OBJECTIVE: The purpose of this study was to analyze the consequences of HDL oxidation for paraoxonase-1 metabolism and function. METHODS AND RESULTS: HDL was oxidized with AAPH, copper ions, and hypochlorite. Secretion studies were performed using human paraoxonase-1-transfected cells lines and primary rat hepatocytes. Stability studies were performed with recombinant paraoxonase. Conditioned medium had significantly reduced paraoxonase-1 when Cu or AAPH-oxidized HDL was the acceptor complex (P<0.01); reduction was dose-dependent on the degree of oxidation. Oxidized HDL had a reduced capacity to stabilize/improve activity of secreted paraoxonase-1. Reduced secretion could not be attributed to enzyme inactivation by lipoperoxides, reduced binding affinity of HDL, or oxidation of the lipid component alone. Hypochlorite oxidation of HDL did not modify HDL-mediated paraoxonase-1 release, but activity of HDL-associated paraoxonase-1 was particularly sensitive to such treatment. CONCLUSIONS: AAPH and copper, but not hypochlorite, oxidation of HDL compromises its ability to promote release of paraoxonase-1 and stabilize enzyme activity. HDL-associated paraoxonase-1 is highly sensitive to hypochlorite. Reducing paraoxonase-1 renders HDL susceptible to oxidation, which may compromise HDL function. It provides a novel example at the HDL level of the detrimental effects of oxidative stress, and underlines the need for further evaluation of the consequences of HDL oxidation.