Apolipoprotein D--an atypical apolipoprotein.

The structure of ApoD and its sites of synthesis have been discovered. These characteristics differ from those of the other apolipoproteins. The role of ApoD in the plasma lipoprotein system remains to be discovered, but the recent, rapid increase in our knowledge of this protein suggests that it plays an important role in the homeostasis or housekeeping of probably all organs. One of its functions is likely to be the transport of a hydrophobic ligand (a lipid) in a one-to-one molar ratio with itself. This transport is likely to occur unidirectionally between neighboring cells in an organ, and between perivascular cells and the blood circulation. The chemical structure of the natural ligand, or ligands, of ApoD in normal cells in vivo or in culture is not known, but ApoD has been shown to bind some steroids and bilirubin. Remarkable upregulation of synthesis of ApoD has been observed during regeneration of injured peripheral nerves. Perhaps the physiologic role of ApoD will prove to be more interesting and of equal importance in biology to the roles of the other apolipoproteins in cardiovascular disease.