Halogenated biphenyls: molecular toxicology.

Area of Science:

• Environmental Chemistry
• Toxicology
• Molecular Biology

Background:

• The biological and toxic effects of polychlorinated biphenyls (PCBs) are strongly influenced by their specific chemical structures.
• Certain PCB congeners exhibit toxicity and biological activity profiles similar to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Purpose of the Study:

• To investigate the structure-activity relationships of polychlorinated biphenyls (PCBs) concerning their toxic effects.
• To determine the role of PCB structure in the induction of hepatic microsomal enzymes, particularly aryl hydrocarbon hydroxylase (AHH).

Main Methods:

• Assessed the induction of hepatic microsomal benzo[a]pyrene or aryl hydrocarbon hydroxylase (AHH) by various PCB congeners in rats and rat hepatoma cells.
• Examined the relationship between PCB structure, AHH induction potency, and binding affinity for the Ah receptor protein.
• Investigated the influence of lateral substituent polarizability on AHH induction activity.

Main Results:

• Highly toxic PCBs, with lateral substitutions and no ortho-chloro substituents, are potent inducers of AHH activity, similar to TCDD.
• Mono-ortho substituted PCBs also induce AHH and phenobarbitone-inducible enzyme activities.
• PCB binding affinities for the Ah receptor correlate with AHH induction potency, indicating a shared mechanism with TCDD.

Conclusions:

• PCB structure is a critical determinant of both toxic effects and AHH induction.
• Lateral substitution patterns and the absence of ortho-chloro substituents are key features for potent AHH induction and toxicity.
• The polarizability of lateral substituents influences AHH induction, but other structural factors also play a significant role.