Biosynthesis of Docosahexaenoic Acid in Trout Hepatocytes Proceeds Via 24-Carbon Intermediates

The role of 24:5n-3 and 24:6n-3 as intermediates in the formation of 22:6n-3 in trout liver was examined. Microsomes prepared from trout liver converted [1-14C]-eicosapentaenoic acid (20:5n-3) to 24:5n-3 and 24:6n-3 but not docosahexaenoic acid (22:6n-3). The radiolabeled 24:5n-3 and 24:6n-3 were isolated from the microsomal incubations by argentation chromatography and used as substrates in incubations with hepatocytes isolated from trout liver. Both 14C-labelled 24:6n-3 and 22:6n-3 were produced by hepatocytes incubated with radiolabelled 24:5n-3. When hepatocytes were incubated with radiolabelled 24:6n-3, the amount of radioactivity recovered in 22:6n-3 over 6 hr increased in direct relation to the decrease observed in the amount of radioactivity recovered in 24:6n-3. The results suggest that the formation of 22:6n-3 in trout liver does not involve Δ4 desaturation of 22:5n-3 but rather proceeds via the Δ6 desaturation of 24:5n-3 with the subsequent chain shortening of the 24:6n-3 produced.