In Situ Monitoring of Cathepsin B-Induced Mitochondrial Permeabilization by Raman Spectroscopy

Cathepsin B is a lysosomal cysteine protease that plays an important role in multiple forms of programmed cell death. The leakage of cathepsin B from the lysosome is believed to be involved in BID truncation, resulting in mitochondrial outer membrane permeabilization (MOMP). Herein, we report that cathepsin B can directly trigger MOMP by interacting with mitochondrial phospholipids. Raman spectroscopy reveals that cathepsin B disrupts the conformational order of phospholipid alkyl chains, thereby enhancing membrane permeability. More importantly, reactive oxygen species (ROS) are generated during the interactions between cathepsin B and unsaturated phospholipids, which further induce lipid peroxidation. Such a process is found to be phospholipid type-specific, and the cathepsin B-cardiolipin interaction is more efficient in generating ROS and oxidizing phospholipid membranes. Furthermore, the ability of cathepsin B to induce the release of cytochrome c from isolated mitochondria is evidenced by in situ resonance Raman spectroscopy. The newly discovered function of cathepsin B has implications for the development of new cancer therapies.