Sadoamides A and B: Bacterial Tripeptides Incorporating Nonproteinogenic Amino Acids as Proteasome Inhibitors

Chemical investigation of an actinomycete isolated from forest soil near a royal tomb site in the Republic of Korea (<i>Streptomyces</i> sp. YNK18) led to the discovery of two peptides: sadoamides A (<b>1</b>) and B (<b>2</b>). Spectroscopic analysis established <b>1</b> and <b>2</b> as all-aromatic tripeptides composed of phenylalanine (Phe) and two nonproteinogenic amino acids, 4-hydroxyphenylglycine (Hpg) and β-methyltryptophan (β-MeTrp). The relative configuration of the β-MeTrp residue was determined by <i>J</i>-based configuration analysis utilizing coupling constants and diagnostic ROESY correlations. The absolute configurations of sadoamides were determined using the advanced Marfey's method. Both compounds effectively inhibited the proteolytic activity of purified proteasomes <i>in vitro</i> and cellular proteasomes in HeLa cells, with <b>1</b> exhibiting greater potency. Sadoamide A (<b>1</b>) stabilized the short-lived antiapoptotic protein MCL1 and exerted significant cytoprotective effects against apoptosis-inducing chemical stimuli. These findings identify sadoamides as new microbial peptides that modulate the ubiquitin-proteasome system and highlight the potential of microbial metabolites to regulate critical eukaryotic signaling pathways, including apoptosis.