Biosynthesis of polyunsaturated fatty acids in the developing brain: II. Metabolic transformations of intracranially administered [3‐ 14 C] eicosatrienoic acid, evidence for lack of Δ 8 desaturasedesaturase

[3‐ 14 C] Eicosatrienoic acid (Δ11,14,17) chemically synthesized from [1‐ 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, Δ5,11,17–20∶4, and its elongated product, Δ7,13,16,19–22∶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, Δ6,9,12,15–18∶4 and Δ8,11,14,17–20∶4, must have been formed from Δ9,12,15–18∶3, formed by retroconversion of the starting material 20∶3, followed by desaturation and elongation. These results suggest a lack of Δ 8 desaturase in the developing brain, leading to formation of Δ5,11,14,17–20∶4 rather than Δ8,11,14,17–20∶4. Howeve, the nonmethylene interrupted double bond isomer does not restrict chain elongation.