Kallikreins are a family of serine protease enzymes, fifteen members strong in humans, that play pivotal roles in biological fluids and tissues. They are best known for two starring roles: KLK1 drives the kallikrein-kinin system, a biochemical cascade central to blood pressure regulation and inflammation, while KLK3, also called prostate-specific antigen, serves as the classic biomarker for prostate cancer. But the family’s reach extends well beyond these famous examples. Deregulated activity and expression of human kallikreins have been tied to numerous pathologies, from cancer progression to inflammatory disorders, making these enzymes attractive targets for molecules that can dial their activity up or down. Because of their structural similarities across the family, the researchers used porcine pancreas kallikrein as a model enzyme, an approach justified by earlier work showing that both human and porcine kallikrein can cleave peptides derived from human and bovine kininogen.

The urgency of finding kallikrein inhibitors gained new momentum during the COVID-19 pandemic. Patients with the disease can develop pulmonary edema early on, and researchers proposed that this occurs because SARS-CoV-2 enters cells through ACE2, which normally inactivates des-Arg9 bradykinin, a potent ligand for the bradykinin 1 receptor in lung endothelial cells. Without ACE2 acting as gatekeeper, the pulmonary environment becomes subject to local vascular leak, culminating in angioedema. Blocking bradykinin receptors and inhibiting kallikrein activity, some researchers argued, might ameliorate early COVID-19 and even prevent acute respiratory distress syndrome. That hypothesis, alongside decades of evidence linking kallikreins to cancer and other conditions, has fueled a growing search for specific inhibitors as therapeutic agents.

The novelty of the new study lies in what the researchers call IMER-pKLK-MB, a bioreactor created by immobilizing porcine pancreas kallikrein on amine-terminated magnetic beads. The chemistry is elegant in its simplicity. The beads are first activated with glutaraldehyde, a bifunctional reagent whose aldehyde groups react covalently with amino groups on the enzyme, anchoring pKLK securely to the bead surface. The immobilization proved exceptionally efficient, capturing 97 percent of the enzyme’s activity. Magnetic beads bring a practical bonus: they can be rapidly separated from reaction mixtures with a simple magnet, eliminating the centrifugation or filtration steps that slow down conventional assays and making the whole workflow compatible with automation and high-throughput screening.

Stability is where this bioreactor truly distinguishes itself. After being incubated in 70 percent methanol for 15 minutes, IMER-pKLK-MB retained over 60 percent of its activity, and even pure methanol left it with 10 percent activity remaining. That tolerance for organic solvents matters enormously in drug screening, where many candidate molecules are poorly soluble in water and must be dissolved in methanol or similar solvents. Even more striking, the bioreactor allowed at least ten consecutive cycles of activity without significant loss, attesting to how securely the enzyme was anchored to the support. When the researchers measured storage stability over time, they found the bioreactor remained highly active in Tris-HCl buffer even 11 months after immobilization. Interestingly, activity actually increased during the first months, probably because the enzyme was adapting to its new environment and adjusting its active conformation in the immobilized state, before settling into a sustained high level.

To turn this bioreactor into a screening tool, the team developed an off-line assay coupled to high-performance liquid chromatography with mass spectrometry detection. The substrate they chose, Z-Phe-Arg-AMC, is cleaved by kallikrein to release a fluorescent AMC group and a product called Z-Phe-Arg-OH, which the mass spectrometer monitors at a mass-to-charge ratio of 456. The chromatographic method was carefully engineered to prevent a common pitfall in LC-MS bioanalysis: ionic suppression, where excess substrate contaminates the ionization source and distorts the signal. A C18 pre-column acts as a trap, retaining the substrate while allowing only the reaction products to reach the mass spectrometer. A ten-port switching valve redirects the flow during a pre-column cleaning step, washing away excess substrate with stronger mobile phase before returning to initial conditions. The entire analysis takes just seven minutes, a speed that makes the method genuinely suitable for screening campaigns.

The researchers systematically optimized the assay conditions, testing buffer concentration, pH, incubation temperature, incubation time, and bioreactor mass. They found optimal activity using 5 millimolar ammonium acetate at pH 9.0, a low buffer concentration that enhances MS sensitivity and reduces matrix-related ionic suppression. Room temperature, around 26 degrees Celsius, proved sufficient, dismissing the need for additional temperature control and simplifying the experimental procedure. A five-minute incubation yielded more than enough product for detection, and just 0.1 milligrams of the bioreactor was sufficient for a measurable signal, ensuring the method stayed sensitive while conserving resources.

Method qualification confirmed the assay’s reliability. The calibration curve showed a linear response for substrate concentrations from 1 to 50 micromolar, with a coefficient of determination of 0.999, and relative standard deviations below 19 percent across triplicates. The limit of detection was 1 micromolar and the lower limit of quantification was 6 micromolar. To verify that the immobilized enzyme still behaved like its free-solution counterpart, the researchers measured two key parameters. The apparent Michaelis-Menten constant, KMapp, came in at 81.2 plus or minus 18 micromolar, indicating the immobilized pKLK could still recognize its substrate, though with somewhat reduced apparent affinity compared to the free enzyme’s 23.5 plus or minus 3.3 micromolar. The reference inhibitor leupeptin, tested across a range of concentrations, yielded an IC50 of 2.15 plus or minus 0.4 micromolar, close to the values previously reported for the free enzyme in solution, confirming that immobilization did not impair the enzyme’s ability to recognize inhibitors.

Compared with traditional kallikrein activity assays that use the enzyme in solution with fluorimetric detection, the new approach offers several advantages. It allows enzyme reuse, reducing costs and increasing storage stability, and it works in the presence of organic solvents, enabling testing of a broader range of samples, including those insoluble in aqueous solutions. Perhaps most importantly, samples that fluoresce can interfere with fluorimetric assays and generate false positives, a problem entirely avoided by mass spectrometry, which brings high selectivity, sensitivity, and efficiency to the analysis. The off-line format also gives researchers flexibility: the bioreactor can be incubated with candidate molecules in microtubes, and the reaction products can then be analyzed by LC-MS at leisure, decoupling the enzymatic reaction from the instrumental analysis.

The developed and qualified method proved an important and reliable approach for screening ligands, and the researchers say it can be used to screen KLK inhibitors. As the pharmaceutical industry continues to pursue kallikrein-targeted therapies for cancer, inflammatory diseases, and beyond, tools like IMER-pKLK-MB could accelerate the identification of active molecules, turning what was once a laborious, single-use assay into a reusable, solvent-tolerant, mass-spectrometry-powered screening platform. With 97 percent immobilization efficiency, eleven months of storage stability, and ten consecutive cycles of activity, this magnetic bioreactor demonstrates how immobilized enzyme technology, thoughtfully engineered, can transform the pace of drug discovery one bead at a time.

Subject of Research: Kallikrein immobilized on magnetic beads for activity-based assays using mass spectrometry

Article Title: Kallikrein immobilized on magnetic beads for activity-based assays using mass spectrometry

Article References: Rocha, C. L., & Cardoso, C. L. (2025). Kallikrein immobilized on magnetic beads for activity-based assays using mass spectrometry. Frontiers in Analytical Science, 5, Article 1514381. https://doi.org/10.3389/frans.2025.1514381

Image Credits: AI Generated

DOI: 10.3389/frans.2025.1514381

Keywords: Kallikrein, immobilized, magnetic, beads, activity-based, assays, mass, spectrometry, scientific research