How to buy 1g of enzyme that is sold in units/mg?

I am trying to buy 1g of horseradish peroxidase. On sigma-aldrich, it is sold as units/mg. I am confused as to what this unit is. It comes in 50-150, 150-250, and 250-330 u/mg. The way I currently understand it is if I buy 25 KU (kilo-units?) of 50-150 u/mg, than I should have at minimum 167 mg of the peroxidase. Is this a correct understanding? The price of 25 KU of horseradish peroxidase is $95, so it seems ridiculous to pay that for only 1 2/3g. 




Sigma Aldrich defines their units as:



> 
> One pyrogallol unit will form 1.0 mg purpurogallin from pyrogallol in 20 sec at pH 6.0 at 20 °C.
> 
> 
> 


Assuming you're doing a biological imaging experiment, these activity values tell you much more about what you're getting than mass.


From a casual internet search it appears that HRP is pretty expensive. I would consider whether you actually need so much of it for your research. 


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The enzyme is sold by activity, not by weight. The different descriptions from Sigma refer to the amount of active enzyme (in units according to their definition) in 1 mg **of the powder or lyophilized residue they sell you**. You will not be buying pure HRP. "Pure" HRP is a bit of a misnomer anyway, since it comes it at least seven isoforms, differing slightly in MW and glycosylation patterns, etc. So if the U/mg number is *higher*, the amount of *active* enzyme per mg of residue is *higher*.


Most HRPs seem to have a molecular weight of around 40,000 g/mol. If you knew the rate constant $k\_{cat}$ of HRP for your substrate in your assay conditions, you could use the rate constant and the MW to calculate a "theoretical" activity in terms of U/mg. My guess would be that this theoretical activity would be higher even than 250 - 330 U/mg. 


The ratio of the U/mg that you buy to this "theoretical" activity is the weight fraction of what you buy that is active enzyme. Probably for the cheapest preparation, more than 90% of the weight is not active enzyme. There are probably salts, water, and inactive/misfolded HRP molecules, and other protein contaminants in there to varying degrees.


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