Wednesday, June 29, 2011

BPA Followup (2/2)

This is a continuation of yesterday’s post regarding absorption of bisphenol A (BPA) through the skin. As was mentioned, an anonymous reader brought to my attention two research articles regarding the subject. One article was discussed yesterday; I’ll now discuss the other research article (open access).

I thought the results found here were quite interesting (I wasn’t expecting to read what they found), but unfortunately the researchers came across as having an expected outcome rather than looking at the results objectively as you will soon see.

The research effort in this article looked at the fate of BPA in human and pig skin. I’ve heard numerous times over the years that pig skin is probably the closest animal skin to humans, so I can see that the authors are trying to (re)establish pigskin as an acceptable model for human skin in future studies, although I’m just going to focus on the human skin results here today. I’m also not going to explain all the details of the setup, but basically they took skin samples (from the abdominal region [*]) and placed them against a collection media. The BPA would be applied to the surface, diffuse through the skin and into the media. The BPA was labeled with 14C atoms so that they could be found later in the surface of the skin, the skin itself and the media.

About 45% of the original 14C was in the media after 3 days, while about 41% was in the skin itself. A very large percentage of the material in the media (39% points) was metabolized either as BPA-glucuronide or BPA-sulfate, while only 12% points of the BPA in the skin was changed.

While numbers do seem to indicate strongly that BPA is absorbed through the skin, the authors make a statement limiting the impact of the research by those wishing to raise the alarm about potential dangers:
”The conjugation of BPA, at least with glucuronic acid, results in the cancellation of the estrogenic activity of the parent compound…it could be concluded that the high biotransformation rate of BPA by the skin does correspond to metabolic detoxification. .”
"Cancellation of the estrogenic activity" and "metabolic detoxification". Why didn't this make it to the headlines? The fact that absorbed BPA is no longer BPA is a game changer. Future research in this area should now focus on what the effects of exposure to BPA-glucuronide and BPA-sulfate are.

I don't understand why these results were downplayed, especially since these sentences are the second and third from the end of the article (overlooking the very short conclusions section). Remember how I said that the authors have an objective in mind? They show it with last sentence of the article:
”This remains highly speculative as BPA conjugates may be converted back in the parent compound at the level of target tissues.”
Somehow it was beyond them (and their reviewers) that this report should change the direction of future research, but they just didn't want to go there. Very sad.

[*] An earlier post showed that abdominal skin was more porous than skin in the palms - looking at the graph in that post, I’d call it about 50% more porous. I don’t see that being much of an issue here since the results were mostly qualitative.


Chemjobber said...

Worth noting that glucuronides are conjugates tagged for disposal via the kidneys.

John said...

How'd you learn so much biochemistry?

Chemjobber said...

Audited a class on medicinal chemistry in grad school, reinforced by working with med chemists during my postdoc. Undergrad degree in biochemistry, too.