Proteinase K: choosing a grade, reading the activity specification, and inactivating it before it ruins the next step

Specifying and using it

1. Grade is about what it does not contain. Molecular biology grade is certified free of detectable nuclease activity, which is the whole point when the enzyme is being used to protect nucleic acid. General or technical grade is cheaper and is appropriate for protein removal where downstream nucleic acid integrity does not matter.
2. Activity units, not mass. Preparations are specified in activity units per milligram against a defined substrate, and lot-to-lot specific activity varies. Dose from the units rather than the mass, particularly when switching supplier or lot, or a protocol calibrated on one preparation will under or over digest on another.
3. Conditions that raise activity. Activity increases with temperature within the working range and is enhanced by denaturants and detergents, which is why lysis buffers pair it with them. Calcium stabilises the enzyme, so chelators in the buffer reduce its working life even though the enzyme remains active.
4. Powder or solution. Lyophilised powder stores longest and needs reconstitution and aliquoting. A ready-made solution is convenient and has a shorter working life. For occasional use, powder is the better buy; for routine use, a stabilised solution aliquoted once saves handling errors.
5. Inactivate or remove before the next step. Heat inactivation, a column clean-up or a phenol extraction all work and all have consequences for yield. Choose deliberately, because residual activity carried into an enzymatic step is a failure that presents as a reagent problem three steps later.

Where the grade decision actually bites

For DNA and RNA extraction, use the certified nuclease-free grade, and treat that certification as the reason you are paying the difference. For general protein digestion, tissue dissociation or destroying protein in a waste stream, the cheaper grade is entirely appropriate.

Suppliers differ on which contaminants they certify against and at what sensitivity. Ask for the lot certificate rather than relying on the catalogue description, particularly for RNA work where a small residual activity is enough to matter.

Handling and hazard information

The powder is a respiratory sensitiser and should be weighed in a way that avoids raising dust, and the container's own hazard information is the authority on handling and personal protective equipment. This is one of the reagents where the label genuinely matters.

Reconstitute and aliquot once rather than freezing and thawing a working stock repeatedly. Repeated cycles cost activity and are a silent source of protocol drift.

Common questions

What grade of Proteinase K do I need?

Molecular biology grade, certified nuclease free, whenever the nucleic acid downstream must stay intact. General grade is fine for protein removal, tissue dissociation and any use where nucleic acid integrity is irrelevant.

How should I dose Proteinase K?

By activity units against the supplier's stated substrate rather than by mass, since specific activity varies by lot and preparation. Re-check when changing supplier or lot.

How do I inactivate Proteinase K?

Heat inactivation, a column clean-up or a phenol extraction, chosen for what the next step tolerates. It survives conditions that stop most enzymes, so removal has to be deliberate rather than assumed.

Powder or ready-made solution?

Powder for occasional use because it stores far longer; a stabilised solution for routine use because it removes a weighing and reconstitution step. Either way, aliquot rather than repeatedly thawing one stock.

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