption; act by contact toxicity

• Mechanism

  • Neurotoxins – modify voltage-gated sodium channels (Type I) prolonging opening → repetitive discharges → conduction block

  • Type II additionally inhibit GABA-gated chloride channels → prolonged depolarisation

  • Disturb neuronal calcium homeostasis; inhibit ATPase, phosphodiesterase, alter catecholamines

• Acute toxicity

  • Contact dermatitis (burning, itching)

  • Ocular/face exposure → pain, lacrimation, photophobia, conjunctival oedema

  • Ingestion → epigastric pain, nausea, vomiting, headache, paresthesia, palpitations, fasciculations, altered consciousness; severe cases → convulsions, opisthotonos, coma

  • Overall severe human intoxication rare despite ubiquitous use

Chlorophenoxyacetic Acid Derivatives

• Chemical traits

  • Odourless, colourless crystals; exposure mainly oral

  • Manufacturing/storage mishaps can yield highly toxic TCDD contamination

• Herbicidal action

  • Act as synthetic auxins → uncontrolled plant growth & death

• Proposed mechanisms in mammals

  • Cell-membrane & BBB damage, neuronal-membrane transport disruption

  • Interference with oxidative phosphorylation & other metabolic pathways

  • Possible role as false cholinergic messenger

• Acute human toxicity

  • Irritation of skin, eyes, GI, respiratory tract

  • GI: burning mouth, nausea, vomiting, diarrhoea, GI bleeding → hypovolaemic shock

  • Chloracne

  • Myalgia, muscular weakness, ataxia

  • Neuro-behavioural: anxiety, irritability, headache

  • Dyspnoea, cold intolerance, arrhythmias, hepatic dysfunction

  • Metabolic disturbances: acidosis, hypo-glycaemia/calcaemia/phosphataemia; ↑ transaminases

Coumarin Derivatives (Anticoagulant Rodenticides)

• Categories

  • First-generation – warfarin derivatives (short-acting)

  • Second-generation “super-warfarins” – brodifacoum, bromadiolone; indandiones (chlorophacinone, pindone) – ↑ potency & half-life

• Mechanism

  • Inhibit vitamin K epoxide reductase → prevent recycling of vitamin K → impaired γ-carboxylation of clotting factors II, VII, IX, X & proteins C, S, Z

  • Clinical effect delayed until hepatic & plasma vitamin K depleted (latency $12-48\text{ h}$)

• Clinical picture (duration: warfarin $2-7\text{ d}$; super-warfarin weeks–months)

  • Gingival/nasal bleeding, easy bruising, muscle & joint pain

  • GI bleeding (melena), haematuria

  • Prolonged prothrombin time/INR

  • Intracranial haemorrhage → shock & death (most common fatal event)

• Management

  • Replace clotting factors & volume: fresh-frozen plasma units

  • Vitamin K₁ (phytonadione) supplementation – dosing guided by INR & agent half-life

General Treatment Principles for Pesticide Poisoning

• Ensure airway & give supplemental oxygen

• Induce emesis (if safe & early post-ingestion)

• Continuous monitoring of vitals & targeted symptomatic therapy

• Specific antidotes

  • Atropine: OPs & carbamates

  • Oximes: OPs only

• Supportive measures as per agent-specific pathophysiology