Effect of homocysteinylation on human high-density lipoproteins: a correlation with paraoxonase activity

We investigated the effect of homocysteine (Hcy)-thiolactone on the activity of the enzyme paraoxonase (PON) associated with human high-density lipoprotein (HDL-PON). HDL were isolated from plasma of normolipidemic subjects. The increase in the levels of sulfhydryl groups (-SH) in HDL incubated with Hcy-thiolactone demonstrates that homocysteinylation of HDL occurs. The increase of -SH groups correlated with the basal values of HDL-PON activity (r = -0.73, P <.001, and r = -0.70, P <.002 using 10 micromol/L and 1 mmol/L Hcy-thiolactone, respectively) suggesting a relationship between the susceptibility of HDL to homocysteinylation and HDL-PON activity. A decrease in the activity of the enzyme HDL-PON was observed in homocysteinylated HDL (Hcy-HDL). The negative correlation established between the basal levels of HDL-PON activity and the percentage decrease of HDL-PON activity (r = -0.76, P <.001, and r = -0.86, P <.001 using 10 micromol/L or 1 mmol/L Hcy-thiolactone, respectively) suggests that subjects with higher HDL-PON activity have a lower decrease in PON activity with respect to subjects with lower HDL-PON activity. The positive correlation established between the percentage decrease of PON activity and the percentage increase of -SH groups in Hcy-HDL (r = 0.80, P <.001, and r = 0.76, P <.001 in HDL incubated in the presence of 10 micromol/L and 1 mmol/L Hcy-thiolactone, respectively) suggests that the modifications of HDL-PON activity are likely related to the compositional changes at the lipoprotein surface of Hcy-HDL. The enzyme PON contributes to the protective role of HDL against the oxidative damage and against toxicity exerted by Hcy involved in the development of atherosclerosis. Therefore the significant decrease of the enzyme activity in HDL incubated with Hcy-thiolactone suggests that homocysteinylation could render HDL less protective against oxidative damage and against toxicity of Hcy-thiolactone.