Identification of plasma Haptoglobin forms loosely binding Haemoglobin

Haptoglobin (Hpt) is known to capture circulating free Haemoglobin (Hb), and bind apolipoprotein (Apo) A-I or E. We report here that Hb can be tightly bound by most of Hpt molecules (TB-Hpt, 80%), whereas loosely bound by a minor part of them (LB-Hpt, 20%). LB-Hpt amount was significantly increased (over 60%) in patients with acute coronary syndrome. LB-Hpt bound ApoA-I and ApoE worse than TB-Hpt (8- and 4-fold less, respectively), and did not affect their activity of stimulating the enzyme LCAT. LB-Hpt and TB-Hpt displayed comparable levels of nitrotyrosine residues, but differences in glycan chains. Changes in the LB-Hpt level might be associated to changes in Hpt functions.