Metabolism and control of lipid structure modification.

Abstract

The lipid composition characteristic of a particular cellular membrane can become significantly altered, sometimes quite suddenly, when the cell is placed under environmental stress. In the majority of cases examined, the alterations seem to return the membrane's physical state towards that existing prior to imposition of the stress. The compositional changes are often diverse in their nature and also in their site of origin within the cell. Certain modifications, such as changes in the degree of phospholipid acyl chain unsaturation and in the reordering of fatty acid pairing on specific phospholipids, are now recognized as crucial first responses to stress, while others (e.g., fluctuations in relative proportions of different phospholipid classes and in the sterol:phospholipid ratio) develop more slowly and may represent secondary adjustments to the initial lipid changes. The factors directly responsible for modifying membrane lipid composition are generally unknown at the molecular level, but recent advances provide new clues favoring involvement, in some cases, of the ubiquitous mediator Ca2+. In other cases, the physical state of a membrane may directly modulate the activity of lipid-metabolizing enzymes embedded therein.