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Molecular targeting of alpha‐2‐macroglobulin by homocysteine: stoichiometry and possible implications in inflammatory diseases
Author(s) -
Catanescu Carmen Otilia,
Barbato John C,
DiBello Patricia M,
Willard Belinda,
Kinter Mike T,
Zhloba Alexander A,
Jacobsen Donald W
Publication year - 2007
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.21.5.a629-c
Subject(s) - homocysteine , cysteine , macroglobulin , trypsin , chemistry , biochemistry , proteolysis , thiol , inflammation , biology , enzyme , immunology
Alpha‐2‐macroglobulin (a2M), a 720‐kDa plasma protein, functions as an extracellular proteinase inhibitor and carrier of cytokines, growth factors and hormones. Disulfide bonds are susceptible for thiol‐exchange reactions with homocysteine (Hcy). The structure of a2M contains 96 cysteine residues and one can assume that some of these disulfide bridges might interact with Hcy. Using 35 S‐D,L‐homocysteine, we show that a2M is homocysteinylated in both its native and in the protein‐proteinase complex (“activated” a2M). The stoichiometry of Hcy binding to native and activated a2M is dose‐dependent: treatment of native and activated a2M with 25 and 50 μM 35 S‐D,L‐Hcy at 37°C for 4 h yielded approximate Hcy:a2M ratios of 2:1 and 4:1 for native a2M, and 3:1 and 6:1 for activated a2M. Trypsin‐protein esterase activity of a2M was shown to be decreased by 12–35% in homocysteinylated‐a2M compared to untreated a2M, suggesting that high levels of Hcy in the blood may affect the proteinase‐binding function of a2M. This might explain why proteolytic activity remains high in inflammatory tissue fluids and why cytokines and growth factors that usually bind to a2M continue to play important roles in the progression of inflammation and atherosclerosis.