Extracellular catalase activity protects cysteine cathepsins from inactivation by hydrogen peroxide.

The resistance of secreted cysteine cathepsins to peroxide inactivation was evaluated using as model THP-1 cells. Differentiated cells released mostly cathepsin B, but also cathepsins H, K, and L, with a maximum of endopeptidase activity at day 6. Addition of non-cytotoxic concentrations of H(2)O(2) did not affect mRNA expression levels and activity of cathepsins, while the catalase activity remained also unchanged, consistently with RT-PCR analysis. Conversely inhibition of extracellular catalase led to a striking inactivation of secreted cysteine cathepsins by H(2)O(2). This report suggests that catalase may participate in the protection of extracellular cysteine proteases against peroxidation.