Magic Red® Fluorescent Cathepsin K Assay Kit

Quantitatively monitor intracellular cathepsin K activity over time in vitro. The Magic Red® substrate fluoresces red upon cleavage by active cathepsin enzymes. The signal can be analyzed using fluorescence microscopy or a fluorescence plate reader.

SKU: 939

Size: 25 Tests
Sale price$206.00

Our Magic Red® Cathepsin assay kits enable researchers to quantitate and monitor intracellular cathepsin B, K, or L activity over time in vitro. The Magic Red® (MR) reagent is a non-cytotoxic substrate that fluoresces red upon cleavage by active cathepsin enzymes.

Elevated cathepsin enzyme activity in serum or the extracellular matrix often signifies a number of gross pathological conditions. Cathepsin-mediated diseases include: Alzheimer’s; numerous types of cancer; autoimmune related diseases like arthritis; and the accelerated breakdown of bone structure seen with osteoporosis. Up-regulated cathepsin B and L activity has been linked to several types of cancer. These include cancer of the colon, pancreas, ovaries, breast, lung, and skin (melanoma). Upregulation of cathepsin K has been shown in lung tumors. Increased cathepsin K activity has also been linked to degenerative bone diseases including osteopetrosis and post-menopausal osteoporosis.

Cathepsins are usually characterized as members of the lysosomal cysteine protease (active site) family and the cathepsin family name has been synonymous with lysosomal proteolytic enzymes. In actuality, the cathepsin family also contains members of the serine protease (cathepsin A and G) and aspartic protease (cathepsin D and E) families as well. These enzymes exist in their processed form as disulfide-linked heavy and light chain subunits with molecular weights ranging from 20-35 kDa. Cathepsin C is the noted exception, existing as an oligomeric enzyme with a MW ~200 kDa. Initially synthesized as inactive zymogens, cathepsins are post-translationally processed into their active configurations after passing through the endoplasmic reticulum and subsequent incorporation into the acidic environment of the lysosomes.

Magic Red® detection substrates utilize the photostable red fluorophore, cresyl violet. When bi-substituted via amide linkage to two cathepsin target peptide sequences, such as (leucine-arginine)2, the bi-substituted cresyl violet is non-fluorescent. Following enzymatic cleavage at one or both arginine (R) amide linkage sites, the mono and non-substituted cresyl violet fluorophores generate red fluorescence when excited at 550-590 nm. Our Magic Red® cathepsin B substrate, MR-(RR)2, is comprised of cresyl violet coupled to two pairs of the amino acid sequence, arginine-arginine (RR), which is the preferential target sequence for cathepsin B. In our cathepsin K substrate, MR-(LR)2, cresyl violet is coupled to two pairs of leucine- arginine (LR). Our MR cathepsin L substrate, MR-(FR)2, contains two pairs of phenylalanine-arginine (FR) coupled to cresyl violet. Cathepsins, like most other crucial cell survival enzymes, are somewhat permissive in the target amino acid sequence they will recognize and cleave. Although Magic Red® substrates contain the amino acid target sequence preferred by a particular cathepsin enzyme, they can also recognize other active cathepsins or proteases when they are present. We encourage validation of cathepsin activity by an orthogonal technique.

To use Magic Red®, add the substrate directly to the cell culture media, incubate, and analyze. Because MR is cell-permeant, it easily penetrates the cell membrane and the membranes of the internal cellular organelles - no lysis or permeabilization steps are required. If cathepsin enzymes are active, they will cleave off the two dipeptide cathepsin targeting sequences and allow the cresyl violet fluorophore to become fluorescent upon excitation. The red fluorescent product will stay inside the cell and will often aggregate inside lysosomes (cathepsins are lysosomal) and other areas of low pH, such as inside the mitochondria. As protease activity progresses and more MR substrate is cleaved, the signal will intensify as the red fluorescent product accumulates within various organelles, enabling researchers to watch the color develop over time and quantify cathepsin B, K, or L activity. By varying the duration and concentration of exposure to the MR substrate, a picture can be obtained of the relative abundance of cathepsin enzymatic activity. Positive cells will fluoresce red and have pronounced red lysosomes and mitochondria. Negative cells will exhibit very low levels of background red fluorescence evenly distributed throughout the cell. This background level of substrate activity could be the result of constitutively synthesized serine proteases that target analogous amino acid sequences for hydrolysis. Please note that Magic Red® substrates can undergo spontaneous hydrolysis over time, resulting in increased background fluorescence. Appropriate controls are necessary for accurate interpretation of the results. There is no interference from pro-cathepsin forms of the enzymes. If the treatment or experimental condition stimulates cathepsin activity, cells containing elevated levels of cathepsin activity will appear brighter red than cells with lower levels of cathepsin activity.

