Eight naturally occurring marine-sponge derived sesquiterpenoid quinones were evaluated as potential inhibitors of pyruvate phosphate dikinase (PPDK), a C4 plant regulatory enzyme. Of these, the hydroxyquinones ilimaquinone, ethylsmenoquinone and smenoquinone inhibited PPDK activity with IC50's (reported with 95% confidence intervals) of 285.4 (256.4 - 317.7), 316.2 (279.2 - 358.1) and 556.0 (505.9 - 611.0) μM, respectively, as well as being phytotoxic to the C4 plant Digitaria ciliaris. The potential anti-inflammatory activity of these compounds, using bee venom phospholipase A2 (PLA2), was also evaluated. Ethylsmenoquinone, smenospongiarine, smenospongidine and ilimaquinone inhibited PLA2 activity (% inhibition of 73.2 ± 4.8 at 269 μM, 61.5 ± 6.1 at 242 μM, 41.0 ± 0.6 at 224 μM and 36.4 ± 8.2 at 279 μM, respectively). SAR analyses indicate that a hydroxyquinone functionality and a short, hydroxide/alkoxide side-chain at C-20 is preferred for inhibition of PPDK activity, and that a larger amine side-chain at C-20 is tolerated for PLA2 inhibitory activity.