The effects of coenzyme a and carnitine on steady-state ATP/ADP ratios and the rate of long-chain free fatty acid oxidation in liver mitochondria

Abstract

1. 1. Conditions for the optimal coupled oxidation by rat liver mitochondria of long-chain free fatty acids were defined. The fatty acids studied were in the ω9 series: oleic (18 : 1), gondoic (20 : 1) and erucic (22 : 1) acids. Carnitine (about 0.1 mM) maximally stimulated State 3 respiration due to oleic and gondoic acids about three-fold (coenzyme A present), and coenzyme A (10–20 μM) stimulated about two-fold (carnitine present). When neither coenzyme A nor carnitine was added, respiration was very slow. 2. 2. When respiration was limited by ADP, concentrations of added CoA only slightly in excess of that required for fatty acid oxidation very significantly decreased the ATP/ADP ratio maintained at a given rate of respiration imposed by externally added ATPase, and increased the level of membrane-associated acyl-CoA. This effect was most pronounced with oleic acid, and least with erucic acid. When excess ADP was present, higher concentrations of added coenzyme A (50–200 μM) inhibited the oxidation of oleic acid in a concentration-dependent manner, whereas the oxidation of substrates other than fatty acids was essentially unaffected. 3. 3. It is concluded that, in addition to its requirement for fatty acid oxidation, coenzyme A exerts two independent effects on mitochondrial metabolism as here determined in vitro: (a) under conditions mimicking those in the intact cell with respect to phosphorylation-dependent respiration (ADP limiting), acyl-CoA formed from added coenzyme A and fatty acid inhibits the adenine nucleotide translocase, resulting in a lowering in the extramitochondrial ATP/ADP ratio obtained at any given rate of phosphorylation-limited respiration, and (b) under State 3 conditions (ADP in excess) coenzyme A (<50–200 μM) specifically suppresses oxidation of long-chain fatty acids by limiting the rate of formation of intramitochondrial acyl-CoA.