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), and 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 a high similarity with members of the genus Actinomycetospora. The structure of 1 was thoroughly investigated for the first time by the extensive use of 1D and 2D NMR experiments while its absolute configuration was determined by Marfey’s analysis. The anti-tyrosinase effects of aforementioned compounds were investigated in vitro by using kojic acid as positive control (IC50 14.07μΜ). Compound 3 exhibited the highest activity (IC50 28.69), followed by compound 4 (IC50 98.29). 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.