Unraveling the binding mechanism of asiatic acid with human serum albumin and its biological implications

Asiatic acid (AsA), a naturally occurring pentacyclictriterpenoid found in Centella asiatica, plays a major role in neuroprotection, anticancer, antioxidant, and hepatoprotective activities. Human serum albumin (HSA), a blood plasma protein, participates in the regulation of plasma osmotic pressure and transports endogenous and exogenous substances. The study undertaken to analyze the drug-binding mechanisms of HSA is crucial in understanding the bioavailability of drugs. In this study, we analyzed the cytotoxic activity of AsA on HepG2 (human hepatocellular carcinoma) cell lines and its binding, conformational, docking, molecular simulation studies with HSA under physiological pH 7.2. These studies revealed a clear decrease in the viability of HepG2 cells upon exposure to AsA in a dose-dependent manner with an IC50 of 45 mu M. Further studies showed the quenching of intrinsic fluorescence of HSA by AsA with a binding constant of K-AsA = 3.86 +/- 0.01 x 10(4) M-1, which corresponds to the free energy of (Delta G) -6.3 kcal M-1 at 25 degrees C. Circular dichroism (CD) studies revealed that there is a clear decrease in the alpha-helical content from 57.50 +/- 2.4 to 50% +/- 2.3 and an increase in the beta-turns from 25 +/- 0.65 to 29% +/- 0.91 and random coils from 17.5% +/- 0.95 to 21% +/- 1.2, suggesting partial unfolding of HSA. Autodock studies revealed that the AsA is bound to the subdomain IIA with hydrophobic and hydrophilic interactions. From molecular dynamics, simulation data (RMSD, Rg and RMSF) emphasized the local conformational changes and rigidity of the residues of both HSA and HSA-AsA complexes.