The Calorigenic Nature of Hepatic Ketogenesis: An Explanation for the Stimulation of Respiration Induced by Fatty Acid Substrates
Authors: BERRY Michael N. , CLARK Dallas G. , GRIVELL Anthony R. , WALLACE Patricia G. 
Published: March 2005

The rates of O2 consumption during fatty acid oxidation to acetyl‐CoA and to CO2 were determined for isolated liver cells from starved rats on the basis of measured rates of respiration and ketogenesis. About 60% of the endogenous O2 uptake was associated with acetyl‐CoA formation. The remainder was assumed to represent total combustion of fatty acid to CO2 and H2O through the Krebs cycle. In the absence of added fatty acid, 1.90 μmol · min‐1· g‐1 of acetyl‐CoA was generated, of which 1.40 μmol · min‐1· g−1 gave rise to ketone bodies. O2 consumption was stimulated about 30% by the addition of 2 mM palmitate or 4 mM hexanoate. This increase was entirely due to stimulation of O2 consumption related to oxidation of fatty acid to acetyl‐CoA. The extra acetyl‐CoA produced was channelled into ketone body formation.

The gluconeogenic substrate lactate, which increases hepatic demand for ATP, stimulated O2 consumption almost 40%, but in this case the increase was due to an enhancement of Krebs cycle activity, and acetyl‐CoA generation was depressed 25%. In the absence of added fatty acid, 83% of the total acetyl‐CoA produced by cells incubated with lactate was oxidized in the Krebs cycle. Addition of fatty acid caused a further stimulation of O2 uptake which reflected a promotion of acetyl‐CoA production with an associated increase in ketone body formation.

The uncoupling agent, 2,4‐dinitrophenol, stimulated total O2 consumption of cells incubated without exogenous substrate or with added fatty acid. In each circumstance this stimulation of O2 uptake was related entirely to enhancement of Krebs cycle activity and depression of ketogeneis. The large increment in O2 consumption induced by the combination of lactate and 2,4‐dinitrophenol in the presence of added fatty acid was likewise attributable solely to a marked stimulation of Krebs cycle flux. Added ethanol caused only a small inhibition of total acetyl‐CoA production and had little effect on respiration, but considerably depressed Krebs cycle activity. This depression was relieved by 2,4‐dinitrophenol.

Oligomycin and antimycin had markedly different effects on fatty acid catabolism although both inhibitors depressed O2 consumption to the same extent. Oligomycin caused only a small diminution of rates of acetyl‐CoA formation but induced a strong depression of cellular ATP concentrations and of acetyl‐CoA oxidation to CO2. In contrast antimycin had no effect on acetyl‐CoA oxidation and brought about only a small decline in cellular ATP levels, but considerably depressed formation of acetyl‐CoA.

These results are interpreted as indicating that addition of fatty acid to liver cells induces various energy‐dependent processes, including ATP synthesis, reversed electron transfer and metabolite translocation, all of which contribute to the observed stimulation of O2 uptake. It is inferred that oxidation of acetyl‐CoA to CO2 is preferentially coupled to ATP synthesis whereas oxidation of fatty acid to acetyl‐CoA is obligatorily associated with reversed electron transfer. Thus, ketogenesis is not constrained by the hepatic phosphorylation potential and provides a mechanism for the rapid disposal of fatty acids, independent of the ATP demands of the liver.