Biosynthesis of polyunsaturated fatty acids in the developing brain: I. Metabolic transformations of intracranially administered 1-14C linolenic acid.

Thirteen-day old rats were given intracranial injections of 1-14C linolenic acid (all cis 9, 12, 15 octadecatrienoic acid) and were sacrificed after 8 hr. Analysis of brain fatty acids showed that 16:0, 18:0, 18:1, 18:3, 20:3, 20:4, 20:5, 22:5, and 22:6 were labeled. The total fatty acid methyl esters were separated into classes according to degree of unsaturation on a AgNO3:SiO2 impregnated plate. The bands were scraped off and the eluted fatty acids were first analyzed by radio-gas liquid chromatography and then subjected to reductive ozonolysis to determine double bond position. The saturated acids, 16:0 and 18:0, as well as the monosaturated 18:1, must have been formed from radioactive acetate produced by beta oxidation of the injected linolenate. Among the polyunsaturated fatty acids, the triene fraction was characterized and identified as 18:3 omega3 (delta9, 12, 15), the starting material, and 20:3 omega3 (delta11, 14, 17); the tetraene fraction was identified as 20:4 omega3 (delta2, 11, 14, 17); the pentaene fraction was identified as 20:5 omega3 (delta5, 8, 11, 14, 17) and 22:5 omega3 (delta7, 10, 13, 16, 19); and, finally, the hexaene fraction was shown to be 22:6 omega3 (delta4, 7, 10, 13, 16, 19). The biosynthesis of these omega3 family fatty acids in the brain in situ is discussed.