Lipoproteins

Overview

Lipid transportation through aqueous media requires the presence of amphipatic mediators between the aqueous and the lipid phase. In animals the lipoproteins, a class of evolutionary old and highly conserved molecules, serve this purpose. Lipoproteins consist of a lipid part and a protein part, termed apolipoproteins, which stabilize the particle. They have been described and characterized in nearly all animal phyla. Most attention have received the vertebrate, particularly the mammalian lipoproteins, due to their role in cardiovascular diseases and the insect lipoproteins (lipophorins), due to their energy demanding flight metabolism. But also on other arthropod and invertebrate lipoproteins, such as crustaceans, chelicerates, molluscs and polychaetes studies are available.

Depending on their buoyant density lipoproteins are categorized into different classes: very low density lipoproteins (VLDL), low density lipoproteins (LDL), high density lipoproteins (HDL) and very high density lipoproteins (VHDL). While in vertebrates all density classes are present, the majority in invertebrates are the HDL.

Insect lipophorins consist of two integral, non-exchangeable lipoproteins with molecular masses of 220-250kDa and 75-85kDa. Respectively, these can be accompanied by an exchangeable apolipoprotein with a mass of 17-20kDa. In crustaceans the lipoproteins seem to consist of a single homodimer of a ~100kDa subunit, although most recently a unique lipoprotein with a apolipoprotein pattern similar to that of the insect lipophorins has been discovered in a freshwater crayfish. A similar apolipoprotein pattern has been reported for the only described polychaete lipoprotein until now, which consists of two apolipoproteins of ~290 and 85kDa.

Most invertebrate lipoproteins have a lipid content of ~30-50%. The most abundant lipid class in any lipoprotein are phospholipids, due to their polar characteristics. Among these, phosphatidylcholine is dominating. However in many insects phosphatidylethanolamine is the major phospholipid while in most arthropod lipoproteins diacylglycerols are the predominating neutral lipid species.

Morphologically the lipoproteins can be grouped in to two distinct classes. On the one hand there are those that are globular in shape (i.e. the mammalian LDL or the insect lipophorins), on the other hand are those who are discoidal in appearance. Among the the latter two groups can be distinguished. There are those that are discoidal in a transitory state during maturation (mammalian HDL) and those that are discoidal in their mature state, which are considered to represent a basal form.

Discoidal lipoproteins

by S. Schenk

Discoidal lipoproteins have been described from vertebrates and invertebrates alike. Discoidal lipoproteins in vertebrates are only present as transitory states in lipoprotein maturation. In invertebrates discoidal lipoproteins have been described and characterized from a variety of species, such as the lobster, the rock crab and the horseshoe crab, the Galizian Crayfish and the polychaetous annelid Nereis virens. However, only the hemolymph lipoproteins of the latter are so called large discoidal lipoproteins, which are characterized by their large diameter of ~40 nm which overcomes that described for the other discoidal lipoproteins so far by the factor of three. As these large discoidal lipoproteins have been only described in aquatic invertebrates it has been speculated that they might be restricted to those animals, however their occurrence in terrestrial animals cannot be ruled out since studies regarding lipoprotein architecture in basal terrestrial invertebrates are lacking.