9 - Lipid Transport

Abstract

This chapter discusses how specialized proteins function in redistribution of hydrophobic lipid molecules. A wide variety of distinct lipid transfer proteins have been characterized and their metabolic roles investigated. In 1986, a lipid transfer particle (LTP) was isolated from M. sexta larvae and shown to facilitate vectorial redistribution of lipids among plasma lipophorin subspecies. The concept that LTP functions in flight-related lipophorin conversions correlates well with observed increases in LTP concentration in adult hemolymph compared with other developmental stages. When compared to other lipid transfer proteins, however, LTP displays unique structural characteristics. The large size of LTP permitted examination of its structural properties by negative stain electron microscopy. The lipid component resembles that of lipophorin in that it contains predominantly phospholipid and DAG. An important question arising from these physical characteristics relates to the requirement of the lipid component as a structural entity and its involvement in catalysis of lipid transfer. Studies employing lipoproteins containing radiolabeled lipids in incubations with LTP have revealed that the lipid component of the particle is in dynamic equilibrium with that of lipoprotein substrates. Thus, it is evident that the lipid moiety is not merely a static structural component of LTP; rather, it can be considered as a functional element in the mechanism of lipid transfer.