Camellia nitidissima Chi flower extracts inhibit α-amylase and α-glucosidase: In vitro by analysis of optimization of addition methods, inhibitory kinetics and mechanisms

[Display omitted] •Adding methods had significantly different inhibition effects on enzyme activity.•All ingredient interacted with starch, causing the inhibition of enzyme activity.•The extract changed the microenvironment and conformation of enzyme. We optimized four addition methods for Camellia nitidissima Chi flower extracts (Cnfe). Among four methods, the first (before meals) had the greatest inhibitory effect via binding of Cnfe with starch. Cnfe bound starch, and reduced inhibition of the enzymes. The higher binding percentage of Cnfe and the substrate starch led to less free Cnfe interaction with enzymes in solution. To determine the underlying inhibitory mechanism(s), we used enzymatic inhibition kinetics, fluorescence quenching and circular dichroism spectroscopies. We found that Cnfe possessed potential inhibitory activity against α-amylase (IC50 300 μg/mL, Ki 162 μg/mL, static quenching constant KFQ 1.5 mL/mg) and α-glucosidase (45 μg/mL, 89 μg/mL and 8.5 mL/mg). Cnfe changed the microenvironment and conformation of the enzyme, making it a better digestive enzyme inhibitor. These experimental results suggest that Cnfe has the potential to control blood sugar, and that it may be used to improve diabetes by reducing postprandial hyperglycemia. Cnfe should be taken before meals and separately from foods with higher binding intensities.