2. LITERATURE REVIEW 13
2.1.3. Toxicity of PAHs
Many PAHs have toxic, mutagenic and/or carcinogenic properties, but numerous studies have indicated that one-, two- and three-ring compounds are acutely toxic (Sims and Overcash, 1983) and HMW PAHs are considered to be genotoxic (Lijinsky, 1991;
Mersch-Sundermann et al., 1992). Being highly lipid-soluble, these are readily absorbed from the gastro-intestinal tract of mammals (Cerniglia, 1984) and are rapidly distributed in a wide variety of tissues with a tendency for localization in body fat. PAH intermediates produced by incomplete degradation pose further potential risk for humans (Kazunga and Aitken, 2000).
The photo-oxidized PAHs are in many cases more toxic than the parent compounds (McConkey et al., 1997). Metabolism of PAHs occurs via the cytochrome P450-mediated mixed function oxidase system with oxidation or hydroxylation as the first step (Stegeman et al., 2001).
A variety of PAHs taken up in the human body undergo metabolic activation. The initial step in the metabolism of PAHs involves the multifunctional P-450 enzyme system forming different epoxides, which are short-lived compounds and may rearrange spontaneously to phenols or undergo hydrolysis to dihydrodiols. These products may then be conjugated with glutathione, glucuronic acid or sulfuric acid for easy excretion.
However, the dihydrodiols may also act as a substrate for cytochrome P-450 again to form new dihydrodiol epoxides, which are unfortunately poor substrates for further hydrolysis. These dihydrodiol epoxides may instead react with proteins, RNA and, most seriously, DNA, thus causing mutations and possibly cancer (Xue and Warshawsky, 2005).
PAHs are seldom encountered individually in the environment and many interactions occur within a mixture of PAHs whereby the potency of known genotoxic and carcinogenic PAHs can be enhanced (Kaiser, 1997). PAHs may also interact in the carcinogenic process, for example, promoting cellular proliferation (Delistraty, 1997). As a group, PAHs have shown varying ability to induce cancer; hence it is difficult to identify the structural features associated with their carcinogenic activity. However, for unsubstituted PAHs, it seems that a minimum of four benzene rings is required to exhibit carcinogenic activity (Pickering, 1999), but it still unproved that all PAHs with four benzene rings are carcinogenic. Some PAHs are very weak while others are strongly
carcinogenic, e.g. benzo[a]pyrene. Structure-activity relationships become even more complex when substitution of the molecular structure occurs; for example, although benz[a]anthracene is a fairly weak carcinogen, 7,12-dimethylbenz[a]anthracene is a very potent carcinogen. Furthermore, some environmental transformation products of PAHs may react directly with DNA causing mutations and possibly cancer without the need for metabolic activation (Moller et al., 1985).
The toxicity of naphthalene has been reported in laboratory animals (Goldman et al., 2001), and is shown to bind covalently with molecules in liver, kidney and lung tissues, thereby enhancing its toxicity from being a simple inhibitor of mitochondrial respiration (Falahatpisheh et al., 2001). Acute naphthalene poisoning in humans can lead to haemolytic anaemia and nephrotoxicity, which is in addition to dermal and ophthalmologic changes in populations occupationally exposed to this compound.
Phenanthrene, a three member ring PAH, is known to be a photosensitizer of human skin, a mild allergen and mutagenic to bacterial systems under specific conditions. It is a weak inducer of sister chromatid exchanges and a potent inhibitor of gap junctional intercellular communication (Weis et al., 1998). The toxicity of benzo[a]pyrene, benzo[a]anthracene, benzo(b)fluoranthene, benzo(k)fluranthene, dibenz(a,h)anthracene and indeno(1,2,3-c,d)pyrene are also studied and shown that they are carcinogenic (Liu et al., 2001). Information on toxicity of other PAHs such as acenaphthene, fluranthene and flourene with respect to their toxicity to mammals is still unknown.
Pyrene has often been considered as a model compound for toxicity studies as its structure is often found in the molecules of other highly carcinogenic PAHs.
Brown et al. (2004) studied toxic effect of pyrene on survival, reproduction, ethoxyresorufin-o-deethylase (EROD) activity (to measure the catalytic activity of cytochrome P4501a) and catalase activity of earthworm Lumbricus rubellus in contact and soil tests. The authors observed that at higher pyrene concentrations there was a steady concentration related decrease in survival and calculated LC50 values of 6.8 mg l-1 for the contact test and 283 mg kg-1 in the soil test from the survival data. Cocoon production rate was significantly reduced compared to controls in the soil test at higher concentrations and was completely ceased at 640 mg kg-1. Though, no EROD activity could be detected, catalase activity in the soil test was also significantly lower at the above concentration compared to all other treatments and the control.
Krasnov et al. (2005) studied toxic effect of pyrene on the transcriptomes of juvenile rainbow trout kidneys and livers by exposing the fish to sub-lethal doses for 4 d and measuring expression of 1273 genes using a cDNA microarray. The authors reported chemical toxicity in metallothionein and mitochondrial proteins of oxidative phosphorylation. Expression of mitochondrial and heat shock proteins were stimulated, whereas genes involved in humoral immune response and apoptosis were suppressed.
Pyrene affected mainly genes implicated in the maintenance of the genetic apparatus, immune response, glycolysis and iron homeostasis.
Incardona et al. (2006) studied mechanism of developmental toxicity of pyrene in zebrafish, and they observed activation of the aryl hydrocarbon receptor (AHR) pathway by pyrene resulting in induction of cytochrome P4501A (CYP1A). The authors observed induced CYP1A expression throughout the vascular endothelium, including the majority of blood vessels in the head and trunk, and in developing hepatocytes, when exposed to
pyrene. Continuous exposure to pyrene (soon after fertilization) resulted in a syndrome of systemic toxicity in early larval stages. The visible signs of pyrene toxicity included dorsal curvature of the body axis, reduced peripheral circulation, anemia, pericardial edema that evolves into yolk sac edema, and cell death beginning in the brain and later involving the spinal cord. Pyrene-exposed larvae began to die during the fifth day when the liver of pyrene exposed larvae appeared opaque with irregular margins, appeared congested with enlarged vacuolated hepatocytes.
Petersen and Dahllöf (2007) studied potential toxicity of pyrene on natural algae from an arctic sediment from shallow-water marine bay. The authors observed direct toxicity of pyrene affecting the algal community reflected in decreased 14C-incorporation.
The decrease was most pronounced under light where a decrease to 34% compared to that of control (without pyrene) was observed. Ammonium, nitrate and silicate uptake by the algae and its total DNA content was significantly decreased in the presence of pyrene as a result of decreased growth leading to increased algal death.
Stabenau et al. (2008) studied effect of pyrene exposure in the leopard frog (Rana pipiens). After exposure for seven days in pyrene saturated water aquaria, the authors measured exercise duration, muscle contractile ability, glycogen levels and mitochondrial respiration, which revealed that pyrene exposure produced many adverse effects in leopard frogs including significant reductions in exercise performance, muscle contractile ability and alterations to muscle mitochondrial oxygen consumption.