Factors Affecting S-Homocysteinylation of LDL Apoprotein B (Lipids, Lipoproteins and Cardiovascular Risk Factors‪)‬

Increased plasma homocysteine (Hcy) [3] is an independent risk factor for vascular disease in humans (1, 2). Although the exact mechanism of Hcy toxicity is unknown, several mechanisms have been investigated that may explain its role in atherosclerosis pathogenesis (3, 4), including endothelial injury, reduction of vascular nitric oxide (NO) production and bioavailability, mitotic effect on smooth muscle cells, influence on leukocyte behavior and hemostasis, and oxidative modification of LDL. Recent study results have suggested that Hcy-induced vascular damage could be a result of Hcy-thiolactone (HcyT), an Hcy-reactive product formed in several cell types as a result of editing reactions of some aminoacyl-tRNA synthetase (5, 6). The synthesis of HcyT is directly proportional to the plasma Hcy:methionine ratio (6). Because HcyT binds protein lysyl residues by amide linkage, individual proteins are homocysteinylated at rates proportional to their lysine contents. Jakubowski found that protein N-homocysteinylation occurs even at HcyT concentrations as low as 10 nmol/L, which is a physiologic concentration (7). N-homocysteinylation leads to protein damage consisting of multimerization and precipitation of extensively modified proteins. Model enzymes, such as methionyl-tRNA synthetase and trypsin, are inactivated by N-homocysteinylation (7). The interaction between HcyT and LDL causes LDL aggregation and higher uptake of N-homocysteinylated LDL (Hcy-LDL) by cultured macrophages (8). Ferretti et al. (9) theorized that N-homocysteinylated LDLs are internalized by membrane receptors with intracellular release of Hcy after hydrolytic degradation. Even at low concentrations (up to 10 [micro]mol/ L), Hcy is cytotoxic and induces cell injury and oxidative damage in cultured cells, with generation of reactive oxidative species, formation of superoxide and hydrogen peroxide, and increased lipid peroxidation product concentrations (10, 11). LDL incubation with 100 /[micro]mol/L HcyT for 2 h was reported to cause the N-homocysteinylation of ~10% of apoB100 lysyl residues (9). Assuming a relative molecular mass (Mr) of -500 000 for apoB and ~200 lysyl residues per apoB molecule (12), ~20 new -SH groups are introduced in the sequence of apoprotein, thus tripling the number of "free sulfydryl" groups of native protein, reported to be 9 (13). The term free sulfydryl, however, used to define sulfydryl groups of proteins that are not involved in intramolecular bridges, may be improper in this context. We demonstrated (14,15) that in vivo low-Mr thiols such as Hcy, cysteine (Cys), cysteinylglycine (CysGly), glutamylcysteine, and glutathione link these "free" -SH groups by disulfide bonds. Therefore lipoproteins may be N-homocysteinylated by HcyT but also S-homocysteinylated by Hcy. Increased total plasma Hcy concentrations lead to increased concentrations of HcyT that reacts with lysine proteins, leading to increased lipoprotein N-homocysteinylation, which increases the number of -SH sites for S-homocysteinylation. Other plasma LMW thiols compete for the same sites, however, depending on their concentration. Therefore, the interaction between physiologic plasma thiols and apoprotein may be related to the balance among the different thiols. No data have been reported on variables affecting the concentrations of Hcy bound to LDL by disulfide linkage. To understand the mechanisms involved in the S-homocysteinylation of LDL, considered a risk factor for vascular disease (16), we measured total plasma thiol and LDL-bound thiol concentrations in a healthy population and investigated the relationships between total and apoprotein-bound thiols. Materials and Methods