Compartmentation of Fatty Acid Oxidation in Liver Cells

When isolated liver cells from starved rats were incubated with fatty acids, the rates of O2 uptake and ketone body production in the presence of hexanoate were somewhat greater than with added palmitate. However, the 3‐hydroxybutyrate/acetoacetate ratio was consistently higher in the presence of palmitate. Moreover, palmitate was much more effective than hexanoate in slowing the restoration to normal of an elevated lactate/pyruvate ratio. This action of palmitate was counteracted by inhibitors of long‐chain fatty acid oxidation to ketone bodies. Palmitate, but not hexanoate, partially inhibited lactate‐stimulated ethanol oxidation. These effects of palmitate were in most instances exaggerated in cells from clofibrate‐treated rats, which had somewhat higher rates of ketogenesis.When cells were incubated with rotenone, ketone bodies were produced at about 30% of control rates and accumulated almost entirely as 3‐hydroxybutyrate. In the presence of antimycin, ketone body accumulation from added palmitate again occurred at about 30% of control rates, but ketogenesis from endogenous substrates or hexanoate was inhibited to a greater extent. Cells from clofibrate‐treated rats were equally sensitive to antimycin but considerably less inhibited by rotenone than control cells, although the 3‐hydroxybutyrate/acetoacetate ratio remained very high.The depression of fatty acid oxidation in cells incubated with rotenone could be relieved to a considerable degree by coupling the dehydrogenation of hydroxyacyl‐CoA to the reduction of acetoacetate. This observation was found to be valid only for longer chain fatty acids (C10–C18). The coupling of octanoate oxidation to acetoacetate reduction was promoted by the addition of carnitine. Carnitine did not increase the coupling of the oxidation of other fatty acids tested to acetoacetate reduction.The formation of labelled ketone bodies from [1‐14C]palmitate and [1‐14C]hexanoate was also examined. When the two fatty acids were presented together to rotenone‐poisoned cells in the presence of acetoacetate, hexanoate caused some inhibition of 14CO2 formation and ketone body production from [1‐14C]palmitate, but the converse did not apply. In fact palmitate stimulated the oxidation of [1‐14C]hexanoate to 14CO2 and labelled ketone bodies. This stimulation was abolished by low levels of 2,4‐dinitrophenol.The results of these experiments are explained on the basis of compartmentation of fatty acid oxidation between mitochondrial matrix and peroxisomes. Whereas both long‐chain and short‐chain fatty acids can be oxidized within the mitochondrial matrix, an additional site of long‐chain acyl‐CoA oxidation exists in the peroxisomes. Thus, peroxisomes and mitochondria co‐operate in the oxidation of long‐chain fatty acids. The oxidation of long‐chain acyl groups within the peroxisomes and the subsequent transfer of reducing equivalents to the mitochondria, probably via a glycerol 1‐phosphate shuttle, explains why long‐chain fatty acids induce a more reduced ‘redox state’ than short‐chain species in isolated liver cells.