Biosynthesis of polyunsaturated fatty acids in the developing brain: II. Metabolic transformations of intracranially administered [3-(14)C] Eicosatrienoic acid, evidence for lack of delta8 desaturase.

[3-(14)C] Eicosatrienoic acid (delta11,14,17) chemically synthesized from [-(14)C] linolenic acid was injected intracranially into 14-day old rats and sacrificed 8 hr later. The analysis of brain fatty acids by radio-gas liquid chromatography before and after ozonolysis showed that the tetraene fraction consisted of a desaturated product, delta5,11,17-20:4, and its elongated product, delta7,13,16,19-22:4. Both of these products, with a combined total of 61% of the total radioactivity recovered in the tetraene fraction, contain a nonmethylene interrupted double bond system and, therefore, are unsuitable for further desaturation. The other two components, delta6,9,12,15-18:4 and delta8,11,14,14-20:4, must have been formed from delta9,12,15-18:3, formed by retroconversion of the starting material 20:3, followed by desaturation and elongation. These results suggest a lack of delta8 desaturase in the developing brain, leading to formation of delta5,11,14,17-20:4 rather than delta8,11,14,17-20:4. However, the nonmethylene interrupted doulbe bond isomer does not restrict chain elongation.