Benzidine (1,1′-biphenyl-4,4′-diamine) is an organic compound that has the formula of (C6H4NH2)2
Benzidine is not a naturally occurring substance. Instead, it is manufactured and used in the dying process of cloth and paper. It was also used to detect blood and in the production of plastic films (1). However, benzidine is no longer produced or used commercially in the US.
Benzidine-based dyes in paper, cloth, leather (Major exposure)
Contaminated water, air, soil near hazardous waste sites (Rare)
Food dyes (Rare – benzidine may be available as a byproduct)
Biotransformation
Biotransformation of benzidine in the body is through N-acetylation, which is likely to be done by N-acetyltransferases (2). Benzidine generally requires P-450 cytochrome for metabolism and to manifest genotoxicity and carcinogenicity (2). However, the isozymes involved in the process have remained unknown.
Toxicokinetics
Absorption: there are data that show the absorption of benzidine through inhalation and oral ingestion. Significantly elevated benzidine levels were detected in the urine samples of workers who had been exposed to benzidine during the work shift. In addition, inhaled particles that are coughed up by the ciliary activity can also be swallowed back down, leading to an increase in oral ingestion (2). Benzidine can also be absorbed through the dermal route (2).
Distribution: there are very limited information on benzidine information. It is unclear whether or not benzidine can cross the placenta or enter breastmilk (2).
Metabolism: benzidine is potentially involved in multiple metabolic pathways in vivo
Elimination: a small amount of benzidine is eliminated in the urine and feces. The elimination process can occur pretty quickly. In fact, benzidine and its metabolite can be detected as early as 1 hour after exposure (2). Without continuous exposure to benzidine, most of the absorbed amount can be eliminated in a week (3).
Carcinogenicity
Benzidine is categorized as carcinogenic. It has been known to increase the risk of developing bladder cancer. It has been hypothesized that the mechanism of its carcinogenicity involved the formation of reactive species from the biotransformative processes. These reactive species can produce DNA adducts and produce mutations as the result (3).
Target organ
Skin
Bladder (cancer)
Stomach, kidney, brain, mouth, esophagus, liver, gallbladder, bile duct, and pancreas can also be affected
Bladder cancer symptoms: frequent/painful urination, lower back pain, blood in the urine
Routes of exposures
Benzidine has been detected in the system after being exposed through:
Inhalation
Oral ingestion
Dermal absorption
Treatments
Biomarkers
A number of benzidine metabolites can be detected and serve as the biomarkers for benzidine exposure. These compounds are hydroxybenzidine, N-acetylbenzidine, and N,N’-diacetylbenzidine, and various glucuronic acid conjugates of these compounds (3).
Manganese is a naturally occurring transition metal and an important trace element in the body. Manganese chemical symbol is Mn and the atomic number is 25. In the body, manganese is a cofactor of many important enzymes that are involved in the synthesis of polysaccharide, fatty acids, and urea from ammonia, regeneration of red blood cells, the reproductive cycle, and chondroitin synthesis in bone matrix (1). Despite several health benefits, excessive exposure to manganese in an extended period of time can lead to manganese induced neurotoxicity, which can present as physical and/or psychological disorders (2,3,4).
In nature, manganese is a chemical element that is often found in mineral combination with iron or as oxides. Manganese compounds have many industrial applications and can be found in:
Steel industry uses manganese as an alloying constituent to improve forging qualities
Alloying agent for aluminum to increase resistance to corrosion
Alloying agent for copper to improve its mechanical strength
Batteries
Chemical industry (e.g. purifying drinking water, medicine)
There has been no evidence showing that high dietary manganese intake can result in toxicity. However, chronic exposure to manganese dust through inhalation has been reported to cause neurotoxicity, especially in the welding and mining industry (4,6).
According to the CDC, the extend of inhaled manganese absorption is based on the particle size. Smaller particles of manganese can be deposited in the lower airways and be absorbed into the blood and lymph fluids. On the other hand, larger particles can be deposited in the nasal mucosa and be transported to the brain by the olfactory or trigeminal nerves (7). Circulating manganese particles are distributed throughout the body with the highest levels found in the pancreas, liver, and kidneys (7).
