Variabines A and B: new β-carboline alkaloids from the marine sponge Luffariella variabilis

Two new β-carboline alkaloids, variabines A (1) and B (2), were isolated from the Indonesian marine sponge Luffariella variabilis. Their structures were elucidated from spectral data, and 1 was found to be a sulfonated derivative of 2. Although numerous β-carboline alkaloids have been isolated from natural sources to date, 1 is the first β-carboline derivative containing a sulfate group. Compound 2 inhibited chymotrypsin-like activity of the proteasome and Ubc13 (E2)–Uev1A interaction with IC50 values of 4 and 5 μg/mL, respectively, whereas 1 had little effect on the activity or interaction.