Diossine, Dibenzofurani, PCBs The term Dioxin is commonly used to refer to a family of toxic chemicals that all share a similar chemical structure and a common mechanism of toxic action. This family includes: seven of the polychlorinated dibenzo dioxins (PCDDs) ten of the polychlorinated dibenzo furans (PCDFs) twelve of the polychlorinated biphenyls (PCBs).

Dibenzodiossine clorurate (diossine) 2,3,7,8-Tetrachlorodibenzo-p-dioxin TCDD

Dibenzofurani clorurati 2,3,7,8-Tetrachlorodibenzo-furan

Policloro Bifenili (bifenili policlorurati, PCBs) Clx Cly

2,3,7,8-Tetrachlorodibenzo-p-dioxin 2,3,7,8-Tetrachlorodibenzofuran 3,3',4,4',5,5'-Hexachlorobiphenyl

PCBs were produced commercially in large quantities until production was stopped in 1977. PCDDs and PCDFs are not commercial chemical products but are trace level unintentional byproducts of most forms of combustion and several industrial chemical processes. Dioxin levels in the environment have been declining since the early seventies and have been the subject of a number of federal and state regulations and clean-up actions; however, current exposures levels still remain a concern.

Meccanismo d’azione delle diossine. 1 TCDD (Dioxins) acts via the intracellular Ah receptor, which is a ligand-dependent transcription factor that functions in partnership with a second protein (known as Arnt); TCDD's effects appear likely to reflect sustained alterations in gene expression.

Composti che agiscono tramite legame al recettore Ah. PCBs coplanari

Altri composti che si legano al recettore Ah

Meccanismo d’azione e valutazione del rischio Knowledge of the mechanisms of dioxin action may facilitate the risk assessment process by imposing bounds on the assumptions and models used to describe possible responses to exposure to dioxin. In addition, mechanistic knowledge of the biochemical pathways that are altered by TCDD may identify novel targets for the development of drugs that can antagonize dioxin's adverse effects.

Polimorfismo di Ah Electrophoretic studies to evaluate the properties of specific proteins from inbred mouse strains reveal the existence of several forms of the TCDD-binding protein (Ah receptor). These observations imply the existence of multiple alleles at the Ah locus in mice. The biochemical properties of the different forms of the Ah receptor remain to be described. In particular, the extent to which the different receptor forms affect the sensitivity to TCDD is not known.

Il recettore Ah nell’uomo Human cells contain an intracellular protein whose properties resemble those of the Ah receptor in animals. Binding studies and hydrodynamic analyses have identified an Ah receptor-like protein(s) in a variety of human tissues. Functional Ah receptors have been found in many human tissues, including lymphocytes, liver, lung, and placenta.

By analogy with the existence of multiple receptor forms in mice, it is reasonable to anticipate that the human population will also be polymorphic with respect to Ah receptor structure and function. Therefore, it is also reasonable to expect that humans may differ from one another in their susceptibilities to TCDD. Both in humans and in mice, two forms of Ah receptor have been identified which exhibit a 5-10-fold difference in binding affinity for 2,3,7,8-TCDD. In humans, one form of the Ah receptor exhibits a Kd (a measure of binding affinity) for 2,3,7,8-TCDD of 0.4 nM, whereas the other form binds 2,3,7,8-TCDD with a Kd of about 2 nM.

In congenic mouse strains, expression of the lower or higher affinity forms of receptor has been extensively demonstrated to result in proportional differences in sensitivity to 2,3,7,8-TCDD with regard to biochemical changes and toxic effects. Thus, congenic mice expressing the lower-affinity form of receptor require higher doses of 2,3,7,8-TCDD to elicit these effects than strains expressing higher-affinity forms. A similar difference in sensitivity to PCDDs has also been demonstrated in tumour promotion studies in skin of congenic mouse strains. Knowledge of genetic polymorphisms that influence TCDD responsiveness may allow the identification of individuals at particular risk from exposure to dioxin.

The evidence to date implies that the Ah receptor participates in most biological response to TCDD. For example, studies of structure-activity relationships among congeners of TCDD reveal a correlation between a compound's specific binding affinity and its potency in eliciting biochemical responses, such as enzyme induction. Furthermore, inbred mouse strains in which TCDD binds with lower affinity to the receptor exhibit decreased sensitivity to dioxin's biological effects, such as thymic involution, cleft palate formation, and hepatic porphyria.

The binding and hydrodynamic properties of the Ah receptor differ relatively little across species and tissues yet responses vary widely; it is impossible, therefore, to account for the diversity of TCDD's biological effects by characteristics of the receptor alone. Mechanistic studies also indicate that several proteins contribute to TCDD's gene regulatory effects and that the response to TCDD probably involves a relatively complex interplay between multiple genetic and environmental factors.

