Vitamin A effects of PCBs and dioxins.

Vitamin A effects of PCBs and dioxins

It has been observed that certain halogenated chemicals -- among them polychlorinated biphenyls (PCBs), polybrominated biphenyls (PBBs) and dioxins -- can reduce concentrations of vitamin A circulating in the blood or stored in the liver, creating potentially serious deficiencies. What hasn't been understood is the mechanism. Now Dutch researchers working with the PCB known as 3,4,3',4'-tetrachlorobiphenyl (TCP) have unraveled its effect in rats. Their work, reported in a recent issue of TOXICOLOGY AND APPLIED PHARMACOLOGY (Vol. 85, No. 3), indicates that a metabolite of the PCB binds to a blood protein, transthyretin. This binding interferes with formation of the blood-protein complex used to transport both vitamin A and the thyroid hormone thyroxin, according to the researchers, A. Brouwer and K.J. van den Berg at the Radiobiological Institute in Rijswijk.

Like the Dutch team, Steven Aust, a toxicologist at Michigan State University in East Lansing, has found ties linking these chlorinated compounds, vitamin A and thyroxin. However, he largely discounts the applicability of the Dutch findings to most of the other related compounds of toxicological concern.

The 3,4,3',4'-TCP used in the Dutch study "is detoxified" when it is broken down in the body's metabolism process, Aust notes. "Such is not the case for dioxin" and many other PCBs, his own research indicates. He told SCIENCE NEWS that "it appears that this particular chemical -- 3,4,3',4'-TCP -- affects vitamin A differently than the other chemicals." In fact, he says, the TCP's nonmetabolizing relatives appear to raise, rather than lower, blood serum concentrations of vitamin A -- perhaps an indication that they are pulling vitamin A from its storage in the liver.

Aust's newest findings also indicate that feeding thyroxin to animals that have ingested TCDD -- the most toxic of the dioxins -- will increase the TCDD's toxicity and magnify its effects on vitamin A, causing a more severe decrease in the body level of vitamin A. Similarly, he says, he's showing that rats whose thyroid glands have been removed -- leaving them with less thyroxin -- are far less severely affected by dioxins.