Conversion of 18:3 Delta 9cis, 12cis, 15trans in rat liver microsomes

Several years ago, it was established that the Delta 15 trans isomer of alpha-linolenic acid is converted in vivo into fatty acids containing 20 and 22 carbons (geometrical isomers of eicosapentaenoic and docosahexaenoic acids). The present study focused on the in vitro Delta 6 desaturation, the first step of the biosynthesis of the n-3 long-chain polyunsaturated fatty acids from 18:3n-3. For that purpose, rat liver microsomes were prepared and incubated with radiolabeled 18:3 Delta 9 cis, 12 cis, 15 cis (18:3 c,c,c) or 18:3 Delta 9cis, 12cis, 15 trans (18:3 c,c,t) under desaturation conditions. The data show that 18:3c,c,t is converted at a lower rate compared with alpha-linolenic acid. The product of conversion of 18:3 c,c,t may be 18:4 Delta 6cis, 9cis, 12cis, 15 trans resulting from a Delta 6 desaturation of the trans substrate. Moreover, the conversion of radiolabeled 18:3 c,c,t was strongly decreased by the presence of 18:3 c,c,c (up to 48%) while the 18:3 c,c,t only slightly decreased the conversion of radiolabeled 18:3c,c,c. Thus, the desaturation enzyme presented a higher affinity for the native all-cis n-3 substrate.