Modes of Inhibition of Human Protein-Tyrosine Phosphatase 1B and Aldose Reductase by Moringa oleifera Lam Leaves Extract

Abstract

Insulin resistance is the main feature of Type 2 diabetes, due to attenuated or diminished signaling from insulin receptors, resulting in hyperglicemia. Inhibition of protein tyrosine phosphatase 1B (PTP1B) has been suggested as an attractive target to improve insulin sensitivity in different cell types. In addition, hyperglycemia activates the intracellular polyol pathway causing accumulation of sorbitol. This results in cellular water and electrolyte imbalance and oxidative injury. Aldose reductase inhibitors block the rate-limiting enzyme of the polyol pathway, decreasing the accumulation of sorbitol. Moringa oleifera is a plant widely used in traditional Indian and African medicine, e.g. for its antidiabetic effect. However, studies about the inhibitory effect of M. oleifera on diabetic complications mediated via diverse enzymes as PTP1B and aldose reductase have not been carried out yet. To potentially identify targets responsible for this antidiabetic effect, we conducted kinetic studies to investigate the inhibitory capacity of M. oleifera extract (MOE) over PTP1B. MOE possess the highest inhibitory activity against PTP1B (IC 50 value of 346.8 µg/mL). We also found that the extract inhibited the recombinant human aldose reductase (rhAR) activity with an IC 50 of 3.55 µg/mL. Subsequent kinetic analysis revealed that MOE behaved as an uncompetitive inhibitor (K i of 19.65 µg/mL) against PTP1B, since it markedly modified K m and also modified V max to a lesser extent. As for the rhAR, the extract behaved as mixed inhibitor (Ki = 18.6 µg/mL, alpha = 1.3). Our results help to understand the inhibition mechanism of PTP1B and rhAR by phenolic compounds and flavonoids present in MOE, which is essential for the development of improved natural inhibitors.