4.4. Measurement of Conjugated Diene Formation

The oxidation of LDL was assessed by the formation of conjugated dienes, determined as the change in UV absorbance at 234 nm [15]. Briefly, LDL (100 μg/mL) in PBS (pH 7.4, final volume of 1 mL) was pre-incubated with ethanolic extract of P. pterocarpum stem barks (SPP: 10, 50, 100 mg/mL) for 30 min at 37 °C, and then 5 μM CuSO₄ was added to initiate the oxidation process. The lag time (minute) was determined as the intercept of baseline and the tangent of the absorbance curve during the propagation phase by those of absorbance at 234 nm that continuously monitoring at 10 min intervals for 5 h at 37 °C using a spectrophotometer (Shimadzu UV-1240, Tokyo, Japan). Quercetin and caffeic acid was used as positive controls [16,27].