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The conversion of primary structures of novel, highly selective and sensitive, internally quenched peptide substrates of the human 20S proteasome into peptide aldehydes is presented. Such covalent and reversible inhibitors differ in their primary structures and chemical moieties attached to their N‐terminal amino group. Inhibitory potency is primarily tested against proteolytic subunits of human constitutive 20S proteasome (β1c, β2c, and β5c subunits), but in some cases, also against 20S immunoproteasome β5i. Most of the peptide aldehydes act nonspecifically and bind equipotently to the corresponding proteolytic subunits of both forms of proteasome (β5c and β5i). Two inhibitors are 2.7‐times more specific for immunoproteasome over its constitutive counterpart. The antitumor activity of the selected inhibitors and MG132 (used here as the reference) is analyzed using MTT assay against nonmalignant human fetal osteoblastic hFOB 1.19, malignant human osteosarcoma MG‐63, human pancreatic cancer MiaPaCa‐2, and human acute leukemia Jurkat T cell lines. All inhibitors tested are able to decrease cell viability in a concentration‐dependent manner. One of the peptide aldehydes exerted stronger, as compared to the MG132, activity against human glioblastoma cell line U87‐MG. Moreover, its combination with dinaciclib, which is a novel, potent inhibitor of cyclin‐dependent kinases, reduces cellular metabolic activity more potently as compared to either proteasome inhibitor or dinaciclib alone. [ABSTRACT FROM AUTHOR]

Sunflower trypsin inhibitor (SFTI-1) is recognized as an attractive scaffold to designed potent inhibitors of various proteases. We have recently found that its analogues inhibit noncovalently both human and yeast 20S proteasomes. Here, a set of novel and more potent in vitro inhibitors is presented. The inhibitory potency of the peptides was assessed with human 20S proteasome in the presence or absence of sodium dodecyl sulfate and with human 26 proteasome. Their antiproliferative action against tumor (human melanoma cells A375) and normal cells (46 BR.1N human fibroblasts and HaCaT keratinocytes) was determined. The selected fluoresceine-labeled inhibitors were able to internalize into A375 cells and were sometimes present as foci in the cells. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 685-696, 2016., (© 2016 Wiley Periodicals, Inc.)