Manganese – Occupational and Environmental Exposure
Carcinogenicity
Manganese and its inorganic compounds are considered to have low carcinogenic risks compared to other heavy metals. Studies have shown that manganese demonstrated weak mutagenic risk in vitro with unknown mechanisms of action. Evidence has remained insufficient to conclude that manganese can increase cancer risks in animals and humans. However, one should be cautious when being exposed to manganese chronically with respect to the impact on the central nervous system and potential harm to the fetus (8,9).
Mechanism of Action
The mechanism of action of manganese in producing neurotoxicity has not been proved yet. The most commonly reported manganese toxicity is neurotoxicity. However, inhaled manganese can also cause adverse health effects on respiratory function, reproductive system, and development in children (10). One of the hypothesized mechanisms of manganese toxicity involves the effects on cholinergic signaling.
Overview of Manganese (Mn) effects on cholinergic signaling. a Mn promotes an increase in reactive oxygen species production through of mitochondrial dysfunction. In addition, Mn impairs the synthesis of precursors for acetylcholine neurotransmitter production. b Mn induces up-regulation of nicotinic and muscarinic receptors. c Mn has a controversial effect on acetylcholinesterase since it is able to increase, reduce or not alter the activity of this enzyme across diferent models of Mn exposure
Target organs
Manganese particles are widely distributed throughout the body and can be found in:
Brain – Basal ganglia: Target for toxicity
Pancreas
Liver
Kidneys
Signs and Symptoms
Manganese can accumulate in the body and exhibit clinical signs and symptoms slowly over months or years. It can cause a permanent neurological disorder known as manganism and present as:
Irritability
Aggressiveness
Hallucinations
Tremors
Muscle spasms
Tinnitus, hearing loss
Mania
Insomnia
Depression
Delusions
Anorexia
Headaches
Changes in mood and short-term memory
Altered reaction times
Reduced hand-eye coordination
Parkinson’s-like symptoms
Besides, inhaled manganese can cause damages to the respiratory system such as lung inflammation and impaired lung function.
Routes of exposures (7)
Inhalation – Predominant route of exposure for occupational populations
Minor route of exposure for the general population:
Oral – Predominant route of exposure for the general population.
Dermal – Minor route of exposure.
Biomarkers
Manganese is often present in blood and urine samples. The urinary excretion level of manganese is considered the most indicative of the most recent manganese exposure. Thus, blood and urine levels can be used to detect manganese levels that are above the normal ranges, which are 4–15 μg/L in blood, 1–8 μg/L in urine, and 0.4–0.85 μg/L in serum (10).
Essentiality and deficiency
Manganese is needed in the body for (11):
Amino acid, cholesterol, glucose, and carbohydrate metabolism
Reactive oxygen species scavenging
Bone formation
Reproduction
Immune response
Blood clotting and hemostasis in conjunction with vitamin K
Deficiency in manganese is rare but may lead to (11):
Bone demineralization
Poor growth in children
Skin rashes
Hair depigmentation
Decreased serum cholesterol and increased alkaline phosphatase activity in men
Altered mood and increased premenstrual pain in women
Altered lipid and carbohydrate metabolism, causing abnormal glucose tolerance
Molina-Poveda, C. (2016). Nutrient requirements. In Aquafeed Formulation (pp. 75-216). Academic Press.
Buchman AR. Manganese. In: A. Catharine Ross BC, Robert J. Cousins, Katherine L. Tucker, Thomas R. Ziegler ed. Modern Nutrition in Health and Disease. 11th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2014:238-44.
Nielsen FH. Manganese, Molybdenum, Boron, Chromium, and Other Trace Elements. In: John W. Erdman Jr. IAM, Steven H. Zeisel, ed. Present Knowledge in Nutrition. 10th ed: Wiley-Blackwell; 2012:586-607.
Aschner JL, Aschner M. Nutritional aspects of manganese homeostasis. Mol Aspects Med 2005;26:353-62. [PubMed abstract]
Finley JW, Penland JG, Pettit RE, Davis CD. Dietary manganese intake and type of lipid do not affect clinical or neuropsychological measures in healthy young women. J Nutr 2003;133:2849-56. [PubMed abstract]
Assem, F. L., Holmes, P., & Levy, L. S. (2011). The mutagenicity and carcinogenicity of inorganic manganese compounds: a synthesis of the evidence. Journal of Toxicology and Environmental Health, Part B, 14(8), 537-570.
