FATTY ACID OXIDATION IN SOLUBLE SYSTEMS OF ANIMAL TISSUES

Summary 1. The half-century of investigations directed towards an understanding of the mechanism of β-oxidation of fatty acids may be divided into three periods: (a) from 1904 to 1939 when the oxidation could be studied at the level of the intact animal or isolated organ or tissue slice; (b) from 1939 to 1952 when it could be studied at the mitochondrial level; and (c) from 1952 onwards when it could be reconstructed in non-mitochondrial and soluble enzyme systems. 2. The Knoop-Dakin theory of β-oxidation could not be directly confirmed owing to the non-accumulation of any intermediates. The theory was based on deductions from the nature of the end-products of the metabolism of phenyl fatty acids. 3. The study of fatty acid oxidation at the mitochondrial level led to the recognition that the fatty acids are not oxidized as such but only in the form of some derivative whose formation is tied up with oxidative phosphorylation and the production of adenosine triphosphate (ATP). 4. The transition from the mitochondrial system to soluble enzymes was facilitated first by the discovery that coenzyme A (CoA) was concerned in acyl transfer reactions and later by the recognition that the active fatty acids are indeed the fatty acyl derivatives of CoA. 5. There are four known enzymatic processes by which fatty acyl CoA's are formed: (a) oxidation of pyruvate to acetyl CoA; (b) conversion of fatty acids to fatty acyl CoA's by ATP; (c) replacement of the succinyl group of succinyl CoA by short-chain fatty acids; and (d) cleavage of β-ketoacyl CoA's by CoA with formation of a fatty acyl CoA and acetyl CoA. 6. Two separate enzymes are known to catalyse the oxidation of fatty acyl CoA's to their corresponding transα, β-unsaturated derivatives. The first is a green dehydrogenase containing copper and flavin as prosthetic groups which is active upon acyl CoA's from C3 to C8. The metal is essential for the interaction of this dehydrogenase with cytochrome c. The second is a yellow flavoprotein which is active upon acyl CoA's from C4 to C18. 7. Unsaturated fatty acyl CoA hydrase catalyses the hydration of transα, β- or β, γ-unsaturated CoA's to their corresponding l(+)-β-hydroxyacyl CoA derivatives. The enzyme acts upon all unsaturated derivatives from C4 to at least C12. At equilibrium (pH 9, 250) the ratio β-hydroxyacyl CoA:total unsaturated acyl CoA is 1. 4:1. 8. The β-hydroxyacyl CoA dehydrogenase catalyses the oxidation of l(+)-β-hydroxyacyl CoA by DPN+. The product of oxidation is the corresponding β-ketoacyl CoA. The enzyme is active over the entire range of fatty acid chain length. The E0of the reaction couple at pH 7.0 and 220 is – 0.224 V. The equilibrium point of the oxidation is strongly pH dependent. 9. The β-ketoacyl CoA cleavage enzyme catalyses the reversible cleavage of β-ketoacyl CoA's by another molecule of CoA to form acetyl CoA and a new acyl CoA with two carbon atoms less than the parent β-ketoacyl CoA. 10. The new fatty acyl CoA generated in the cleavage reaction undergoes a repeat cycle of β-oxidation while the C2 unit (acetyl CoA) undergoes condensation with oxalacetate to form citrate. 11. Each of the component reactions in the β-oxidation cycle has been shown to be reversible. 12. The asymmetric labelling of acetoacetate formed during oxidation of labelled fatty acids by liver homogenates or mitochondrial suspensions is a phenomenon which can readily be explained in terms of the mechanism of the β-ketoacyl CoA cleavage enzyme. 13. The factors which militate against the accumulation of intermediates during fatty acid oxidation are discussed. 14. The accumulation of acetoacetate in any tissue requires a combination of two essential conditions: (a) presence of acyl CoA deacylase; and (b) absence of a β-ketoacid activation enzyme. 15. Assuming that in the diabetic the operation of the citric acid cycle is subnormal by virtue of reduced conversion of glucose to pyruvate, it is possible to explain the accumulation of ketone bodies and its abolition by insulin in terms of the known enzyme reactions of the β-oxidation cycle. 16. The predominance of C16 and C18 fatty acids in lipids may be due to the fact that only the acyl CoA's of these particular acids dissociate to a sufficient degree from combination with the enzymes of the fatty acid oxidizing system as to become available for ester synthesis. It is a great pleasure to acknowledge my indebtedness to Dr Helmut Beinert for his advice and assistance in the preparation of the manuscript and to Drs H. R. Mahler, D. R. Sanadi and S. J. Wakil for their suggestions.