molecular and radioactivity

[Dr Engelbert Buxbaum]

Am 14.12.2009, 15:15 Uhr, schrieb Allan Jones <allan.jones from gmx.de>:


> Take the 35S methionine i used for example: The ALI (annual limit on
> intake) for this isotope is 10 mCi. I used in total for all my  
> experiments
> less than 1 mCi. As I was not eating the stuff, worked with gloves and
> tried to be reasonably careful, I assume not to have incorporated any
> radiation, but even if I had 'eaten' the entire aliquot I would still  
> only
> have doubled my annual background radiation, as it would not even amount  
> to
> 10% ALI.
>
> 32P is of course a stronger source and actually requires adequate
> shielding, but still you have a dosimeter monitoring whether or not you
> have been overexposed.

And even that danger is overrated. Leukemia patients used to get 10 mCi  
32P i.v., that is enough to run a major lab for a year or so. And nobody  
worried about their wastes, while in the lab we carefully collect  
everything!

It does make sense to replace 32P by 33P though, not only is the radiation  
softer, but you get a longer half life too.

There are procedures where radioactivity is irreplaceble, I used to do  
enzyme kinetics on ATPases which required measuring hydrolysis as function  
of [ATP] down to 10 nM or so. Given that you need to work under initial  
rate conditions you can have a maximum of 10% hydrolysis, that is 1 nM in  
10 uL of sample, and you need a c.v. of less than 2% for meaningful  
results. Measuring phosphate liberation from gamma-33P-ATP is actually  
simple, I did up to 400 time-courses a day, each with 7 data points (by  
hand, not with a robot!).