Protease inhibitors and when they interfere

A protease inhibitor is a name for a class of enzyme you intend to stop, plus a list of enzymes you might stop by accident. Lysis releases proteases that nick the protein you wanted. It is reasonable to block them. It is unreasonable to pour in every inhibitor you have ever heard of and then wonder why the His-tag column is empty or the kinase assay is dead. This glossary is organised around that collision. Fragments on a gel are interpreted with reading a protein gel. The metal-resin collision is spelled out in His-tags and why a band is not pure protein. Activity that vanishes while the band remains is the neighbouring problem in enzymes lose activity before they look degraded.

A short glossary of classes

Serine proteases use a serine in the catalytic step. Phenylmethylsulfonyl fluoride, usually called PMSF, and AEBSF are names you will see aimed at this class. Cysteine proteases use a cysteine nucleophile. Leupeptin inhibits a subset of serine and cysteine proteases. More selective cysteine-directed names exist in protocols. Metalloproteases need a metal, often zinc, and EDTA or EGTA is the blunt instrument used against them. Aspartyl proteases are the class pepstatin is associated with. A commercial cocktail is a mixture across some of these classes, sometimes offered with or without EDTA. The "with EDTA" version is a different reagent from the version without it. Read the lid.

None of those sentences is a dose. Working concentrations vary by protocol and by how long the sample will sit. Follow the manufacturer's protocol or the methods paper you are actually reproducing. Public protocol libraries such as protocols.io show the range of practice. They do not authorise a blend you invented at the bench.

PMSF in water is a short story

PMSF is typically dissolved in a dry organic solvent such as isopropanol or ethanol because it does not live long in aqueous buffer. Once you add it to a lysate, it starts to hydrolyse. The higher the pH and the warmer the tube, the less of it remains. Adding PMSF at the beginning of a two-hour lysis in a warm beaker, and then calling the sample protected at the end, overstates the case. AEBSF is used when a serine inhibitor needs to persist in water. It is still a serine-directed reagent. If your downstream step is cleavage by thrombin or another serine protease, you may have just inhibited the cutter. Dialysis or another buffer exchange may be required before cleavage. Plan for that, or choose a cleavage enzyme the inhibitors do not hit.

Collateral damage that looks like a failed prep

EDTA is the clearest double agent. It quiets metalloproteases. It also inactivates metalloenzymes whose activity you wanted to measure, and it strips immobilised nickel or cobalt so a histidine-tagged protein never binds. If the elution is empty, check whether the inhibitor tablet contained EDTA before you blame the clone. EGTA is a related chelator with its own metal preferences. Treat it as a chelator, not as a harmless buffer salt.

Leupeptin and pepstatin are smaller stories with the same shape. They are appropriate when their class is the threat, and they are contaminants when the protein you purified is itself a protease of that class. A cysteine protease assayed in the presence of leftover leupeptin can look inactive while the gel looks intact. That is inhibitor carryover, not unfolding. Remove small inhibitors before the assay if the protocol says they will interfere. Do not assume a spin concentrator removed them unless you checked.

Inclusion-body protocols sometimes skip inhibitors because the pellet is considered tough. Proteolysis still happens at the lysis and wash, and again during a long refold. Add only the classes that match the risk and that the later activity can tolerate. A gel full of fragments after a refold is a reason to revisit that list, not a reason to add every bottle.

How to reason when the gel and the assay disagree

If the gel shows fragments, inhibition was too late or the wrong class. Add the inhibitor at lysis, cold, and confirm the class. If the gel shows one full-length band and the activity is gone, suspect carryover, a missing cofactor, or EDTA in a metalloenzyme before you suspect total unfolding. Exchange the buffer and rerun the assay with a control protein that has never seen the cocktail. If a His-tag flow-through contains the product, test a load without chelator.

Record the cocktail identity, whether it included EDTA, and when it was added. "Inhibitors added" is not a reproducible phrase. UniProt entries sometimes name cofactors and catalytic classes for the protein you are protecting. Read that before you chelate the metal the entry lists.

Safety, heat and an enquiry

PMSF is toxic. Treat the solid and the solvent stock as the hazard sheet describes. This article is not a dosing guide, not a clinical recommendation, and not biosafety approval for a pathogenic lysate. Your institution sets containment.

A warm laboratory shortens the aqueous life of PMSF further and encourages you to leave lysates out. Keep the lysis cold if the protocol says cold, and do not dissolve fresh inhibitor into buffer an hour before you use it unless the reagent is one that tolerates water. After a power cut, a "cold" room may not be cold. Note that, and do not claim the inhibition timeline you planned.

For a reagent enquiry, name the protease class you must block, any enzyme or resin you must not block, and whether the cocktail may contain a chelator. Those constraints belong with a reagents and chemicals request and a quote. If inhibitors are part of a larger expression plan, the custom protein expression and purification page is an enquiry reference only. A method can be discussed. It is not a statement that lysis or purification is performed as a service.