Xenobiotic Compounds Overview

Xenobiotic Compounds

• Xenobiotics are foreign molecules not normally ingested or used by an organism.
• Examples include antibiotics, environmental pollutants, food additives, pesticides, and other foreign substances.
• Humans encounter thousands of xenobiotics, which require metabolism before excretion.

Xenobiotics - What?

• Xenos = Foreigner
• Bios = Life
• Xenobiotics can be exogenous or endogenous, both with similar effects.

Exogenous Xenobiotics

• Exogenous xenobiotics are foreign molecules that enter through food, medicine or inhalation.
• Examples: Drugs, food additives, pollutants, insecticides, chemical carcinogens.

Endogenous Xenobiotics

• Endogenous xenobiotics are not foreign substances, but have effects similar to exogenous xenobiotics.
• They are synthesized within the body or result from metabolic processes.
• Examples: Bilirubin, Bile acids, Steroids, Eicosanoids, certain fatty acids.

Metabolites

• Metabolites are low-molecular-weight organic and inorganic chemicals that are reactants, intermediates, or products of enzyme-mediated biochemical reactions.
• They are produced by plants for growth and development (primary metabolites) or as a part of their defense mechanisms (secondary metabolites).
• Metabolites are usually small molecules and have various functions including fuel, structure and signaling, catalytic activity and interactions with other organisms.
• Ethylene is an example of a primary metabolite.
• Many secondary metabolites have important ecological function, such as antibiotics and pigments like resins and terpenes.

Recalcitrant Xenobiotics

• These compounds resist biodegradation, persist in the environment, and accumulate over time thus posing a hazard.
• Microorganisms are able to degrade many xenobiotics but they are unable to degrade many others, these resistant compounds are called recalcitrant.
• Factors contributing to recalcitrance:
  • Lack of enzymes for degradation;
  • Large molecular size hindering entry into microbial cells;
  • Chemical structure making compounds highly stable.
  • Insoluble in water
  • High Toxicity

Sources of Xenobiotic Compounds

• Petrochemical industry (e.g., vinyl chloride, benzene)
• Pesticide industry (e.g., benzene derivatives)
• Paint industry (e.g., solvents like xylene, toluene)
• Plastic industry (e.g., complex organic compounds, antioxidants)
• Other sources (e.g., electronic, textile, cosmetic industries)

Types of Recalcitrant Xenobiotic Compounds

• Polychlorinated biphenyls (PCBs)
• Synthetic polymers (e.g., polyethylene, polystyrene, polyvinyl chloride)
• Alkylbenzyl sulfonates
• Oil mixtures
• Pesticides

Halocarbons (PCBs)

• Halocarbons are compounds containing a variable number of halogen atoms.
• PCB are a significant group of halocarbons and are formed by covalent linkages of two benzene rings, with substituent halogens.
• They are used as plastics, insulator coolants, solvents, and other uses.

Synthetic Polymers

• Produced as plastics, including polyethylene, polystyrene, and polyvinyl chloride.
• Their insolubility and high molecular weight contribute to their recalcitrant nature.

Other Xenobiotics

• Many pesticides contain aliphatic structures with substituents like nitro, sulphonate, methoxy, amino, and carbamoyl groups.
• Alkylbenzyl sulfonates are another group of xenobiotics.

Xenobiotic Hazards

• Toxicity to various organisms (bacteria, eukaryotes, and even humans).
• Carcinogenicity (capable of causing cancer).
• Accumulation in the environment (causing pollution).
• Bioaccumulation and biomagnification (increased concentrations in organisms higher up in the food chain).
• Mutagenic and allergenic effects, reproductive issues in humans.

Food Chain Effects

• Bioaccumulation: Xenobiotics accumulate in primary producers and then increase as they're consumed through the food chain.
• Biomagnification: As higher organisms consume organisms on lower trophic levels, the concentration of the contaminant increases.

Effects of Xenobiotics

• Health Effects: Mutagenicity, teratogenicity (issues in development), carcinogenicity, and allergic responses.
• Soil Effects: Assimilation by plants, transportation of compounds through soil erosion and accumulation in the food chain.
• Water Effects: Sedimentation of hydrophobic pollutants; changes in the physicochemical properties of water.

Human Health

• Carcinogenic effects in humans, including liver cancer, and melanoma.
• Immune system suppression, swelling thymus gland in infants.
• Nervous system effects (neurological functions, short-term memory, learning issues).
• Reproductive system effects, reduced birth weight, decrease gestational ages, abortions.
• Endocrine system effects (thyroid function).
• Other possible health issues.

Marine and Animal Health

• Inhibition of plankton growth and photosynthesis impacting the food chain.
• Toxicity to marine organisms at low concentrations.

Fate of Xenobiotics

• Ingestion, inhalation or dermal contact.
• Absorption into bloodstream.
• Distribution to tissues and organs.
• Biotransformation (chemical modification).
• Storage
• Excretion.

Transformations of Xenobiotics

• Mineralization: complete decomposition of an organic compound into inorganic elements.
• Biodegradation: decomposition of organic compounds into inorganic elements with the participation of living organisms, often with biomass accretion.
• Biotransformation: the change of the structure of a xenobiotic compound.