Unmetabolisable polyhalogenated compounds: Implications for mammalian and other vertebrate species

Abstract

The drug metabolising enzyme endowment of the higher vertebrates has a blind spot. There is no general capacity for dehalogenating polybrominated or polychlorinated aromatic compounds. Exposure to such compounds where metabolic transformation is blocked by the particular isomeric disposition of Cl- or Br-substituents, leads to their persistence and accumulation in the tissues.

The attendant health hazards of such accumulation in the tissues for various vertebrate species are now recognised, especially since the localised accidental overexposure of cattle to polybrominated biphenyls in the U.S.A., and of humans to polychlorinated biphenyls and dioxin in Japan and Italy, respectively, changed the outlook even of those not directly affected.

These sentinel events served as warnings of the long-term dangers of ambient chemicals that are inadvertently bioconcentrated as the world's ecosystem becomes increasingly contaminated with such synthetic polyhalogenated residues, which unlike the naturally occurring polyiodinated aromatic compounds related to the thyroid hormones, are resistant to biodegradation.

The females of the higher vertebrates can more readily reduce their body burdens of the unmetabolisable lipophilic polyhalogenated compounds at the expense of the next generation, by sharing the burden with their progeny via the milk or their clutch of eggs.