Compensatory changes, which occur in response to TCDD's primary effects, can complicate the analysis of dioxin action in intact animals. For example, TCDD can produce changes in the levels of steroid hormones, peptide growth factors, and/or their cognate cellular receptors. In turn, such alterations have the potential to produce a series of subsequent biological effects, which are not directly mediated by the Ah receptor. Furthermore, the hormonal status of an animal appears to influence its susceptibility to the hepatocarcinogenic effects of TCDD. Likewise, exposure to other chemicals can alter the developmental toxicity of TCDD.

Tossicità equivalente: TEF, TEQ Toxicity Equivalency Factors (TEF) compare the toxicity of different dioxins. Furan (F) congener TEF Dioxin (D) congener 2,3,7,8-TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF 1,2,3,4,6,7,8,9-OCDF 0.1 0.05 0.5 0.01 0.001 2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD 1,2,3,4,6,7,8,9-OCDD 1.0 .01 .001 EPA

La tossicità equivalente (TEQ) è data da: [concentrazione] x TEF La TEQ di una miscela è pari alla somma dei prodotti [concentrazione] x TEF dei singoli congeneri. La TEQ equivale alla quantità di 2,3,7,8 TCDD che darebbe lo stesso effetto della miscela (EPA). I valori di TEF sono stati derivati principalmente da studi di tossicità acuta (in vivo e in vitro).

Sample Calculation for Toxicity Equivalent Values

L’approccio TEF/TEQ si basa su due presupposti: tutti i congeneri condividono un unico meccanismo d’azione; presenti in miscele, i loro effetti sono additivi. L’approccio TEF/TEQ non è valido per le risposte tossiche che non coinvolgono il recettore Ah.

Problemi dell’approccio TEF/TEQ 1) I valori di TEF possono variare in relazione al parametro di tossicità considerato Es. TEF 1,2,3,4,7,8 HxCDD: Perdita di peso: 0,03 Atrofia timica: 0,08 Induzione enzimatica (AHH): 0,125

2) Le differenze farmacocinetiche tra i congeneri risultano in diversi valori di TEF in dipendenza dal protocollo di esposizione. In particolare, se l’esposizione non è sufficientemente protratta, i congeneri con emivita più lunga non raggiungono lo stato stazionario  potenza sottostimata.

3) Esistono degli agonisti naturali del recettore Ah (il cui TEF è ignoto), che possono determinare un occupazione persistente, di basso livello, del recettore. Alcuni composti potrebbero agire da antagonisti.

Diverse Autorità regolatorie di diversi Paesi possono valutare valori di TEF diversi per i singoli congeneri. I valori internazionali attualmente più adottati sono quelli proposti da: WHO; commissione NATO.

Alcuni pochi congeneri contribuiscono in modo preponderante alla TEQ totale nei vari media ambientali (aria, terreno, piante, acqua).

ARIA

SUOLO

PIANTE

MARE

Contaminazione ambientale Dioxins can be commonly detected in air, soil, sediments and food. Dioxins are transported primarily through the air and are deposited on the surfaces of soil, buildings and pavement, water bodies, and the leaves of plants. Most dioxins are introduced to the environment through the air as trace products of combustion. The principal route by which dioxins are introduced to most rivers and lakes is soil erosion and storm water runoff (deflusso) from urban areas. Industrial discharges can significantly elevate water concentrations near the point of discharge to rivers and streams.

Major contributors of dioxin to the environment include: Incineration of Municipal Solid Waste Incineration of Medical Waste Secondary Copper Smelting (fusione del rame) Forest Fires Land Application of Sewage Sludge (fanghi di segheria) Cement Kilns (fornaci) Coal Fired Power Plants Residential Wood Burning Chlorine Bleaching of Wood Pulp Backyard burning of household waste may also be an important source.

Valori di TEQ nell’aria in: 1) città dell’Europa occidentale; 2) una stazione di rilevamento in Siberia.

3/4-Phase Model Input Parameters Distribution Coefficient (Kd) Definition and Use Kd is a measure of the chemical mass that partitions to both the solid and liquid phases. It is used to predict chemical partitioning and to estimate retardation.

3/4-Phase Model Input Parameters Soil Organic Carbon / Water Partitioning Coefficient (Koc) Definition Koc is Kd “normalized” to soil foc. It is the ratio of (1) the of the amount of chemical adsorbed in the soil per unit weight of organic carbon per (2) equilibrium chemical concentration in solution. Koc = g adsorbed / g organic carbon per  g/mL solution = ml/g or L/kg.