Trypsin-EDTA

Trypsin is a serine protease. In the pancreas it digests dietary protein; in cell culture it does the same job to the proteins that attach a cell to its neighbours and to the plastic. It cleaves peptide bonds specifically on the carboxyl side of lysine and arginine residues, which are common enough in adhesion proteins that a few minutes of exposure is sufficient to release a monolayer.

The trypsin used in cell culture is normally crude porcine pancreatic trypsin, supplied as an irradiated powder and specified by an activity designation rather than a purity: 1:250 means one part of the powder will digest 250 parts of casein under defined conditions. It is not a purity grade, and a 1:250 preparation contains other pancreatic proteases alongside trypsin. That mixture is part of why crude trypsin works well on tough primary tissue and part of why it is harsher on delicate cells than a purified or recombinant enzyme.

EDTA is in the formulation for a specific reason that is worth understanding rather than memorising. The major cell-adhesion systems are divalent-cation-dependent. Cadherins, which mediate cell-to-cell adhesion, require calcium to hold their extracellular domains in the rigid conformation that allows binding; strip the calcium and the domains become floppy and the adhesion fails. Integrins, which mediate cell-to-substrate adhesion, require divalent cations at their metal-ion-dependent adhesion site. EDTA chelates calcium and magnesium and disables both systems.

There is a second, less obvious effect. Divalent cations in the extracellular matrix physically shield the peptide bonds trypsin needs to reach. Removing them exposes those sites. So EDTA does not merely loosen adhesion in parallel with trypsin - it makes the trypsin itself more effective. This synergy is why trypsin-EDTA works faster and at lower enzyme concentrations than trypsin alone.

The same logic explains the third component you may not have thought about: the base solution is always calcium- and magnesium-free. Trypsin-EDTA is formulated in Ca/Mg-free HBSS or PBS precisely so that the buffer does not put back the ions the EDTA just removed. Washing a monolayer with a calcium-containing PBS immediately before trypsinising defeats the design.

The percentage refers to the trypsin content by weight per volume. 0.05% is 0.5 g/L; 0.25% is 2.5 g/L, a fivefold difference in enzyme.

0.05% trypsin-EDTA is the routine choice for most established adherent cell lines - HeLa, CHO, Vero, NIH-3T3, MDCK and similar - and for anything you want to treat gently. Gibco's 0.05% product is formulated with 0.5 g/L trypsin (1:250) and 0.2 g/L EDTA-4Na in calcium- and magnesium-free HBSS with phenol red.

0.25% trypsin-EDTA is used for cells that resist detachment: firmly adherent lines, cells that have gone past confluence and deposited heavy extracellular matrix, some primary cultures, and tissue fragments. Gibco's 0.25% product contains 2.5 g/L trypsin (1:250) with 0.38 g/L EDTA-4Na in the same base. Sigma's equivalent uses 2.5 g/L porcine trypsin with 0.2 g/L EDTA-4Na in HBSS with phenol red - so EDTA content varies between suppliers, roughly in the range of 0.4-0.9 mM, and is not a standardised figure.

The practical rule is to use the lowest concentration and the shortest time that releases the cells. Higher enzyme is not a solution to a slow detachment; it is usually a symptom of one of the problems listed further down (serum carryover, cold reagent, calcium in the wash). Cells that need 0.25% for ten minutes are cells you are damaging.

Some laboratories keep a 0.25% stock and dilute it into Ca/Mg-free PBS to make 0.05% as needed. This works, is cheaper, and is perfectly acceptable provided the diluent is sterile and calcium-free. Note that diluting the trypsin also dilutes the EDTA, which changes the balance of the reagent.

Trypsin without EDTA exists and has real uses: work where chelating divalent cations would disrupt the biology under study, and applications where EDTA carryover would poison a downstream divalent-cation-dependent enzyme. It detaches cells more slowly than the combined reagent.

