Cardiolipin (CL) is a complex lipid molecule that is found in animals, plants, and bacteria. In animals, it is primarily located in the inner mitochondrial membrane. This lipid plays an important structural role in maintaining the proper configuration for mitochondrial enzymes that are involved in respiration and generating a proton gradient. Recent research has shown that CL is also found in smaller amounts in the outer mitochondrial membrane, where it is thought to link the two membranes together. Many human diseases are caused by defects or decreases in the concentration of this compound.
Lipids can be very complicated molecules because they are comprised of fatty acid chains linked to a unit of glycerol with two phosphate units on it. Cardiolipin is so named because it was first identified in cow heart tissue. It is technically a diphosphatidylgycerol lipid. Its fatty acid groups are less complex than those of other molecules, since they are comprised of four chains of linoleic acid, an 18 carbon polyunsaturated fatty acid. The two phosphate groups can have different charges on them, which does permit a great degree of structural variability with this lipid.
Cardiolipin has a number of functions and comprises roughly 25% of the lipids in the inner mitochondrial membrane. Mitochondria are the powerhouses of the cell and serve as the location of respiration and much of the generation of the high-energy compound adenosine triphosphate (ATP). These structures have an outer and inner membrane, which each contain lipids, proteins, and many enzymes, and a compartment between the membranes. CL was originally only thought to be present in the inner membrane, but has since been shown to comprise about 4% of the lipids in the outer membrane. It is thought to form a union between the two membranes that is essential for the functioning of a number of highly important mitochondrial proteins.
Anti-cardiolipin antibodies are used for a variety of medical purposes. One such purpose is in the diagnosis of autoimmune diseases, such as lupus. If a positive result is obtained, the patient is retested six weeks later. They are also used to diagnose syphilis because the bacteria that cause this disease have CL in their membranes. Thrombotic events, such as blood clots in the circulatory system or a history of recurring miscarriages, are additional reasons to use cardiolipin antibodies as a clinical tool.
A number of medical conditions result from defects in CL production or changes in its concentration or structure during aging. Cardiolipin can be oxidized to produce toxic derivatives that are thought to accumulate in the brain. These compounds are hypothesized to be a factor in the development of Parkinson’s disease and Alzheimer’s disease.