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Abstract: Background: Actinidin, a protease from kiwifruit, belongs to the C1 family of cysteine proteases. Cysteine proteases were found to be involved in many disease states and are valid therapeutic targets. Actinidin has a wide pH activity range and wide substrate specificity, which makes it a good model system for studying enzyme–substrate interactions. Methods: The influence of inhibitor (E-64) binding on the conformation of actinidin was examined by 2D PAGE, circular dichroism (CD) spectroscopy, hydrophobic ligand binding assay, and molecular dynamics simulations. Results: Significant differences were observed in electrophoretic mobility of proteolytically active and E-64-inhibited actinidin. CD spectrometry and hydrophobic ligand binding assay revealed a difference in conformation between active and inhibited actinidin. Molecular dynamics simulations showed that a loop defined by amino-acid residues 88–104 had greater conformational mobility in the inhibited enzyme than in the active one. During MD simulations, the covalently bound inhibitor was found to change its conformation from extended to folded, with the guanidino moiety approaching the carboxylate. Conclusions: Conformational mobility of actinidin changes upon binding of the inhibitor, leading to a sequence of events that enables water and ions to protrude into a newly formed cavity of the inhibited enzyme. Drastic conformational mobility of E-64, a common inhibitor of cysteine proteases found in many crystal structures stored in PDB, was also observed. General significance: The analysis of structural changes which occur upon binding of an inhibitor to a cysteine protease provides a valuable starting point for the future design of therapeutic agents. [Copyright &y& Elsevier]

Background The intestinal epithelium forms a barrier that food allergens must cross in order to induce sensitization. The aim of this study was to evaluate the impact of the plant-derived food cysteine protease — actinidin (Act d1) on the integrity of intestinal epithelium tight junctions (TJs). Methods Effects of Act d1 on the intestinal epithelium were evaluated in Caco-2 monolayers and in a mouse model by measuring transepithelial resistance and in vivo permeability. Integrity of the tight junctions was analyzed by confocal microscopy. Proteolysis of TJ protein occludin was evaluated by mass spectrometry. Results Actinidin (1 mg/mL) reduced the transepithelial resistance of the cell monolayer by 18.1% (after 1 h) and 25.6% (after 4 h). This loss of barrier function was associated with Act d 1 disruption of the occludin and zonula occludens (ZO)-1 network. The effect on intestinal permeability in vivo was demonstrated by the significantly higher concentration of 40 kDa FITC-dextran (2.33 μg/mL) that passed from the intestine into the serum of Act d1 treated mice in comparison to the control group (0.5 μg/mL). Human occludin was fragmented, and putative Act d1 cleavage sites were identified in extracellular loops of human occludin. Conclusion Act d1 caused protease-dependent disruption of tight junctions in confluent Caco-2 cells and increased intestinal permeability in mice. General significance In line with the observed effects of food cysteine proteases in occupational allergy, these results suggest that disruption of tight junctions by food cysteine proteases may contribute to the process of sensitization in food allergy. [ABSTRACT FROM AUTHOR]