Inhibition of protein tyrosine phosphatase 1B (PTP1B) and α-glucosidase by xanthones from Cratoxylum cochinchinense, and their kinetic characterization.

Cratoxylum cochinchinense displayed significant inhibition against protein tyrosine phosphatase 1B (PTP1B) and α-glucosidase, both of which are key target enzymes to attenuate diabetes and obesity. The compounds responsible for both enzymes inhibition were identified as twelve xanthones (1-12) among which compounds 1 and 2 were found to be new ones. All of them simultaneously inhibited PTP1B with IC ₅₀ s of (2.4-52.5 µM), and α-glucosidase with IC ₅₀ values of (1.7-72.7 µM), respectively. Cratoxanthone A (3) and γ-mangostin (7) were estimated to be most active inhibitors against both PTP1B (IC ₅₀  = 2.4 µM for 3, 2.8 µM for 7) and α-glucosidase (IC ₅₀  = 4.8 µM for 3, 1.7 µM for 7). In kinetic studies, all isolated xanthones emerged to be mixed inhibitors of α-glucosidase, whereas they behaved as competitive inhibitors of PTP1B. In time dependent experiments, compound 3 showed isomerization inhibitory behavior with following kinetic parameters: K _i ^app  = 2.4 µM; k ₅  = 0.05001 µM ^-1  S ^-1 and k ₆  = 0.02076 µM ^-1  S ^-1 .
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