Design of a new peptide substrate probe of the putative biomarker legumain with potential application in prostate cancer diagnosis ex vivo

The lysosomal endoprotease legumain (asparaginyl endoprotease) has been proposed as a putative biomarker in prostate tumours, in which the enzyme is markedly overexpressed. Overexpression, coupled with highly selective specificity for cleavage of substrates at the C-terminus of asparagine (Asn) residues, make legumain an attractive biochemical target for potential diagnosis, prognosis and treatment. We report the design, synthesis, characterisation and preliminary evaluation of a new rhodamine-B (Rho-B)-labelled legumain peptide substrate probe 5 [Rho-Pro-Ala-Asn-PEG-AQ(4-OH)] and its selective targeting to lysosomes in PC3 prostate cancer cells. Probe 5 was efficiently activated by recombinant human legumain to afford the high quantum yield reporter fluorophore tripeptide 4b (Rho-Pro-Ala-Asn-OH) with concomitant release of intense fluorescence. Furthermore, probe 5 was activated upon incubation with homogenates derived from fresh-frozen tissue material of prostatectomy specimens. Probe 5 represents a new viable biochemical tool for probing the activity of legumain with the potential to be used in ex vivo diagnostics in the cancer pathology laboratory.