Paraoxonase protection of LDL against peroxidation is independent of its esterase activity towards paraoxon and is unaffected by the Q -> R genetic polymorphism

High density lipoprotein (HDL)-associated paraoxonase (PON) seems to play a major role in the protection of low density lipoprotein (LDL) against pero xidation by HDL, and the partly purified enzyme exerts a dose-dependent pro tective effect. A common polymorphism of the human gene (192 Q-->R) modulat es paraoxonase activity but purified enzyme from either genotype is equally effective against LDL peroxidation. The inhibition of Cu2+-induced LDL per oxidation by HDL was monitored by lipid peroxide assay and change in LDL el ectrophoretic mobility. We show that HDL from type 2 diabetic patients with the QQ or RR genotype (n = 12 for each) reduce, to the same extent, both p eroxide production (by 60.6 +/- 20.0 and 63.9 +/- 23.5%) and relative chang e in mobility (61.3 +/- 21.8 and 61.4 +/- 26.5%) despite a 6-fold differenc e in paraoxonase activity (47.4 +/- 4.4 vs. 299.7 +/- 23.7 U/l, P < 0.0001) . Protection was, however, related to paraoxonase activity, but with a diff erent efficiency in each group corresponding to a better protection per uni t of enzyme in the QQ genotype group, Inactivation of PON activity by heati ng (56 degrees C, 10 min) or by EDTA was totally without effect on protecti on, which remained correlated with the paraoxonase activity measured prior to inactivation. In summary, these results suggest that the protein bearing both paraoxonase and arylesterase activities also possesses a third thermo stable property, closely associated with paraoxon hydrolysis activity and u naffected by PON genetic variability.