Biosynthesis of fatty acids in cell-free preparations. 4. Synthesis of fatty acids from acetate by a partially purified enzyme system from rabbit mammary gland

Abstract

A partially purified enzyme preparation from lactating rabbit mammary gland catalyzes the synthesis of even-num-bered-chain fatty acids from C4 to C18, with the shorter-chain fatty acids preponderating. This synthesis is achieved from acetate, CoA, adenosine triphosphate and stoichiometric amounts of reduced diphos-phopyridine nucleotide (DPNH). Intermediates were studied by paper chromatography, as their hydroxamates. The findings suggest that fatty acid synthesis in these enzyme preparations takes place by a stepwise condensation of C2 units, through the reversal of the now well-established [beta]-oxidation of even-numbered-chain fatty acids. TPNH and lipoic acid had no stimulatory effect upon the synthesis of even-numbered-chain fatty acids by the mechanism outlined above. DPNH appears a specific electron donor for both the reduction of B-ketoacyl-CoA to B-hydroxyacyl-CoA and of unsaturated fatty acyl-CoA to saturated fatty acyl-CoA. This latter reduction step appears stimulated in some manner, as yet undefined, by a metabolic derivative of adenosine 5-phosphate, which is not hypoxanthine, and which may possibly be inosine monophosphate.