ABSTRACT
The complexity of HDL provides a clue to the diverse nature of the biological events with which it is associated and at least an expectation that it is likely to be involved in many functions rather than just one physiological process. The degree of complexity of this lipoprotein class has been addressed in detail in another chapter; suffice it to remind the reader that HDL comprises several subclasses varying in size and composition and that the existence of these various particles is inextricably related to an organised lipid transport system. A key feature of the transport system is remodelling and probable recycling of the macromolecules in response to the action of lipolytic enzymes and lipid transfer proteins. These plasma factors orchestrate an elaborate exchange and transfer of both lipid and apolipoprotein moieties producing a number of identifiable subclasses of HDL particles that appear to have different functions in the body. The rearrangement of HDL molecules occurs concomitantly with changes in the composition of the other lipoprotein classes resulting from an exchange or transfer of components between VLDL, LDL or chylomicrons: these interactions are integral to the transport of lipid in the blood, producing and maintaining a balance between a supply of energy to all tissues, and the removal of excess fat from the body. The plasma factors referred to above play a major role in modulating these pathways, and much information is now available about their regulation by hormones and diet.
