New Sulfur-Containing Diketopiperazine from Marine-Derived Bacteria Streptomyces rochei sp. 81 with Potent Carbonic Anhydrase II Inhibition

The ethyl acetate extract of the marine bacterial strain Streptomyces rochei sp. 81 was purified by silica gel column chromatography to obtain three diketopiperazines (1–3) and one straight chain fatty acid (4). The compounds were identified as cyclo(L-leu-N-ethenethiol-L-pro) (1), cyclo(L-Leu-L-Pro) (2), cyclo(L-Pro-D-Tyr) (3), and 2,4-dimethyldocasonoic acid (4) by HR-ESI-MS and NMR techniques. One new (1) and three known compounds (2–4) were screened for carbonic anhydrase-II (CA-II) inhibition and as a result 1 and 3 were found to be the most active compounds with IC₅₀ of 9.8 ± 0.56 and 10.9 ± 0.88 μM, respectively. The results indicated that these two compounds (1 and 3) can be promising candidates for anticancer drugs.