Gerber, G. B., Leonard, A., & Hantson, P. H. (2002). Carcinogenicity, mutagenicity and teratogenicity of manganese compounds. Critical reviews in oncology/hematology, 42(1), 25-34.
Fipronil (5-amino-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-(trifluoromethylsulfinyl)pyrazole-3-carbonitrile) (Figure 1), a compound that belongs to the phenylpyrazole family, is a broad-spectrum insecticide that is commonly used to eliminate fire ants, beetles, rootworms, ticks, termites, fleas (both larval and adult stages), grasshoppers, cockroaches, and many other kinds of insects. Fipronil has been known to cause central nervous system toxicity to insects. Due to its effectiveness, fipronil has been widely used in many commercial products that help eliminate common insects and pests. The safety concerns with fipronil involve the potential off-target effects of products which should be frequently observed to monitor long-term side effects and resistance development.
Fipronil can be metabolized by P450 enzymes. In general, there are three different pathways of fipronil metabolism. Fipronil can be reduced to fipronil sulfide, oxidized to fipronil sulfone, or hydrolyzed to fipronil amide (Figure 2). Animal studies have shown that fipronil sulfone is the major metabolite that can be detected in multiple organs such as brain, liver, and adipose tissues. Fipronil in the brain can be locally converted to the sulfone metabolite within 2-4 hours.
Fipronil itself can induce oxidative stress via the molecular pathways that lead to antioxidant alteration, DNA and mitochondrial toxicities, and eventually apoptosis (Figure 3).
Regarding the metabolites of fipronil, fipronil sulfone is the major metabolite in the human and rat liver. Studies have shown that fipronil and its metabolites may be able to cross the blood-brain barrier. Fipronil-desulfinyl can reach a 10-fold higher potency in mammals, which raises a concern for neural toxicity in humans who have been exposed to fipronil. In addition, fipronil can also be metabolized to form hydroxylamine metabolites via another pathway that can generate reactive intermediates.
Fipronil is absorbed very slowly through the skin. It can remain on stratum corneum, viable epidermis, and pilo-sebaceous units with a longer half-life. If fipronil is administered orally, it can have slow absorption but a large volume of distribution. A study with orally administered fipronil showed that fipronil and its metabolites are mostly excreted in feces (45-75%) and in urine (5-25%).
In animals, a two-year study in rats testing the highest dose showed evidence of benign and malignant follicular cell tumors. Rats fed with fipronil-desulfinyl only showed signs of toxicity without any evidence of carcinogenicity.
In humans, fipronil is classified as “Group C- possible human carcinogen” by the U.S EPA. Mutagenicity studies have not shown any evidence of mutations in human lymphocytes. Carcinogenic effects of fipronil have not been demonstrated in any studies in humans.
Mechanism of Action
Fipronil binds and blocks GABAA -gated chloride channels in the central nervous system. This causes the accumulation of GABA in the synaptic junctions and the inhibition of nerve transmission, resulting in neuronal hyperexcitability state, paralysis, and eventually death (Figure 4). Fipronil has a higher affinity for insect GABA receptors, which helps enhance the selectivity for insects and decrease the toxicity risks for mammals.
Wang X, Martínez MA, Wu Q, Ares I, Martinez-Larranaga MR, Anadón A, Yuan Z. Fipronil insecticide toxicology: oxidative stress and metabolism. Critical reviews in toxicology. 2016 Nov 25;46(10):876-99.
Robea MA, Nicoara M, Plavan G, Strugaru SA, Ciobica A. Fipronil: mechanisms of action on various organisms and future relevance for animal models studies. Survey in Fisheries Sciences. 2018 Aug 10;5(1):20-31.
Islam R, Lynch JW. Mechanism of action of the insecticides, lindane and fipronil, on glycine receptor chloride channels. British journal of pharmacology. 2012 Apr;165(8):2707-20.
Mohamed F, Senarathna L, Percy A, Abeyewardene M, Eaglesham G, Cheng R, Azher S, Hittarage A, Dissanayake W, Sheriff MR, Davies W. Acute human self‐poisoning with the n‐phenylpyrazole insecticide fipronil—a gabaa‐gated chloride channel blocker. Journal of Toxicology: Clinical Toxicology. 2004 Jan 1;42(7):955-63.
