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Clinical Chemistry 52: 2258-2264, 2006. First published October 26, 2006; 10.1373/clinchem.2006.074302
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Right arrow Lipids, Lipoproteins, and Cardiovascular Risk Factors
(Clinical Chemistry. 2006;52:2258-2264.)
© 2006 American Association for Clinical Chemistry, Inc.

Lipids, Lipoproteins, and Cardiovascular Risk Factors

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

Glen L. Hortin1,a, Nitin Seam2 and Gerard T. Hoehn2

Departments of1 Laboratory Medicine and 2 Critical Care Medicine, Warren Magnuson Clinical Center, National Institutes of Health, Bethesda, MD.

aAddress correspondence to this author at: Laboratory Medicine, Warren Magnuson Clinical Center, National Institutes of Health, Bethesda, MD 20892. Fax 301-402-1885; e-mail ghortin{at}mail.cc.nih.gov.

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 ~16% 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 {alpha}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 {alpha}1-acid glycoprotein.

Conclusions: Approximately 10%–30%, of protein-bound Hcy, Cys, and CysGly are disulfide-linked to globulins. Amino acids disulfide-linked to albumin are rapidly exchangeable, while exchange of disulfide-linked amino acids from globulins, such as {alpha}1-acid glycoprotein, is much slower. Consequently, the pools of Hcy, Cys, and CysGly bound to albumin and globulin may represent kinetically and functionally distinct pools. Plasma concentrations of total Hcy and Cys, which are dominated by albumin-bound pools, may not reflect the abundance of functionally significant modifications of globulins.






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Copyright © 2006 by the American Association for Clinical Chemistry.