Expression of a Recombinant Kringle V of Human Apolipoprotein(a): Antibody Characterization and Species Specificity

Abstract

Lipoprotein(a) is a macromolecular complex consisting of a low-density lipoprotein-like particle with an additional glycoprotein, apolipoprotein(a) [apo(a)], linked to apolipoprotein B-100 via a disulfide bond. Apo(a) is highly homologous to plasminogen. We have cloned the sequence corresponding to the kringle V domain of apo(a) from human liver cDNA using an experimental approach involving use of the polymerase chain reaction. The protein product of this clone was expressed in the cytoplasmic compartment ofEscherichia colias a MalE fusion protein. Fusion apo(a) Kr V was isolated from cytoplasmic extracts and purified by amylose–agarose affinity chromatography by eluting with 10 mMmaltose. The fusion protein was injected into sheep in order to generate a polyclonal anti-apo(a) Kr V antibody. The antibody raised reacted against both reduced Lp(a) and the C-terminal domain of apo(a), corresponding to a sequence extending from Kr 33 to the C-terminal residue, but did not react with the N-terminal domain containing the repeated Kr IV sequences. The presence of the Kr V sequence was detected in every human apo(a) size isoform tested but only in apo(a) from human and chimpanzee among a panel of apo(a) proteins derived from different animal species.