Comparison of 20-, 22-, and 24-carbon n-3 and n-6 polyunsaturated fatty acid utilization in differentiated rat brain astrocytes

Astrocytes convert n-6 fatty acids primarily to arachidonic acid (20:4n-6), whereas n-3 fatty acids are converted to docosapentaenoic (22:5n-3) and docosahexaenoic (22:6n-3) acids. The utilization of 20-, 22- and 24-carbon n-3 and n-6 fatty acids was compared in differentiated rat astrocytes to determine the metabolic basis for this difference. The astrocytes retained 81% of the arachidonic acid ([ 3H]20:4n-6) uptake and retroconverted 57% of the docosatetraenoic acid ([3- 14C]22:4n-6) uptake to 20:4n-6. By contrast, 68% of the eicosapentaenoic acid ([ 3H]20:5n-3) uptake was elongated, and only 9% of the [3- 14C]22:5n-3 uptake was retroconverted to 20:5n-3. Both tetracosapentaenoic acid ([3- 14C]24:5n-3) and tetracosatetraenoic acid ([3- 14C]24:4n-6) were converted to docosahexaenoic acid (22:6n-3) and 22:5n-6, respectively. Therefore, the difference in the n-3 and n-6 fatty acid products formed is due primarily to differences in the utilization of their 20- and 22-carbon intermediates. This metabolic difference probably contributes to the preferential accumulation of docosahexaenoic acid in the brain.