High density lipoproteins (HDL) exert a protective effect against homocysteinylation due to the activity of the
enzyme paraoxonase/thiolactonase associated to the
lipoprotein surface. However, a small amount of N-homocysteinylated HDL (N-Hcy-HDL) is present in human plasma, suggesting that homocysteinylation of plasma
lipoproteins occurs in vivo. Aim of the present study was to investigate the effect of homocysteinylation on
apoprotein structure and physico-chemical properties of HDL using the analysis of the fluorescent emission spectra of
tryptophan and
Laurdan (6-dodecanoyl-2-dimethyl-aminonaphthalene). Our results demonstrated that the increase in -SH groups in HDL homocysteinylated in vitro (Hcy-HDL) was associated with
apoprotein conformational changes and modifications of physico-chemical properties. A significant decrease of
paraoxonase and lactonase activity of HDL bound PON1 has also been observed in Hcy-HDL. A significant decrease of the
enzyme activity has been observed also in purified PON1 homocysteinylated following the same experimental conditions used for HDL. Moreover, we demonstrated that oxidized HDL were more susceptible to homocysteinylation with respect to control HDL. The modifications of
apoprotein conformation and physico-chemical properties in Hcy-HDL and the decrease of
paraoxonase-1 activity could affect the protective effect of HDL against oxidative damage and/or homocysteinylation and could contribute to accelerated
atherosclerosis in patients affected by diseases associated with oxidative damage, in renal disorders and in patients affected by genetic or
nutritional disorders of
homocysteine or
folate metabolism.