Bound Homocysteine, Cysteine, and Cysteinylglycine Distribution between Albumin and Globulins

Abstract

Background: Major portions of homocysteine (Hcy), cysteine (Cys), cysteinylglycine (CysGly), and glutathione in serum are covalently bound to proteins via disulfides. Albumin has been considered the dominant binding protein. Methods:Pooled serum and plasma from healthy adults were fractionated into albumin and globulins by affinity columns. Content of Hcy, Cys, CysGly, and glutathione was determined for serum and plasma fractions and purified proteins by an HPLC method before and after incubation with excess CysGly, Hcy, or glutathione Results: Of protein-bound amino acids in pooled serum, 12% of Hcy, 21% of Cys, and 33% of CysGly were bound to globulins, with the remainder bound to albumin. Slightly higher proportions were bound to globulins in pooled plasma. Globulins had ϳ16% of total exchangeable disulfide and thiol groups in serum based on results of loading with CysGly. These results agree with expected abundance of unpaired Cys residues in globulins relative to albumin. Significant amounts of disulfide-linked amino acids were detected for HDL and ␣ 1 -acid glycoprotein but not for transferrin. Exchange of disulfide-linked amino acids on exposure to excess Hcy or glutathione was much faster for albumin than for ␣ 1 -acid glycoprotein.Conclusions: Approximately 10%-30%, of proteinbound Hcy, Cys, and CysGly are disulfide-linked to globulins. Amino acids disulfide-linked to albumin are rapidly exchangeable, while exchange of disulfidelinked amino acids from globulins, such as ␣ 1 -acid glycoprotein, is much slower. Consequently, the pools of Hcy, Cys, and CysGly bound to albumin and globulin

Increased Low-Density Lipoprotein S-Homocysteinylation in Chronic Kidney Disease

“…Steric constraints might favor reactivity of smaller reactants such as Cys and Hcy vs. Cys-Gly, or unpaired Cys in other proteins. Our data agree with recent observations in which higher ratios of Hcy:Cys-Gly for albumin than globulins have been found [36]. This may reflect differences in the reactivity or accessibility of the unpaired Cys in albumin and globulins.…”

“…We found that the relative quantity of Cys-Gly is higher in the apoprotein fraction than in plasma both in controls and CKD patients, thus confirming our previous data on the high affinity of apoB for Cys-Gly. On the contrary, it has been amply reported that albumin shows an elevated affinity principally for Cys and Hcy [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]. Unusually, the unpaired Cys34 of albumin have a p K a <6 [15].…”

Current Issues in Measurement and Reporting of Urinary Albumin Excretion

“…It has an unusually low pK of about 5 that results in increased reactivity and rates of disulfide-bond formation and exchange with other sulfhydryl-containing compounds in plasma (57,74,75 ). Albumin can form disulfide-linked dimers, with 2 albumin monomers linked by cysteine-34 residues, or the cysteine sidechain can be oxidized to a sulfonic acid (57,76 ).…”

Optimized Method for Determining FreeL-Cysteine in Rat Plasma by High-Performance Liquid Chromatography with the 4-Aminosulfonyl-7-fluoro-2,1,3-benzoxadiazole Conversion Reagent

“…It is known that a significant amount of plasma cysteine is present in protein-bound forms such as those forming S-S bonds with cysteine residues in albumin and globulins. 22) The plasma samples in the present study were de-proteinized prior to measurement, so the amount in the proteinbound form can be neglected.…”