Catalyzing Through Confusion: Making (Some) Sense of Enzyme Units

On the surface, it would seem easy enough to pick an enzyme (or an amount of enzyme) for an experiment. Just look at the concentration on the label, adjust accordingly, and you’re on your way. Alas, not with enzymes. The number of units used to measure enzymes is dizzying. However, it’s better now than it used to be. For enzymes, just knowing the initial amount of enzyme and substrate, and the amount of product at the end the reaction isn’t enough. The real value comes from measuring the enzyme’s activity. How much product was produced by how much enzyme in how much time? So, just looking at the enzyme unit in milligrams (mg) doesn’t tell you very much, even though a lot of consumer supplements simply list any enzyme ingredients by mg.

Enter the unit

Judging from the literature of enzymology over the past century, it would appear that there are as many enzyme measurement units as there are enzymes. There was the King-Armstrong unit for alkaline phosphatase, an Anson-Mirsky Unit for peptides or carbonic anhydrase, a Somogyi Unit to measure amylase activity, and a choice of either the Wilbur-Anderson Unit or Roughton-Booth Unit that depends on the assay you were using. For consumer products, the types of units can be even more dizzying. The main reference for consumer and commercial food units is the Food Chemical Codex (FCC). In the United States, the FCC is part of the U.S. Pharmacopeial Convention (USP). However, some enzymes here (such as trypsin and chymotrypsin) are expressed in mg, while others appear with a number of different units. But to their credit, the USP wants to harmonize all this. They won’t be the first. Since the early 1960s, biochemists have been trying to corral these various units into one. Then, the International Union of Biochemistry introduced what is now the most widely used enzyme unit: “the Unit,” or “U” for short. Its formula is simply the amount of enzyme that will catalyze the conversion of one micromole of substrate to product per minute. It’s also assumed that the reaction takes place under standard conditions (which, naturally, can significantly affect enzyme activity—Taq polymerase, for example, does not function at room temperatures). So, the Unit has been with us a long time, and can be converted to certain types of enzymes. It also can be crudely made into fractions. This has presented some problems, however. A “milliUnit” or “picoUnit” isn’t very helpful. Also, minute isn’t used under SI (Systeme Internationale), the international standards for scientific measurements.

A better Kat?

For more than 35 years, the International Union of Biochemistry has been trying to wean enzymology measurements away from the “Unit,” in favor of a measurement that better fits with SI. Called the katal (Kat), it expresses enzymatic activity as the amount of enzyme that increases reaction rates by one mole per second in a specified assay. While “moles” and “seconds” are indeed part of SI, even the IUB recognized that moles per second is probably much too large for most enzymatic reactions. But at least micromoles per microsecond, picomoles per picosecond, nanomoles for nanosecond, or any mix of these, still confirms to SI. The conversion from Unit to Katal is also relatively simple. One U (unit) correspondents to 16.67 nanokatals (nkat).

1 U catalyses a rate of 1 micromole/min 1/60 pmol/s corresponds to 16.67 nmol/s 16.61 nkat

However, the “U” continues to be favored as the more practical measurements. Manufacturers also tend to prefer the Unit to the Kat, simply because it’s more convenient to avoid fractions of units in the product’s label. Now, next time you reach for an enzyme from the lab fridge, you can calculate the actual amount that you need for the experiment.  

References

US Pharmacopeia. Presentations from enzyme workshop. https://www.usp.org/sites/default/files/usp_pdf/EN/meetings/workshops/morris.pdf International Union of Biochemistry. Units of Enzyme Activity Recommendations 1978 Eur. J. Biochem. 97, 319-320(1979) https://onlinelibrary.wiley.com/doi/10.1111/j.1432-1033.1979.tb13116.x/pdf King, E.J., and Moss, D.W. International enzyme units and isoenzyme nomenclature. J Clin Pathol 1963;16:4 391-393. https://jcp.bmj.com/content/16/4/391.citation