Modulating the Cellular Uptake of Fluorescently Tagged Substrates of Prostate-Specific Antigen before and after Enzymatic Activation

A series of peptides based on the prostate-specific antigen (PSA)-specific sequence histidine-serine-serine-lysine-leucine-glutamine were functionalized with an anthraquinone fluorophore at the C-terminal residue side chain using the copper(I)-catalyzed azide–alkyne cycloaddition reaction. The effect of incorporating a negatively charged N-terminal tetra-glutamic acid group into the substrate and the effect of masking the negatively charged C-terminal carboxylic acid functionality of the substrate were investigated using confocal fluorescence microscopy in two cell lines, DLD-1 and LnCaP. The addition of a tetra-glutamic acid group to the N-terminus of the intact sequence was shown to reduce cellular uptake of the intact substrate prior to activation by PSA. In contrast, masking the C-terminal carboxylic acid group of the substrate as a methyl ester was shown to improve cellular uptake of the peptide fragment after activation by PSA. The synthesized C-terminal methyl ester substrates with the anthraquinon...