Stereospecificity of the Transfer of Hydrogen from Reduced Nicotinamide Adenine Dinucleotide Phosphate to the Acyl Chain in the Dehydrogenase-catalyzed Reactions of Fatty Acid Synthesis

Tritium from NADP3H was incorporated into the fatty acids (mainly palmitic acid) synthesized from acetyl and malonyl coenzyme A by purified pigeon and rat liver fatty acid synthetases. The quantity of tritium incorporated into the product was nearly the same from the A and B forms of NADP3H. These compounds were prepared from tritiated isocitric and glutamic acids, respectively. The quantity of radioactivity incorporated into fatty acids per mole of NADPH oxidized was slightly less than half the quantity of radioactivity present per mole of isocitric or glutamic acid. On the assumption that the two types of reduction in fatty acid synthesis (β-ketoacyl-enzyme and α,β-unsaturated acyl-enzyme) proceed stereospecifically, as do all other pyridine nucleotide dehydrogenase reactions, these results indicate that one reduction proceeds by the transfer of hydrogen from the A side of position 4 of the pyridine ring of NADPH, and the other reduction proceeds by transfer from the B side. This deduction was confirmed by the performance of partial reactions with model substrates. When acetoacetyl coenzyme A or S-acetoacetyl-N-acetylcysteamine was reduced to the β-hydroxybutyryl ester with fatty acid synthetase and tritiated NADPH, tritium was incorporated into the product only from the B form of the coenzyme. When S-crotonyl-N-acetylcysteamine was similarly reduced to the butyryl ester, tritium was incorporated into product only from the A form of NADPH. Thus, the reduction of β-ketoacyl-enzyme in fatty acid synthesis results in the transfer of hydrogen from the B side of NADPH, and the reduction of α,β-unsaturated acyl-enzyme results in the transfer of hydrogen from the A side of NADPH.