Monitoring compartment-specific substrate cleavage by cathepsins B, K, L, and S at physiological pH and redox conditions

The results revealed distinct cleavage patterns at all conditions analyzed, indicating compartment-specific processing of thyroglobulin by cysteine cathepsins. In particular, proteolytic activity of cathepsin S towards the substrate thyroglobulin can now be understood as instrumental for extracellular thyroid hormone liberation. Our study emphasizes that the proteolytic functions of cysteine cathepsins in the thyroid are not restricted to endo-lysosomes but include pivotal roles in extracellular substrate utilization. We conclude that understanding of the interplay and fine adjustment of protease networks in vivo is better approachable by simulating physiological conditions in protease activity assays.