Mitochondrial respiration on rumenic and linoleic acids

Rumenic acid (cis-9, trans-11-C(18:2)) represents approx. 80% of conjugated linoleic acid (CLA) in dairy products. CLA has been shown to exert beneficial effects on health, but little work has been devoted to the ability to oxidize CLA isomers and the role of these isomers in the modulation of beta-oxidation flux. In the present study, respiration on rumenic acid was compared with that on linoleic acid (cis-9, cis-12-C(18:2)) with the use of rat liver mitochondria. In state-3, respiration was decreased by half with rumenic acid in comparison with linoleic acid. In the uncoupled state, respiration on CLA remained 30% lower. The lower ability to oxidize CLA was investigated through characterization of the enzymic steps. Rumenic acid was 33% less activated by acyl-CoA synthase than was linoleic acid. However, after such activation, the transfer of both acyl moieties to carnitine by carnitine acyltransferase I (CAT I) was of the same order. Moreover, CAT II activity was comparable with either isomer. After prior incubation with rumenic acid, oxidation of octanoic acid by re-isolated mitochondria was unimpaired, but that of palmitoleic acid was impaired unless linoleic acid was used in the prior incubation. The slower respiration on cis-9, trans-11-C(18:2) is suggested to arise from lower carnitine-acylcarnitine translocase activity towards the acylcarnitine form, causing an upstream increase in the corresponding acyl-CoA.