Tuesday, November 19, 2013

A Comparison of the Deleterious Impact of Human Exposure to Methylmercury and PCBs

In a scientific paper entitled, Neurobehavioral toxicity of methylmercury and PCBs Effects-profiles and sensitive populations authored by Dr. Christopher Newland from the Department of Psychology, Auburn University that appeared in the Journal of Environmental Toxicology and Pharmacology, the investigator compares the toxicology of methylmercury and polychlorinated biphenyls (PCBs) in human populations.
Both methylmercury and PCBs are known to be potent neurotoxic agents that have deleterious consequences in terms of the sensory, motor and cognitive abilities of those who have suffered sufficient exposure.

PCBs represent a class of compounds in which over 200 separate substances are members.  There appears to be a wide range of varying toxic effects among these substances depending upon their precise molecular structure.  However, they all bind to the aryl hydrocarbon receptor (ahr) found on the surface of mammalian cells.  Disruption of the usual binding to the Ah receptor can play havoc with the healthy functioning of the nervous system and can account for the wide-ranging toxic effects of PCBs.

Methylmercury is the toxic form of mercury that human populations are usually exposed to.  When ingested it is rapidly absorbed by the gut into the bloodstream and can penetrate the blood-brain barrier by two routes.  It can passively pass through this barrier due to its lipid solubility.  Secondarily, it can actively combine with the amino acid cystein that is one of the 20 amino acids that are the building blocks for the synthesis of proteins.  Once bound it readily penetrates the blood-brain barrier carried by a methionine transporter. The blood-brain barrier is the physiological barrier that usually protects the brain from any foreign and potentially harmful substances that may be circulating in the bloodstream.  Once within the brain the methyl group is enzymatically removed leaving the highly toxic mercury that can persist for a prolonged period of time.
According to the author, “methylmercury exposure affects the visual, auditory, and somatosensory systems.”
In addition, “Methylmercury exposure during adulthood produces a progressive and irreversible constriction of the visual field a pattern of toxicity not associated with developmental exposures. Methylmercury exposure during gestation or lactation affects higher-order visual function.”  Motor damage including nystagmus – rapid involuntary movement of the eyes -  that suggests damage to the cerebellum and cerebral palsy that suggests damage to the cerebral cortex have been shown in victims of the Minamata exposure in Japan as previously described.

Since PCBs and methylmercury contaminants are both found in fish and since seafood is the usual route of exposure of these dangerous chemicals for humans, it would be efficacious, from a public health standpoint, to examine the synergistic effects of these compounds on human health.   

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