A unique tetrameric structure of deer plasma haptoglobin – an evolutionary advantage in the Hp 2-2 phenotype with homogeneous structure

Artikel i vetenskaplig tidskrift, 2008

Similar to blood types, human plasma haptoglobin (Hp) is classified into three phenotypes: Hp 1-1, 2-1 and 2-2. They are genetically inherited from two alleles Hp 1 and Hp 2 (represented in bold), but only the Hp 1-1 phenotype is found in almost all animal species. The Hp 2-2 protein consists of complicated large polymers cross-linked by a2-b subunits or (a2-b)n (where n ‡ 3, up to 12 or more), and is associated with the risk of the development of diabetic, cardiovascular and inflammatory diseases. In the present study, we found that deer plasma Hp mimics human Hp 2, containing a tandem repeat over the a-chain based on our cloned cDNA sequence. Interestingly, the isolated deer Hp is homogeneous and tetrameric, i.e. (a-b)4, although the locations of )SH groups (responsible for the formation of polymers) are exactly identical to that of human. Denaturation of deer Hp using 6 m urea under reducing conditions (143 mm b-mercaptoethanol), followed by renaturation, sustained the formation of (a-b)4, suggesting that the Hp tetramers are not randomly assembled. Interestingly, an a-chain monoclonal antibody (W1), known to recognize both human and deer a-chains, only binds to intact human Hp polymers, but not to deer Hp tetramers. This implies that the epitope of the deer a-chain is no longer exposed on the surface when Hp tetramers are formed. We propose that steric hindrance plays a major role in determining the polymeric formation in human and deer polymers. Phylogenetic and immunochemical analyses revealed that the Hp 2 allele of deer might have arisen at least 25 million years ago. A mechanism involved in forming Hp tetramers is proposed and discussed, and the possibility is raised that the evolved tetrameric structure of deer Hp might confer a physiological advantage.