Phenol red or not is a formulation choice, not a functional one. Phenol red makes it easy to see the reagent and to spot pH problems; it is omitted for fluorescence work where its background matters, and for downstream applications where any dye is unwanted.

Most laboratories buy trypsin-EDTA ready-made, but preparing it from powder is cheaper at volume and is the only option when you need a formulation nobody sells. The published Gibco formulations are the target to reconstruct.

0.25% trypsin-EDTA, per litre

For 0.05% trypsin-EDTA use 0.5 g of the same powder per litre, with 0.2 g/L EDTA tetrasodium.

Method

1. Chill the base solution. Bring the Ca/Mg-free HBSS or PBS to 2-8 degrees C before you start. Trypsin is a protease and digests itself; every minute it spends warm and concentrated during preparation is activity you lose before the bottle is even finished.
2. Dissolve the EDTA first. It goes into solution readily and needs no special handling.
3. Add the trypsin powder slowly with gentle stirring, keeping the vessel cold. Do not warm to speed dissolution, and do not stir vigorously enough to foam - proteins denature at air-liquid interfaces.
4. Check the pH. Trypsin-EDTA solutions are normally formulated at pH 7.2-8.0. Adjust with dilute sodium hydroxide or hydrochloric acid if needed.
5. Bring to final volume, then sterile-filter through a 0.22 micron membrane. Never autoclave - heat destroys the enzyme completely. Trypsin solutions foul filters, so use a pre-filter and allow more membrane area than the volume suggests.
6. Aliquot immediately into single-use volumes and freeze once at -30 to -5 degrees C, protected from light. This is not optional advice: a bottle that gets thawed and refrozen repeatedly loses activity steadily and invisibly, and it is the main reason home-made trypsin acquires a reputation for being unreliable.

Weighing the powder needs care. Trypsin powder is a respiratory sensitiser. Weigh it in a fume hood or with local extraction and avoid generating dust.

Water quality carries straight through. If you are making the base solution as well as the enzyme solution, use cell-culture-grade water with a stated low endotoxin specification.

A note on what you are buying. The 1:250 designation on the powder is an activity statement, not a purity grade, and different lots of crude porcine trypsin vary. If reproducibility across months matters more than cost per litre, ready-made solution with a specified activity is the better choice, and recombinant trypsin-like reagents remove the lot-variability problem altogether.

Volumes below are for a T-75 flask at 80-90% confluence. Scale proportionally to surface area.

1. Warm the reagents. Bring trypsin-EDTA, Ca/Mg-free DPBS and complete medium to room temperature or 37 degrees C. Trypsin activity at 4 degrees C is a fraction of its activity at 37 degrees C, and cold reagent is the single most common cause of a detachment that "will not work".
2. Remove the spent medium completely. Aspirate it all. Serum in residual medium contains protease inhibitors that will neutralise the trypsin the moment you add it.
3. Wash the monolayer. Add 5-10 mL of calcium- and magnesium-free DPBS, rock gently, aspirate. This removes both residual serum and residual calcium. Use a Ca/Mg-free wash - DPBS with calcium and magnesium works against the EDTA. One wash is usually sufficient; two if the culture was in high-serum medium.
4. Add trypsin-EDTA. 2-3 mL of 0.25%, or 1-2 mL of 0.05%. Rock the flask so the whole surface is covered, then remove most of the excess if you are working with sensitive cells - a thin film is enough, and less reagent means less to neutralise later.
5. Incubate at 37 degrees C. 2-5 minutes for most lines. Do not walk away. Check the flask under an inverted microscope at 2 minutes: cells should be rounding up and beginning to detach.
6. Release the cells. When most cells are rounded, tap the side of the flask sharply with the heel of your hand. Cells should sheet off. If they do not, return to the incubator for 1 minute and check again rather than extending blindly.
7. Neutralise immediately. Add 6-8 mL of complete medium containing at least 5-10% serum, or an equivalent volume containing a defined trypsin inhibitor if you are working serum-free. Pipette gently up and down against the growth surface to break up clumps and collect the cells.
8. Centrifuge. 200-300 x g for 5 minutes. Higher speeds damage cells without improving recovery.
9. Resuspend and count. Discard the supernatant, which carries the trypsin and the inhibitor with it, resuspend the pellet in fresh complete medium, count with trypan blue and seed at the required split ratio.

