Distribution of homocysteine and cysteine in plasma lipoprotein fractions

S-thiolation of plasma proteins have been detected in healthy humans, in patients with cardiovascular diseases and it is a recurrent phenomenon in oxidative stress elicited by reactive oxygen species. The low-molecular-weight aminothiols homocysteine (Hcy) and cysteine (Cys) can exist in the plasma either free or bound to thiol-combining groups, many of which are present in proteins (Pb), particularly albumin. Recent studies have demonstrated that also plasma lipoproteins (LP) are susceptible to form disulfide-linked products with Hcy and Cys and that the N-homocysteinylation is accompanied by structural and functional alteration and could increase the atherogenicity of LDL. However, the mechanism through which Hcy contributes to these abnormalities remains however still undefined. In order to ascertain the link between physiologic thiols and lipoproteins (LP), we determined simultaneously the distribution of Hcy and Cys, in vivo, bound to different plasma protein fractions by disulfide linkage. Human VLDL, LDL, HDL and LPDS (lipoprotein-free protein fraction) were obtained from plasma of normolipemic subjects volunteers (University workers) prepared by differential ultracentrifugation. The purity of the isolated subfractions were controlled by electrophoresis on agarose gel and no significant albumin content was found in the isolated lipoprotein fractions. The Hcy and Cys bound to single plasma protein fraction was calculated as difference between total and free aminothiols. Hcy and Cys in plasma and in lipoprotein fractions were analyzed by the HPLC method (1). Protein-bound Hcy and Cys (Pb-Hcy and Pb-Cys) (9.10 and 196.79 \ub5mol/L, respectively) represents about 76% and 79% of total plasma Hcy and Cys (11.98 and 249.33 \ub5mol/L, respectively). The absolute Pb-Hcy and Pb-Cys distribution between the single LP fractions was as follows (\ub5mol/L): VLDL=0.59 (6.4%) and 36.78 (18.7%) respectively; LDL=0.77 (8.5%) and 9.13 (4.6%) respectively; HDL=1.44 (15.8%) and 6.63 (3.4%) respectively; LPDS=6.30 (69.3%) and 144.24 (73.3) respectively. The Hcy and Cys/protein ratios in each single LP fraction (nmol/mg of protein) suggest a higher binding capacity for Hcy and Cys by VLDL and LDL probably due to their content in Apo B. The detection of all aminothiols (Hcy, Cys, Cys-Gly, GSH) linked to Apo B may be important to understand the mechanisms of thiol-disulfide exchange reactions and to evaluate if this binding may have a complex role in promoting lipoprotein oxidation or structural and/or functional modifications in lipoprotein thiolation both in vivo and in vitro