Toxicity of Dibenz[a,h]anthracene

Dibenz[a,h]anthracene is an ‘alternant’ class of polycyclic aromatic hydrocarbons (PAHs) with five fused benzene rings. It has a molecular weight of 278.33, with a melting and boiling point of 266 and 524°C, respectively. It generally exists as colorless crystalline plates or leaflets. Being a pure organic compound, dibenz[a,h]anthracene is soluble in organic solvents like petroleum ether, benzene, ether, and toluene but insoluble in aqueous media. The measured log octanol/water partition coefficient is 6.84. Dibenz[a,h]anthracene was the first pure PAH demonstrated to be carcinogenic. Due to incomplete combustion of organic matters, it is widespread in the environment.

Environmental Fate

Dibenz[a,h]anthracene is largely associated with particulate matters, soils, and sediments. Its presence in places distant from primary sources indicates that it is reasonably stable in the atmosphere and capable of long-distance transport.
Dibenz[a,h]anthracene can be adsorbed very strongly if released to the soil. However, no leaching to the groundwater or hydrolization or evaporation from soils surface is expected.With half-lives of 18 and 21 days, it is generally subjected to biodegradation in soil systems. Volatilization of dibenz[a,h] anthracene from wet soil surfaces is not expected to be an important fate process based on an estimated Henry’s Law constant of 7.3×10^-8 atm-m3 mol^-1. A biodegradation halflife of 750 days at 20°C after incubation with unacclimated soil microcosms indicates that biodegradation is not an important environmental fate process in soil. 

Dibenz[a,h]anthracene released to the atmosphere will likely be associated with particulate matter and may be subjected to moderately long-range transport, depending mainly on the particle size distribution and climatic conditions, which will determine the rates of wet and dry deposition. The estimated vapor pressure of 9.5×10¹⁰ mm Hg at 25°C of dibenz[a,h] anthracene indicates that this compound will exist solely in the particulate phase in the ambient atmosphere if released into air. Its presence in areas remote from primary sources demonstrates the potential for this long-range transport as well as dibenz[a,h]anthracene’s considerable stability in the air. Dibenz[a,h]anthracene absorbs solar radiation strongly, suggesting that it may be susceptible to direct photolysis in the environment. The estimated vapor phase half-life in the atmosphere is 1 day as a result of reaction with photochemically produced hydroxyl radicals.

Mechanism of Toxicity 

Dibenz[a,h]anthracene is enzymatically attacked by the cytochrome P450-dependent monooxygenase system at three distinct molecular regions, that is, the bay, M, and K regions, yielding the arene oxides at the 1,2; 3,4; and 5,6 positions. The arene oxides may spontaneously aromatize to phenols or undergo epoxide hydrolase-catalyzed hydration to transdihydrodiols. The bay-region theory predicts that dibenz [a,h]anthracene 3,4-diol-1,2-epoxide is an ultimate carcinogenic metabolite of dibenz[a,h]anthracene. Because dihydrodiol formation precedes epoxidation at the adjacent double bond, dibenz[a,h]anthracene 3,4-dihydrodiol is the immediate metabolic precursor of the bay-region diolepoxide of dibenz[a,h]anthracene. The interaction between such species and the nucleophilic centers in cellular DNA can result in the formation of covalent DNA adducts, a process generally considered to lead to an initiating event in tumorigenesis.

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