Evaluation of Inhibitory Actions of Flavonols and Related Substances on Lysophospholipase D Activity of Serum Autotaxin by a Convenient Assay Using a Chromogenic Substrate

Overproduction of lysophosphatidic acid (LPA) by lysophospholipase D/autotaxin (lysoPLD/ATX) is postulated to be involved in the promotion of cancer and atherosclerosis. A lysoPLD inhibitor may be utilized to ameliorate the LPA-related pathological conditions. In this study, a new assay was devised to quantify p-nitrophenol from hydrolysis of chromogenic substrate by serum lysoPLD without tedious lipid extraction procedures. Flavonols, phenolic acids, free fatty acids, and N-acyltyrosines inhibited lysoPLD activity in a micromolar range. They were classified into competitive, noncompetitive, or mixed type inhibitors. The results show that the low hydrophobicity of an inhibitor is a critical factor in its preference for the binding to a noncatalytic binding site over a catalytic binding site. Considering its reported bioavailability and the low dependency of its inhibitory activity on serum dilution, flavonol is likely to be a more effective lysoPLD inhibitor in human blood circulation in vivo than the other inhibitors including LPA.