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.