The MR fluorophore, cresyl violet, fluoresces red when excited at 550-590 nm. The red fluorescent signal can be monitored with a fluorescence microscope or plate reader. It has an optimal excitation of 592 nm and emission of 628 nm. Hoechst 33342 is included with the kit to concurrently label nuclei after labeling with MR. It is revealed under a microscope using a UV-filter with excitation at 365 nm and emission at 480 nm. Acridine orange (AO) is also included in the kit to identify lysosomes and other intracellular organelles. It is revealed under a microscope using excitation at 480 nm and emission at >540 nm, or alternatively with excitation at 550 nm and emission at >610 nm.

Cathepsin K
592 nm / 628 nm
Fluorescence Plate Reader, Fluorescence Microscope
Cell culture
Domestic: Overnight Delivery; International: Priority Shipping
United States
  1. Prepare samples and controls
  2. Reconstitute Magic Red by adding 50 µL DMSO.
  3. Dilute Magic Red 1:10 by adding 450 µL diH2O.
  4. Add 20 µL Magic Red to each sample (~ 500 µL aliquot of cultured cells).
  5. Incubate while protected from light.
  6. Watch color start to develop within 15 minutes.
  7. If desired, label with additional stains, such as Hoechst, DAPI, Acridine orange, or an antibody.
  8. If desired, fix or embed cells.
  9. Analyze with a fluorescence microscope or fluorescence plate reader. Magic Red has an optimal excitation at 592 nm and emission at 628 nm.

If working with adherent cells, please see the manual for additional protocols.

Kit 939: 25 Tests
  • Magic Red Reagent (MR-LR2), 1 25-test vial, #6135
  • Hoechst 33342 Stain, 1 mL, #639
  • Acridine Orange Stain, 0.5 mL, #6130
  • Kit Manual
  • Kit 940: 100 Tests
  • Magic Red Reagent (MR-LR2), 1 100-test vial, #6136
  • Hoechst 33342 Stain, 1 mL, #639
  • Acridine Orange Stain, 0.5 mL, #6130
  • Kit Manual
  • Product Specific References

    PMID Publication
    36810735Ng, PY, et al. 2023. Sugar transporter Slc37a2 regulates bone metabolism in mice via a tubular lysosomal network in osteoclasts. Nature communications, 906.
    37625588Noda, K., et al. 2023. Characterization of Rab32- and Rab38-positive lysosome-related organelles in osteoclasts and macrophages. The Journal of biological chemistry, 105191.

    Question: Can we fix cells after magic red staining and then permeabilize and incubate with LAMP1 antibody to co-stain for lysosomes?

    Answer: We have not done any fixation procedures with this substrate but others have used a 4% paraformaldehyde treatment for 30 minutes with cold storage to extend their viewing time. Cleavage of the aspartic carboxyl frees up two primary-secondary amino groups that can react with the aldehyde functionality of the formaldehyde molecules. They don’t have the nice reactivity of a classic free amino group like you find on a lysine for instance but they are reactive and apparently fixable.

    Question: My second question was if I can permeabilize cells afterfixation followed by labelling cells with LAMP-1/2 antibody to visualize lysosomes, would that work?? I am afraid of using acridine orange which cannot be used in same cells and may overlap with magic red. This LAMP labeling may help me to ascertain lysosomal vs cytoplasmic location of cathepsin since my major interest is to look for lysosomal membrane permeabilization. If you any other product related to LMP or any other advice which can be used for magic red staining in context of LMP, please do let me know.

    Answer: We don’t foresee any issues with your proposed antibody labeling following Magic Red staining provided the fluorophore tag on the LAMP antibody is compatible with the fluorescence emission spectrum of the cleaved Magic Red substrate dye (cresyl violet). Acridine Orange is included in the kit to visualize lysosomal structures, however you are correct that using Magic Red substrate and Acridine Orange in the same cells should be avoided because of the overlap in emissions. We have not personally attempted to co-stain with other lysosomal markers beside Acridine Orange (AO) so unfortunately I can’t offer further advice. We intend to add an additional 2 or 3 lysosomal staining probes to our company product offering listings. But presently, we only included an example of AO staining to support the premise that our Magic Red substrates are capable of penetrating both the cell and lysosomal lipid bilayer membrane structures. Once inside lysosomal structures, our Magic Red cathepsin substrates can be converted into a fluorescent form of the substrate and detected by fluorescence microscopy techniques.

    Question: I am writing you because I have a question and to let you know that I was able to fix the MagicRed CTSK probe in tissue and extend the life a couple of days. My question is, do you know the molecular weight of the MagicRed Cathepsin K probe?

    Answer: The molecular weight of Magic Red Cathepsin K Substrate (MR(LR)2) is 1068.

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