Approximate trypsin-EDTA volumes by vessel

Trypsin does not stop on its own, and cells left in it continue to be digested. There are three ways to end the reaction and they are not interchangeable.

Serum-containing medium. Serum contains alpha-1-antitrypsin and alpha-2-macroglobulin, which inhibit trypsin directly. Adding two to three volumes of medium with 5-10% serum stops the reaction essentially immediately. This is the default and the reason trypsinisation is straightforward in serum-supplemented culture.

Defined trypsin inhibitor. In serum-free culture, adding serum-free medium does not stop trypsin - it only dilutes it, and cells will continue to be damaged. Soybean trypsin inhibitor (SBTI) is the standard defined alternative: a single polypeptide that forms a stable, enzymatically inactive one-to-one complex with trypsin. It is added at an approximately equimolar amount relative to the trypsin present. SBTI inhibits trypsin, factor Xa, plasmin and plasma kallikrein, but has no effect on metallo-, cysteine or aspartic proteases, which is worth knowing if your dissociation cocktail contains collagenase or dispase as well.

Dilution alone, which is sufficient only for reagents designed for it. Recombinant trypsin-like reagents of the TrypLE type are inactivated by dilution and do not require an inhibitor, which is one of the reasons they are convenient in serum-free and animal-origin-free workflows. Do not assume this behaviour for conventional porcine trypsin.

In every case, remove the inhibitor by centrifugation and resuspension in fresh medium. Leaving trypsin and inhibitor in the seeding suspension carries protein and protease-inhibitor activity into the next passage, and residual SBTI in particular can affect subsequent experiments.

Over-trypsinisation. The most common and most damaging error. Symptoms are low post-passage viability, cells that attach poorly or slowly at the next seeding, granular or blebbing cells under the microscope, and clumping from DNA released by lysed cells. Less visibly, trypsin cleaves cell-surface proteins - the receptors and markers on the outside of the cell are substrates like any other. If a flow cytometry panel loses a marker after trypsinisation that was present in the same cells harvested with EDTA alone, the enzyme cleaved the epitope. For surface-marker work, use EDTA-only dissociation or a gentler reagent, or allow cells to recover in medium for 30-60 minutes before staining.

Under-trypsinisation. Patches of cells remain attached. This is not a neutral outcome: the cells that detach first are not a random sample, so repeated under-trypsinisation quietly selects a subpopulation over many passages. Take the whole monolayer or restart.

Serum carryover - the number one reason trypsin "doesn't work". Any residual serum-containing medium neutralises the trypsin on contact. Aspirate thoroughly and wash. If detachment is unexpectedly slow, this is the first thing to check, before increasing concentration or time.

Calcium in the wash buffer. Washing with DPBS containing calcium and magnesium immediately before adding trypsin-EDTA reintroduces exactly the ions the EDTA is there to remove. Use the Ca/Mg-free formulation.

Cold reagent. Trypsin at refrigerator temperature is sluggish. Warm it.

Over-confluent cultures. Cells past confluence lay down substantial extracellular matrix and become progressively harder to lift, tempting longer digestions that damage them. Passage before the monolayer is fully confluent.

Repeated freeze-thaw. Trypsin is a protease and will digest itself. Each freeze-thaw cycle loses activity, and a bottle that has been thawed and refrozen five times behaves unpredictably. Aliquot on receipt into single-use volumes and freeze once.

