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Blood, 15 October 2006, Vol. 108, No. 8, pp. 2846-2849.
Prepublished online as a Blood First Edition Paper on June 15, 2006; DOI 10.1182/blood-2006-05-022327.
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RED CELLS
Brief report
Haptoglobin-related protein is a high-affinity hemoglobin-binding plasma protein
Marianne Jensby Nielsen,
Steen Vang Petersen,
Christian Jacobsen,
Claus Oxvig,
David Rees,
Holger Jon Møller, and
Søren Kragh Moestrup
From the Department of Medical Biochemistry, University of Aarhus, Denmark; the Department of Molecular Biology, University of Aarhus, Denmark; the Department of Paediatric Haematology, King's College School of Medicine, King's College Hospital, London, United Kingdom; and the Department of Clinical Biochemistry, Aarhus University Hospital, Denmark.
Haptoglobin-related protein (Hpr) is a primate-specific plasma protein associated with apolipoprotein L-I (apoL-I)-containing high-density lipoprotein (HDL) particles shown to be a part of the innate immune defense. Despite the assumption hitherto that Hpr does not bind to hemoglobin, the present study revealed that recombinant Hpr binds hemoglobin as efficiently as haptoglobin (Hp). However, in contrast to Hp, Hpr did not promote any high-affinity binding to the scavenger receptor CD163. Binding of hemoglobin to circulating native Hpr incorporated into the HDL fraction was indicated by hemoglobin-affinity precipitation of plasma Hpr together with apoL-I. In conclusion, plasma has 2 high-affinity hemoglobin-binding haptoglobins instead of one, but only Hp-hemoglobin complexes are efficiently recognized by CD163. Circulating Hpr-bound hemoglobin should therefore be taken into consideration when measuring "free" plasma hemoglobin. Furthermore, Hpr-bound hemoglobin might contribute to the biologic activity of the circulating apoL-I/Hpr-containing HDL particles.
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