Formation of acetoacetate from fatty acids by particulate systems of rat liver.

The relative importance of oxidation through the Krebs cycle or of conversion to acetoacetate for the oxidation of fatty acids in liver is unknown. Attempts have been made to answer this question by studies of the products of the oxidation of fatty acids in particulate systems (l-7). In most of the experiments this process has required the presence not only of adenine nucleotides, magnesium ion, and cytochrome c, but also “activators” such as heated extracts of liver (6), intermediates of the Krebs cycle (2, 4, 7), or reduced diphosphopyridine nucleotide (DPN) (3). The type of activator added apparently determines the pathway of oxidation (2, 3). Thus in the presence of reduced DPN the carbon chains of both long and short chain acids are converted to acetoacetate (3), whereas, when sources of oxalacetate are added, the fatty acids, particularly those with long chains (2), have a greater tendency to be oxidized through the Krebs cycle (2, 7, 8). However, little information is available as to the pathway of oxidation of fatty acids in systems which do not require the addition of such activators (1, 5). Since these undoubtedly contain intact activating systems, it seems possible that a closer approximation to the major pathway of fatty acid oxidation in liver might be obtained by further study of such preparations. Therefore, the effect of chain length of the fatty acid on acetoacetate yield was investigated in systems which did not require the addition of activators for maximal rate of oxidation. The fatty acids studied were the homologous series of fatty acids ranging in chain length from Cz to Cl?. The particulate systems used included washed particles from liver, liver mitochondria prepared in hypertonic sucrose, and liver mitochondria prepared in isotonic sucrose. The results obtained emphasize the importance of conversion to acetoacetate in the oxidation by liver of fatty acids of even carbon number.