organohalogens: stable = toxic?

[Tony Tweedale <esact@selway.umt.edu>]

i've been looking for some answers to this q. for a few weeks. glad to 
see this list up and running, it could be an invaluable resource. here's 
hoping it attracts a wide spectrum of pov's because we can rely on the 
facts of the hazards of dioxin like compounds to speak for themselves.



what's the relationship between organohalogen chronic toxicity and their 
stability?

other than the dioxin, etc. produced in natural fires, gribble in his recent 
_es&t_ article had no proof of stable, persistant o-hal's being utilized in 
metabolism anywhere in nature. in fact, labile halogen's (eg CHCl3) seem 
largely (only?) to be used as a toxic agent by plants and microbes. 

on the other side of the question, the C-halogen bond is almost uniquely 
strong. i hear that the C-F bond is not broken in metabolism, tho it is 
made. Cl & Br are not quite that stable. tcdd dioxin has been radio-labled 
and detected attached to dna, after being placed outside of cells. yet 
generally such toxicity seems associated with unstable, reactive metabolites 
with a strong electrical charge (redox) gradient, eg free radicals. [other 
toxicities are, eg, unbalancing hormone systems by binding with receptors].

in sum: are o-hal's stability & their chronic toxicity related; if so, 
doesn't their stability make them unreactive in the presence of biological 
enzyme systems?
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