A Clerodane Diterpene from <i>Aquarius scaber</i> (Rataj) Christenh. &amp; Byng Leaves as Inhibitor of <i>Leishmania mexicana</i> Trypanosomatid Enzymes

The clerodane diterpenoid 2-oxo-5<i>α</i>,8<i>α</i>-cleroda-3,13-dien-16,15-olide (<b>1</b>) isolated from the leaves of <i>A. scaber</i> was identified by IR, ¹H and ¹³C NMR (1D and 2D) and HRESIMS spectrometric analysis. This is the first report of this diterpene in the <i>Aquarius</i> genus. The inhibitory potential of <b>1</b> has been determined against cysteine proteases from <i>Leishmania mexicana</i> rCPB2.8, rCPB3.0 and rH84Y. The inhibitory constants (<i>Ki</i> and α<i>Ki</i>) as well as the mechanism of action were also investigated. Compound <b>1</b> was 7- and 18-fold more potent in the inhibition of rCPB3.0 (IC₅₀ = 4.2 µM) than rCPB2.8 (IC₅₀ = 20 µM) and rH84Y (IC₅₀ = 75 µM). From the kinetic mechanisms of inhibitor binding, we found that compound <b>1</b> has a higher affinity to rCPB3.0 (<i>K</i>_i = 7.5 µM and α<i>K</i>_i = 7.4 µM) than rCPB2.8 (<i>K</i>_i = 15.4 µM, α<i>K</i>_i = 14.7 µM) and rH84Y (<i>K</i>_i = 81.4, and α<i>K</i>_i = 27.9 μM and β<i>K_i</i> = 41.5 µM). Compound <b>1</b> also presented a non-competitive linear simple inhibition mechanism at both enzymes rCPB3.0 and rCPB2.8. On the other hand, <b>1</b> presented a positive cooperativity mechanism of inhibition at rH84Y.