Apolipoprotein D (Apo D) is a glycoprotein of estimated molecular weight 33 kDa that in humans is encoded by the APOD gene. Unlike other lipoproteins, which are mainly produced in the liver, Apo D is mainly produced in the brain and testes. It is a component of high-density lipoprotein (HDL) that has no marked similarity to other apolipoprotein sequences. Apo D is closely associated with the enzyme lecithin:cholesterol acyltransferase – an enzyme involved in lipoprotein metabolism. Apo D has also been shown to be an important link in the transient interaction between HDL and LDL particles and between HDL particles and cells.
APO D is a biomarker of androgen insensitivity syndrome (AIS). It is associated with neurological disorders and nerve injury, especially related to myelin sheath. APO D was shown to be elevated in a rat model of stroke. APO D is elevated in patients with schizophrenia, bipolar disorder, and Alzheimer’s disease.