PLASMA-LIPID TRANSPORT IN THE HEDGEHOG - PARTIAL CHARACTERIZATION OF STRUCTURE AND FUNCTION OF APOLIPOPROTEIN-A-I

Apart from exhibiting the presence of lipoprotein [a] in its plasma, a nother interest of the European hedgehog in lipoprotein research lies in the quantitative prominence of a complex spectrum of high density l ipoproteins (HDL) and very high density lipoproteins (VHDL) as cholest erol transporters in plasma (Laplaud, P. M. et al. 1989. Biochim. Biop hys. Acta. 1005: 143-156). We, therefore, initiated studies in the fie ld of reverse cholesterol transport in the hedgehog. As a first step, we characterized apolipoprotein A-I (apoA-I), the main protein compone nt of hedgehog HDL and VHDL. Proteolytic cleavage of apoA-I (M(r) appr ox. 27 kDa) using two different enzymes resulted in two sets of peptid es that were subsequently purified by high performance liquid chromato graphy, and that allowed us determination of the complete protein sequ ence. Hedgehog apoA-I thus consists of 241 amino acid residues and exh ibits an overall 58% homology to its human counterpart, i.e., the lowe st value observed to date among mammalian species. However, it retaine d the general organization common to all known apoA-Is, i.e., a series of amphipathic helical segments punctuated by proline residues. Circu lar dichroism experiments indicated a helical content of approx. 45%, increasing to approx. 58% in the presence of lecithin unilamellar lipo somes. Apart from other differences, amino acid composition analysis s hows that hedge hog apoA-I contains four isoleucine residues, while th is amino acid is totally absent from the corresponding protein in high er mammals. Polyclonal antibodies raised against hedgehog apoA-I faile d to detect any cross-reactivity between the animal and human proteins , although comparative prediction of the respective antigenic structur es using the Hopp-Woods algorithm indicated that several potentially a ntigenic sites may occur in similar regions of the protein. Finally, h edgehog apoA-I was shown to be able to activate lecithin:cholesterol a cyl transferase, although it was 4 to 5 times less efficient in this r espect than the human protein.