Active site competition is the mechanism for the inhibition of lipoprotein-associated phospholipase A(2) by detergent micelles or lipoproteins and for the efficacy reduction of darapladib

Lipoprotein associated phospholipase A(2) (Lp-PLA(2)) has been characterized for its interfacial activation as well as inhibition by detergent micelles and lipoprotein particles. The enzyme has been shown to bind on the surfaces of hydrophobic aggregates, such as detergent micelles, lipoprotein particles and even polystyrene latex nanobeads. Binding to hydrophobic aggregates stimulates the activity of Lp-PLA(2) but may not be the necessary step for catalysis. However, at higher concentrations, detergent micelles, latex nanobeads or lipoprotein particles inhibit Lp-PLA(2) possibly by blocking the access of substrates to the active site. The competition mechanism also blocks inhibitors such as darapladib binding to Lp-PLA(2) and reduces the efficacy of the drug. Darapladib has very low solubility and mainly exists in solutions as complexes with detergents or lipoprotein particles. The inhibition of Lp-PLA(2) by darapladib is dependent on many factors such as concentrations of detergents or lipoproteins, incubation time, as well as the order of mixing reaction components. The in vitro Lp-PLA(2) activity assays used in clinical studies may not accurately reflect the residual Lp-PLA(2) activity in vivo. Darapladib has been found mainly bound on HDL and albumin when it is incubated with human serum. However, Lp-PLA(2) is more sensitive to darapladib when bound on LDL and relatively resistant to darapladib when bound on HDL. Therefore, high cholesterol levels may decrease the efficacy of darapladip and cause the drug to be less effective in high risk patients. Our study will help to design better inhibitors for Lp-PLA(2). The discoveries also contribute to understanding the mechanism of interfacial activation and inhibition for Lp-PLA(2) and provide a new concept for researchers in building better kinetic model for interfacial enzymes.