Digestive enzymes are sold by activity, not by weight

Buying an enzyme by weight is like buying a battery by its size. What you need is the work it can do, and for enzymes that is measured in activity units — under a defined assay, at a defined pH and temperature.

Why milligrams mislead

An enzyme preparation is active protein plus carrier. The ratio between them varies enormously between suppliers and grades, so 100 mg of one protease can do several times the work of 100 mg of another. Worse, activity declines during storage while weight does not, so a weight-based specification cannot detect a material that has lost potency.

The unit systems

Each enzyme class has its own conventional units, and the same enzyme may be quoted in more than one:

• Protease — HUT, SAPU, PC or FCC units, each measured at a different pH. A protease quoted in HUT (acid conditions) and one quoted in PC (alkaline) are being characterised for completely different parts of the digestive tract.
• Amylase — DU or SKB units.
• Lipase — FIP or LU units.
• Lactase — ALU units.
• Cellulase — CU units.

There is no general conversion between systems, because the assays differ in substrate and conditions. Any supplier offering a clean conversion factor between two unit systems is approximating, and it is worth knowing that before you rely on the arithmetic.

pH is part of the specification

An enzyme has an activity optimum, and a number measured at that optimum can be irrelevant to your application. A protease assayed at pH 4.7 describes stomach-phase behaviour; the same enzyme assayed at pH 7 describes something else. Specify the unit system and the assay pH together, or the figure is not comparable to anything.

What to put on the specification

• Activity per gram, with the unit system and assay method named.
• Source organism — most commercial enzymes are fungal (Aspergillus species) or bacterial, which decides vegan status and allergen declarations.
• Carrier and its proportion.
• Stability data — activity retention over shelf life at defined temperature and humidity.
• Microbiology, including limits on the production organism itself, and mycotoxin testing for fungal-derived enzymes.

Ask for the assay, not just the number

Request the full assay method alongside the activity figure. If two suppliers send the same method and the same units, you can compare them directly. If they send different ones, you have not yet received comparable quotes — however similar the numbers look.