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Name: Tricishanna H.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: July 2003

How, and in what units, is the concentration of enzymes measured?

Dear Tricishanna,

Since enzymes are proteins, their actual concentration can be determined using any of the fairly standard protein assays available commercially. These assays typically compare the color production of a protein-binding reagent, such as Coomasie Blue, from the unknown enzyme with that from a protein "standard" of known concentration, usually to find the milligrams of protein per milliliter (mg/mL) of solution.

However, because enzymes are often not 100% pure, their concentration is more often measured in terms of their enzyme activity. Even if they have been completely purified, their activity is usually a more functionally useful value, since the amount of specific activity per mg of enzyme often varies for different enzyme preparations. Activity is a much less standardized value, however. Its measurement is different for almost every individual enzyme, and it is generically reported simply as units per mL (U/mL). The particular assay used to define these units is often established by the first investigators to purify and characterize the enzyme. However, it is not always strictly adhered to, and other investigators sometimes develop competing assays, often leading to confusion as to which units are being reported.

This is probably a more complicated explanation than you expected, but I hope that it's not too confusing for you. Thanks for the good question,

Jeff Buzby, Ph.D.
CHOC Research Institute

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