Biological Activities of essential fatty acids
Membrane Structure and Function
Omega-6 and omega-3 PUFA are important structural components of cell membranes. When incorporated into phospholipids, they affect cell membrane properties such as fluidity, flexibility, permeability and the activity of membrane bound enzymes. DHA is selectively incorporated into retinal cell membranes and postsynaptic neuronal cell membranes, suggesting it plays important roles in vision and nervous system function.
DHA is found in very high concentrations in the cell membranes of the retina, which conserves and recycles DHA even when omega-3 fatty acid intake is low. Animal studies indicate that DHA is required for the normal development and function of the retina. Moreover, these studies suggest that there is a critical period during retinal development when inadequate DHA will result in permanent abnormalities in retinal function. Recent research indicates that DHA plays an important role in the regeneration of the visual pigment rhodopsin, which plays a critical role in the visual transduction system that converts light hitting the retina to visual images in the brain.
The phospholipids of brain gray matter contain high proportions of DHA and AA, suggesting they are important to central nervous system function. Brain DHA content may be particularly important, since animal studies have shown that depletion of DHA in the brain can result in learning deficits. It is not clear how DHA affects brain function, but changes in neuronal cell membrane DHA content could alter the function of ion channels or membrane associated receptors, as well as the availability of neurotransmitters.
Eicosanoids are potent chemical messengers derived from 20-carbon PUFA that play critical roles in immune and inflammatory responses. During an inflammatory response, DGLA, AA and EPA in cell membranes can be metabolized by enzymes known as cyclooxygenases and lipoxygenases to form prostaglandins and leukotrienes, respectively. In those who consume typical Western diets, the amount of AA in cell membranes is much greater than the amount of EPA, resulting in the formation of more eicosanoids derived from AA than EPA. However, increasing omega-3 fatty acid intake increases the EPA content of cell membranes, resulting in higher proportions of eicosanoids derived from EPA. Physiological responses to AA-derived eicosanoids differ from responses to EPA-derived eicosanoids. In general, eicosanoids derived from EPA are less potent inducers of inflammation, blood vessel constriction, and coagulation than eicosanoids derived from AA.
Regulation of Gene Expression
The results of cell culture and animal studies indicate that omega-6 and omega-3 fatty acids can modulate the expression of a number of genes, including those involved with fatty acid metabolism and inflammation. Although the mechanisms require further clarification, omega-6 and omega-3 fatty acids may regulate gene expression by interacting with specific transcription factors, including peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs).
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