We have investigated the in vitro inhibition of papain, trypsin, and cathepsins B and G by five benzophenone-type compounds, three natural ones isolated from Garcinia brasiliensis and two synthetic derivatives. The activities of pentaprenylated trihydroxy-substituted benzophenone guttiferone A (1) on all assayed enzymes were approximately 2-69 folds higher than that manifested by mono-hydroxylated tetraprenylated and triprenylated compounds epiclusianone (2) and garciniaphenone (3), respectively, the other natural benzophenones that also inhibited significantly the four enzymes. Differently, the synthetic derivatives 2,2',4-trihydroxybenzophenone (4) and diphenylmethanone (5) have inhibited weakly the studied proteases. Furthermore, compound 1 has bonded preferentially to cathepsin G, once its IC(50) value (2.7+/-0.1 microM) on such peptidase is quite similar to that of the classical inhibitor of serine proteases, chymostatin (2.1+/-0.1 microM). Interesting structure-activity relationships (SARs) were confirmed by flexible docking simulations, likewise the potential usefulness of natural compound 1 as antitumoral drug is strengthened by our results concerning the antiproteolytic activity.