Baboon lipoprotein(a) binds very weakly to lysine-agarose and fibrin despite the presence of a strong lysine-binding site in apolipoprotein(a) kringle IV type 10.

Human apolipoprotein(a) kringle IV type 10 [apo(a) KIV(10)] contains a strong lysine-binding site (LBS) that mediates the interaction of Lp(a) with biological substrates such as fibrin. Mutations in the KIV(10) LBS have been reported in both the rhesus monkey and chimpanzee, and have been proposed to explain the lack of ability of the corresponding Lp(a) species to bind to lysine and fibrin. To further the comparative analyses with other primate species, we sequenced a segment of baboon liver apo(a) cDNA spanning KIV(9) through the protease domain. Like rhesus monkey apo(a), baboon apo(a) lacks a kringle V (KV)-like domain. Interestingly, we found that the baboon apo(a) KIV(10) sequence contains all of the canonical LBS residues. We sequenced the apo(a) KIV(10) sequence from an additional 10 unrelated baboons; 17 of 20 alleles encoded Trp at position 70, whereas only two alleles encoded Arg at this position and thus a defective LBS. Despite the apparent presence of a functional KIV(10) LBS in most of the baboons, none of the Lp(a) in the plasma of the corresponding baboons bound specifically to lysine-Sepharose (agarose) even upon partial purification. Moreover, baboon Lp(a) bound very poorly to plasmin-modified fibrinogen. Expression of baboon and human KIV(10) in bacteria allowed us to verify that these domains bind comparably to lysine and lysine analogues. We conclude that presentation of KIV(10) in the context of apo(a) lacking KV may interfere with the ability of KIV(10) to bind to substrates such as fibrin; this paradigm may also be present in other non-human primates.

Identification of a novel apolipoprotein(a)-related protein from the European hedgehog (Erinaceus europaeus).

We have isolated a cDNA encoding an apo(a)-related protein designated HaRP-1 (Hedgehog apo(a) related protein-1). The HaRP-1 cDNA (2114 bp; corresponding to a 2.6 kb transcript) was isolated from a hedgehog liver cDNA library. The HaRP-1 clone corresponded to an open reading frame of 676 amino acids and contains a signal sequence followed by a preactivation domain and 7 kringle domains which exhibit an average of 57% amino acid identity with hedgehog plasminogen kringle III. We expressed HaRP-1 in human embryonic kidney cells; immunoprecipitation of metabolically-labeled conditioned medium from transfected cells showed the presence of a 74 kDa band corresponding to HaRP-1. Of note, we also observed an approximately 72 kDa species present in hedgehog plasma by western blotting using a human anti-apo(a) monoclonal antibody; we speculate that the 72 kDa plasma species corresponds to HaRP-1. Interestingly, although none of the 7 kringle domains contained a canonical lysine-binding site, we found that recombinant HaRP-1 bound specifically to lysine-Sepharose. It is likely that the evolution of the HaRP-1 gene is coincident with the evolution of hedgehog apo(a), both of which occurred by duplication of the plasminogen kringle III motif. The function of HaRP-1 remains unclear at present, but may constititute a member of the family of apo(a) proteins that functions in the regulation of lysine-dependent proteolysis.