Selective targeting of lysosomal cysteine proteases with radiolabeled electrophilic substrate analogs

The lysosomal cysteine proteases of the papain family are some of the best studied proteolytic enzymes. Small-molecule inhibitors and fluorogenic substrate mimics have been used to probe the physiological roles of these proteases. A high degree of homology between family members and overlap in substrate specificity have made elucidating individual protease function, expression and activity difficult.

Using peptide vinyl sulfones and epoxide as templates, we have generated probes that can be tagged with radioactive iodine. The resulting compounds covalently label various cathepsins and several unidentified polypeptides likely to be proteases. MB-074 was found to be a highly selective probe of cathepsin B activity. Probes that labeled several cathepsins were used to examine the specificity and cell permeability of the CA-074 family of inhibitors. Although CA-074 reportedly acts in vivo, we find it is unable to penetrate cells. Esterifying CA-074 resulted in a cell-permeable inhibitor with dramatically reduced activity and specificity for cathepsin B. The probes were also used to monitor protease activity in primary human tumor tissue and cells derived from human placenta.

We have generated a highly selective cathepsin B probe and several less specific reagents for the study of cathepsin biology. The reagents have several advantages over commonly used fluorogenic substrates, allowing inhibitor targets to be identified in a pool of total cellular enzymes. We have used the probes to show that cathepsin activity is regulated in tumor tissues and during differentiation of placental-derived cytotrophoblasts to invasive cells required for establishing blood circulation in a developing embryo.