*2.1. Cell Cultures*

Cell lines cultivated for the allergen treatment included HEK293 and HeLa. HEK293 cells were cultivated in Dulbecco's modified Eagle medium (DMEM, Sigma-Aldrich, St. Louis, MO, USA) supplemented with 10% heat-inactivated fetal bovine serum (FBS), 1% pen/strep (penicillin 10,000 U/mL; streptomycin 10 mg/mL) and 200 mM L-glutamine (Sigma-Aldrich, St. Louis, MO, USA), and HeLa cells were cultivated in Eagle's minimum essential medium (EMEM, Lonza, Basel, Switzerland) supplemented with 20% FBS, 1% pen/strep (penicillin 10,000 U/mL; streptomycin 10 mg/mL) and 1% (*v/v*) 200 mM L-glutamine. Cell lines were grown in a humidified atmosphere of 95% air and 6% CO2 at 37 ◦C, in T-25 flasks (Thermo Scientific, Waltham, MA, USA) until confluence and then were trypsinized (0.25% Trypsin-0.53 mM EDTA). HEK293 (250,000 cells per mL) and HeLa (300,000 cells per mL) cells were seeded at a volume of 1 mL in 12-well plates (Sarstedt, Nümbrecht, Germany) and grown to confluence before allergen treatment.

#### *2.2. Cysteine Protease Treatment*

Two proteolytic enzymes, papain from Carica papaya and actinidin from Actinidia deliciosa, were employed for the induction of acute cellular response in the HEK293 and HeLa cell lines. Commercial papain from papaya was purchased from Sigma–Aldrich (P 4762, Sigma–Aldrich, St. Louis, MO, USA), while actinidin was isolated from kiwifruit (Actinidia deliciosa) according to the previously published procedure [50]. Before the treatment, actinidin (0.9 mg/mL) was incubated for 1 h at 37 ◦C in DMEM (enzyme activation) or DMEM with an equimolar amount of E64 inhibitor (enzyme inactivation). Before the treatment, papain (0.4 mg/mL) was dissolved in the medium without or with an equimolar amount of E-64 cysteine protease inhibitor. Confluent HEK293 and HeLa cell monolayers were treated with papain, inactivated papain, activated actinidin, inactivated actinidin, or DMEM/EMEM without FBS at 37 ◦C for 0 (1 h before the treatment), 3, 6, or 12 h, respectively. The leakage of UA from HEK293 and HeLa cells into the medium was measured using the electrochemical sensor La(OH)**3**@MWCNT/CP.
