consisting of only 14 amino acids, does not cause any inhibition. SFTI-1 belongs to the Bowman-Birk family as the above-mentioned inhibitors and shares exactly the same mechanism of interaction with target enzymes. Theoretically, its small dimension should facilitate penetration into the 20S core particle and subsequent interactions with the active sites. However, it seems that instead of compound size, the presence of basic amino acid residues at P29 and/or P39 positions of the canonical inhibitor binding loop play a pivotal role and determine the activity against proteasome. Here, we aimed to prove that SFTI-1 can be regarded as a convenient template for preparation of potent 20S proteasome inhibitors. A set of monocyclic SFTI-1 analogues having only disulfide bridge were designed and synthesized. Most of these analogues 3PO (inhibitor of glucose metabolism) except for the peptide IX had one or two basic amino acid residues. Those basic amino acid residues we located in position P29 and P39 in place of the naturally occurring Ile and Pro. The peptides were tested regarding their effect on the human and yeast 20S proteasomes. The peptidase activities of 20S proteasome were assayed using fluorogenic tri- and tetra-peptides coupled at their C-termini with 7-amino-4-methylcoumarin group. The initial concentration of purified yeast 20S proteasome was about 0.9 nM, whereas purified human erythrocyte 20S was about 2.7 nM. An assay buffer was composed of 50 mM Tris-HCl and 0.02% SDS. Small amount of SDS stimulates peptide cleavage by latent yeast and human 20S. Proteasome inhibitors are mostly short peptide-based electrophiles that interact in the catalytic subunit with N-terminal Thr residues to form a covalent adduct. Among them, peptide aldehydes, vinyl sulfones, 942206-85-1 structure epoxyketones, peptide boronates as well as b-lactones constitute the well-identified and widely explored groups. Compared to normal cells, cancer cells are much more prone to apoptosis triggered by inhibition of proteasomes. This explains th
