Paraoxonase 1 (PON1, EC 18.104.22.168) is an HDL-associated esterase/lactonase enzyme mainly synthesized in the liver and released into the bloodstream. PON1 hydrolysis highly toxic organophosphorus compounds such as the pesticide, paraoxon and various nerve agents used as chemical weapons. Additionally PON1 has protective effect against oxidative damage. PON1 has anti-atherogenic properties and protects serum HDL and LDL particles against lipid peroxidation. PON1 inhibits N-homocysteinylation of LDL-associated proteins by hydrolyzing the highly reactive pro-oxidant homocysteine thiolactone. Measurement of serum PON1 activity has been proposed as an antioxidant capacity index and biomarker of liver and cardiovascular disease.