Quantification of Thiol-Containing Amino Acids Linked by Disulfides to LDL

Abstract

Several studies have indicated that plasma proteins interact with homocysteine (Hcy) to form stable disulfide-linked products. Hcy in plasma is mainly bound to albumin, but interactions with ceruloplasmin, fibrin, annexin II, and transthyretin have been also reported (1)(2)(3)(4). In 1991, Olszewski and McCully (5) described the presence of Hcy in lipoproteins in patients with hypercholesterolemia. Because the analysis was performed after acidic hydrolysis of apoprotein, the Hcy measured by these authors was the sum of (a) Hcy incorporated in the primary structure of apolipoprotein B-100 (5), (b) Hcy thiolactone bound to lysine residues of protein by amide or peptide linkages and converted to Hcy by acidic conditions after release (6), and (c) Hcy linked to apolipoprotein B-100 (apoB-100) by a disulfide bond. Recent studies have demonstrated that there are at least nine free sulfhydryl groups (−SH) in the apoB-100 primary structure that could potentially bind plasma free aminothiols by disulfide linkage (7). Because lysyl residues of apoB-100 could react in vivo with plasma Hcy thiolactone by an amide bond, the number of free apoprotein sulfhydryl groups could increase considerably, thus increasing the number of sites that may be bound with plasma aminothiols (6). These LDL modifications are accompanied by an increase in density and in electrophoretic mobility of lipoprotein and are associated with functional alterations that make Hcy-LDL more susceptible to aggregation and to spontaneous precipitation. Moreover, higher uptake of Hcy-LDL by membrane receptor and by phagocytosis and a higher accumulation of intracellular cholesterol have been observed in cultured macrophages, suggesting that homocysteinylation could increase the atherogenicity of LDL (8)(9). Thus, to study the association between Hcy and lipid metabolism, a highly sensitive method to measure Hcy and other thiols bound to apoB-100 is required. Here we describe a simple capillary electrophoresis …