Paraoxonase 1 protects against protein N-homocysteinylation in humans

Genetic or nutritional disorders in homocysteine (Hcy) or folate metabolism elevate plasma Hcy-thiolactone and lead to vascular and/or brain pathologies. Hcy-thiolactone has the ability to form isopeptide bonds with protein lysine residues, which generates N-Hcy-protein with autoimmunogenic and prothrombotic properties. Paraoxonase (PON1), carried on high-density lipoproteins (HDLs) in the blood, hydrolyzes Hcy-thiolactone and protects against the accumulation of N-Hcy-protein in vitro. To determine its role in vivo, we studied how natural variation in Hcy-thiolactonase activity of PON1 affects plasma N-Hcy-protein levels in cystathionine beta-synthase-deficient patients (n=28). We found that plasma N-Hcy-protein was negatively correlated with serum Hcy-thiolactonase activity (r=-0.43, P=0.01), i.e., the higher the Hcy-thiolactonase activity, the lower N-Hcy protein levels. This relation was faithfully replicated in vitro in experiments with radiolabeled Hcy-thiolactone. We also found that enzymatic activities of the PON1 protein measured with artificial substrates correlated less strongly (r=-0.36, P=0.025 for paraoxonase activity) or did not correlate at all (phenylacetate hydrolase and TBLase activities) with plasma N-Hcy protein. These findings provide evidence that the Hcy-thiolactonase activity of PON1 is a determinant of plasma N-Hcy-protein levels and that Hcy-thiolactonase/PON1 protects proteins against N-homocysteinylation in vivo, a novel mechanism likely to contribute to atheroprotective roles of HDL in humans.-Perła-Kaján, J., Jakubowski, H. Paraoxonase 1 protects against protein N-homocysteinylation in humans.