Was just pointed out to me off-list that my anchovy data was off, so I
just double-checked the FDA website:
https://www.fda.gov/food/foodborneillnesscontaminants/metals/ucm115644.htm
Turns out the latest number for anchovies is 0.016 ppm, or 0.5 ug per
ounce (28g).
So, if you use a whole 2 oz can, that's still ~ 1 microgram Hg as the
Anchovie Pizza Equivalent.
And it looks like one piece of bigeye tuna sushi could be as much as
~14g*1.816ppm = 25 APEs
-James Holton
MAD Scientist
On 9/6/2017 11:44 AM, James Holton wrote:
Something that could perhaps be of use here is what I like to call the
"Anchovie Pizza Equivalent" (APE), which is about 1 microgram of
mercury. According to the Food and Drug Administration website here
in the USA the average mercury content of anchovies is 0.34 ppm, which
is about 1 microgram per ounce of fish. Tuna can be higher, but
varies a lot from fish to fish. My point here is that most
institutions regard the amount of mercury you bring onsite for
purposes of eating for lunch, be it sushi or pizza, as small enough to
be negligible. I tend to agree. So, one could argue that 1 microgram
of Hg per day is a "safe amount". Especially if you don't eat it.
In terms of protein crystals, a 100 micron wide cube has a volume of 1
nanoliter, and if it were soaked to a final concentration of 50 mM Hg
that is 1e-9 L * 50e-3 mol/L *200 g/mol = 10 ng. So, 100 protein
crystals soaked with Hg add up to roughly 1 APE. Please note that I
am in no way encouraging you to eat your protein crystals, and
especially not the solutions you soak them in. You should do your own
APE calculations for those. But I do think it important to note just
how tiny the amount of metal in our crystals really is.
Now, mercury is purportedly the second-most-toxic metal after
Plutonium. But Pu derivatives are uncommon. In fact, until recently
(4zhd) Pu derivatives were unheard of. The authors I'm sure will tell
you 4zhd involved no small amount of paperwork. But as long as you are
not working with Pu, you can regard every other metal as less toxic
than Hg.
Another good example is selenium; by far the most common metal
derivative. Although toxic, Se is also a dietary requirement. I
suppose this is an excellent demonstration of what "moderation" really
means. The Recommended Daily Allowance (RDA) of selenium in the USA
for adult men and pregnant women is 55-60 micrograms per day. In
crystals, one Se atom per 100 amino acids at 50% solvent comes to an
overall concentration of 50 mM. So, a 100 micron crystal contains
about 4 ng of Se. It would take 15,000 such crystals to add up to the
US RDA. The synchrotrons I work at don't go thought that many
crystals every day. But even if they did, I'd stick to my
commercially available multivitamin to get my dietary selenium.
So, although it is never a good idea to be sloppy with chemicals in
the lab, I think it is also important to do the math and think about
not just the toxicity of the things we work with on the bench, but the
everyday items all around us. It is never a good idea to be
antagonistic with regulators about such things. They are only trying
to do their job, and all they are trained to know about are LD50s and
how to stay as far below them as possible. A little
gently-pointed-out insight into non-lethal applications of heavy
metals can be helpful all around. The over-the-counter drug Pepto
Bismol (bismuth subsalicylate) is almost 50% bismuth by weight, a
metal that is right next to mercury on the periodic table. Brominated
vegetable oil contains no bromine, by the way. And dandruff shampoos
such as Selsun Blue make an excellent and surprisingly radiation-hard
reference for the selenium edge.
-James Holton
MAD Scientist
On 9/4/2017 3:13 AM, Dr Stephen Graham wrote:
Hi all,
(This email is aimed primarily at my UK colleagues, but feel free to
read on and gloat that you don't have to write safety forms in your
lab/country!).
I need to sort out written risk assessments for heavy atom soaking of
crystals in my lab. I wondered whether anyone would be willing to
share the risk assessments they have in their institute/company so
that I can seek inspiration and make sure I'm keeping up with best
practice.
Many thanks,
Stephen
