Application of a Sulfoxonium Ylide Electrophile to Generate Cathepsin X‑Selective Activity-Based Probes

Cathepsin
X/Z/P is cysteine cathepsin with unique carboxypeptidase
activity. Its expression is associated with cancer and neurodegenerative
diseases, although its roles during normal physiology are still poorly
understood. Advances in our understanding of its function have been
hindered by a lack of available tools that can specifically measure
the proteolytic activity of cathepsin X. We present a series of activity-based
probes that incorporate a sulfoxonium ylide warhead, which exhibit
improved specificity for cathepsin X compared to previously reported
probes. We apply these probes to detect cathepsin X activity in cell
and tissue lysates, in live cells and in vivo, and to localize active
cathepsin X in mouse tissues by microscopy. Finally, we utilize an
improved method to generate chloromethylketones, necessary intermediates
for synthesis of acyloxymethylketones probes, by way of sulfoxonium
ylide intermediates. In conclusion, the probes presented in this study
will be valuable for investigating cathepsin X pathophysiology.