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 peroxidation by HDL, and the partly purified enzyme exerts a dose-dependent protective effect. A common polymorphism of the human gene (192 Q-->R) modulates paraoxonase activity but purified enzyme from either genotype is equally effective against LDL peroxidation. The inhibition of Cu2+-induced LDL peroxidation by HDL was monitored by lipid peroxide assay and change in LDL electrophoretic 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 peroxide production (by 60.6 +/- 20.0 and 63.9 +/- 23.5%) and relative change in mobility (61.3 +/- 21.8 and 61.4 +/- 26.5%) despite a 6-fold difference 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 different efficiency in each group corresponding to a better protection per unit of enzyme in the QQ genotype group. Inactivation of PON activity by heating (56 degreesC, 10 min) or by EDTA was totally without effect on protection, 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 thermostable property, closely associated with paraoxon hydrolysis activity and unaffected by PON genetic variability.