Isolation, Spectral Assignments and Absolute Configuration of a Linear Hexapeptide from the Culture Broth of the Plant-Associated Actinomycete Actinomycetospora sp. CA-287887

A high proteolytic-resistant hexapeptide (αs1-CN 181–186) (1) along with two known 2,5-diketopiperazines, namely cyclo-(L-Pro-L-Phe) (2) and cyclo-(L-Pro-L-Tyr) (3), as well as the carboxylic acid 2-hydroxyphenylacetic acid (4), were isolated from the actinomycete strain CA287887. The morphological 16S rRNA gene sequence and phylogenetic data of the strain exhibited high similarity with members of the genus Actinomycetospora. The structure of 1 was thoroughly investigated for the first time through the extensive use of 1D and 2D NMR experiments while its absolute configuration was determined by Marfey’s analysis. The anti-tyrosinase effects of the aforementioned compounds were investigated in vitro using kojic acid as the positive control (IC50 14.07 μΜ). Compound 3 exhibited the highest activity (IC50 28.69 μΜ), followed by compound 4 (IC50 98.29 μΜ). Compound 1 was further evaluated for cytotoxicity against HepG2, A2058, A549, and MiaPaca-2 cell lines. At all the tested concentrations (0.01–200 μg/mL), no cytotoxic effect was observed.