Human serum paraoxonase (PON 1) is inactivated by oxidized low density lipoprotein and preserved by antioxidants

Human serum paraoxonase (PON1) can protect low density lipoprotein (LDL) fr om oxidation induced by either copper ion or by the free radical generator azo bis amidinopropane hydrochloride (AAPH). During LDL oxidation in both o f these systems, a time-dependent inactivation of PON arylesterase activity was observed. Oxidized LDL (Ox-LDL) produced by lipoprotein incubation wit h either copper ion or with AAPH, indeed inactivated PON arylesterase activ ity by up to 47% or 58%, respectively. Three possible mechanisms for PON in activation during LDL oxidation were considered and investigated: copper io n binding to PON, free radical attack on PON, and/or the effect of lipoprot ein-associated peroxides on the enzyme. As both residual copper ion and AAP H are present in the Ox-LDL preparations and could independently inactivate the enzyme, the effect of minimally oxidized (Ox-LDL produced by LDL stora ge in the air) on PON activity was also examined. Oxidized LDL, as well as oxidized palmitoyl arachidonoyl phosphatidylcholine (PAPC), lysophosphatidy lcholine (LPC, which is produced during LDL oxidation by phospholipase A2-l ike activity), and oxidized cholesteryl arachidonate (Ox-CA), were all pote nt inactivators of PON arylesterase activity (PON activity was inhibited by 35%-61%). PON treatment with Ox-LDL (but not with native LDL), or with oxi dized lipids, inhibited its arylesterase activity and also reduced the abil ity of the enzyme to protect LDL against oxidation. PON Arylesterase activi ty however was not inhibited when PON was pretreated with the sulfhydryl bl ocking agent, p-hydroxymercurybenzoate (PHMB). Similarly, on using recombin ant PON in which the enzyme's only free sulfhydryl group at the position of cysteine-284 was mutated, no inactivation of the enzyme arylesterase activ ity by Ox-LDL could be shown. These results suggest that Ox-LDL inactivatio n of PON involves the interaction of oxidized lipids in Ox-LDL with the PON 's free sulfhydryl group. Antioxidants such as the flavonoids glabridin or quercetin, when present during LDL oxidation in the presence of PON, reduce d the amount of lipoprotein-associated lipid peroxides and preserved PON ac tivities, including its ability to hydrolyze Ox-LDL cholesteryl linoleate h ydroperoxides. We conclude that PON's ability to protect LDL against oxidat ion is accompanied by inactivation of the enzyme. PON inactivation results from an interaction between the enzyme free sulfhydryl group and oxidized l ipids such as oxidized phospholipids, oxidized cholesteryl ester or lysopho sphatidylcholine, which are formed during LDL oxidation. The action of anti oxidants and PON on LDL during its oxidation can be of special benefit agai nst atherosclerosis since these agents reduce the accumulation of Ox-LDL by a dual effect: i.e, prevention of its formation, and removal of Ox-LDL ass ociated oxidized lipids which are generated during LDL oxidation. (C) 1999 Elsevier Science Inc.