Zymography is an electrophoretic technique that detects the enzymatic activity of proteases, lipases, glycosidases, and other hydrolytic enzymes. The most common application is detecting matrix metalloproteinase (MMP) activity in biological samples.

Principle

A protein sample is separated by SDS-PAGE under non-reducing conditions (no DTT or β-mercaptoethanol, these would destroy enzyme activity). The separating gel is copolymerized with a substrate, typically gelatin (0.1% w/v) for MMP-2 and MMP-9 detection, or casein for broader protease detection. After electrophoresis, SDS is removed by washing in a Triton X-100 solution, allowing the proteases to renature. The gel is then incubated in a development buffer at 37 °C for 12–48 hours, during which active proteases digest the substrate in their immediate vicinity. After staining with Coomassie Blue, clear bands appear against a dark blue background where the substrate has been digested.

Protocol

1. Prepare the separating gel with 0.1% gelatin or casein added to the acrylamide solution (8–10% acrylamide is standard for MMPs). Include a stacking gel without substrate.
2. Load samples without boiling and without reducing agents. Use a non-reducing sample buffer (62.5 mM Tris-HCl pH 6.8, 25% glycerol, 2% SDS, 0.01% bromophenol blue).
3. Run at 100–150 V at 4 °C to prevent proteolysis during electrophoresis.
4. Wash the gel in renaturation buffer (2.5% Triton X-100 in 50 mM Tris-HCl pH 7.5) for 2 × 30 minutes at room temperature with gentle shaking.
5. Wash briefly in development buffer (50 mM Tris-HCl pH 7.5, 10 mM CaCl₂, 150 mM NaCl, 0.02% NaN₃, 1 µM ZnCl₂).
6. Incubate in development buffer at 37 °C for 12–48 hours.
7. Stain with Coomassie Blue R-250 (0.5% in 30% methanol, 10% acetic acid) for 1 hour.
8. Destain (30% methanol, 10% acetic acid) until clear bands are visible.
9. Image the gel and quantify bands by densitometry.

Interpretation

Bands appear as clear zones on a blue background. Their position indicates the molecular weight (by comparison to protein standards), and their intensity is proportional to the amount of active enzyme. The molecular weight of the clear band should correspond to the pro-form or active form of the expected protease. For example, pro-MMP-9 appears at ~92 kDa, active MMP-9 at ~82 kDa, pro-MMP-2 at ~72 kDa, and active MMP-2 at ~62 kDa.

Reverse Zymography

To detect protease inhibitors (TIMPs, tissue inhibitors of metalloproteinases), the gel is copolymerized with both substrate and a standard protease (e.g., conditioned medium from HT-1080 cells containing active MMPs). After development and staining, inhibitor bands appear as dark (stained) areas where the substrate was not digested, against a clear background where the added protease digested the substrate everywhere else.

Applications

Zymography is used to measure MMP activity in tumor extracts, wound fluid, synovial fluid, and cell culture supernatants. It is a standard assay in cancer biology (MMPs promote invasion and metastasis), cardiovascular disease (plaque remodeling), and tissue remodeling. Quantitative zymography is possible with purified MMP standards loaded on the same gel.

Limitations

Zymography detects total proteolytic activity at that molecular weight, not necessarily a single enzyme, multiple proteases may co-migrate. It is also not truly quantitative in an absolute sense (substrate digestion kinetics vary between enzymes). For specific activity measurements, ELISA-based activity assays or FRET substrate cleavage assays are preferred.