Reusing trypsin. Do not. Beyond the obvious contamination risk of returning a reagent that has contacted cells to a stock bottle, the activity of used trypsin is unknown - it has been partly consumed by substrate and partly inhibited by whatever serum was carried over.

EDTA carryover into downstream enzyme steps. EDTA chelates the divalent cations that metalloproteases require. Carrying trypsin-EDTA-treated cells directly into a collagenase or dispase digestion will inhibit those enzymes, sometimes completely. Wash in a divalent-cation-containing buffer between steps.

Porcine pancreatic trypsin is an animal-derived reagent, which matters for three reasons: regulatory expectations in cell therapy and biomanufacturing, lot-to-lot variability inherent to a crude tissue extract, and the theoretical risk of adventitious agents. Several alternatives address this.

Recombinant trypsin-like proteases. Produced by microbial fermentation rather than extracted from pancreas, these give a single defined enzyme rather than a protease mixture. Dissociation kinetics are comparable to porcine trypsin with lower cytotoxicity, they are stable at room and refrigerated temperatures rather than requiring frozen storage, and dilution alone inactivates them, which removes the need for serum or an inhibitor. They are the most direct substitute for conventional trypsin in a serum-free workflow.

EDTA-only dissociation (Versene). A simple chelator solution, typically 0.5 mM EDTA in Ca/Mg-free PBS, with no enzyme at all. Because nothing is being cleaved, surface proteins are preserved intact - this is the reagent of choice when downstream flow cytometry depends on surface epitopes. It is also the standard method for clump-passaging human pluripotent stem cells, where enzymatic single-cell dissociation causes problems. The trade-offs are that it is slower, it will not lift firmly adherent cells at all, and cells come off in sheets and clumps rather than as a single-cell suspension.

Enzyme-free chemically defined dissociation media. Formulated alternatives that combine chelation with other non-enzymatic mechanisms, offering more consistency than EDTA alone while keeping surface proteins intact.

Accutase-type reagents. Mixed proteolytic and collagenolytic activity of non-mammalian origin. Gentler than crude trypsin on many cell types and widely used for stem cells and neurons, though direct comparisons have reported better viability with recombinant trypsin-like reagents than with Accutase in some systems.

Choosing between them comes down to what you need to preserve. If it is surface epitopes, use EDTA or an enzyme-free reagent. If it is regulatory compliance and reproducibility, use a recombinant enzyme. If it is speed on a stubborn primary culture, crude 0.25% trypsin-EDTA still has the edge - it is harsh precisely because it is a mixture of proteases, and sometimes that is what the tissue requires.

Liquid trypsin-EDTA is normally stored frozen, typically at -30 to -5 degrees C protected from light, with a shelf life of around 24 months. Once thawed, a working bottle held at 2-8 degrees C retains usable activity for a few weeks, though activity declines steadily through autolysis; label the thaw date.

Aliquot on receipt. Thaw the bottle once, split it into single-use volumes sized for how you actually work, and refreeze those. This is the single highest-value habit in handling trypsin, because it eliminates the freeze-thaw activity loss that makes results drift over months.

Recombinant trypsin-like reagents are generally stable at 2-8 degrees C and often at room temperature, which removes the freeze-thaw problem entirely and is one of their practical advantages.

Sterility. Trypsin-EDTA is sterile-filtered and contains no preservative. It contacts cells directly and is a common route for contamination into an otherwise clean culture. Decant rather than pipetting from the stock bottle, do not return unused reagent, and discard any bottle that looks cloudy.

pH and phenol red. Trypsin-EDTA solutions are typically formulated at pH 7.2-8.0. In phenol-red-containing formulations a bottle that has turned yellow has dropped in pH, which usually means it has been contaminated or left warm too long - discard it.

Personal protection. Trypsin powder is a respiratory sensitiser and should be weighed with extraction; the ready-made solutions are low hazard. Standard practice applies to any material of animal origin used in the laboratory.