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Identification of sources of hydrogen atoms in fatty acids synthesized using deuterated water and stereospecifically deuterium labelled NADPH by gas chromatographic mass spectrometric analysis.

The mechanism of hydrogen incorporation into fatty acids with rat liver fatty acid synthetase was investigated. The distributions of deuterium atoms incorporated into the fatty acids from D2O and stereospecifically deuterium labelled NADPH were determined by mass chromatography. In palmitic acid, 14 deuterium atoms were incorporated from D2O and were found on the even-numbered methylene carbon atoms (two deuterium atoms per carbon atom). Seven of these 14 deuterium atoms were incorporated as the result of deuterium–hydrogen exchange between the methylene group of malonyl-CoA and C2O and D2O. Seven deuterium atoms came from the 4α-position of NADPH and were found on the odd-numbered methylene carbon atoms (one deuterium atom per carbon atom). Seven deuterium atoms from the 4β-position of NADPH were also found on the odd-numbered methylene carbon atoms (one deuterium atom per carbon atom). The stereospecificities of β-ketoacyl reductase and enoyl reductase for NADPH were determined with S-acetoacetyl-N-acetyl-cysteamine and S-crotonyl-N-acetylcysteamine as substrates. The products, S-hydroxybutryl-N-acetyl-cysteamine and S-butyryl-N-acetylcysteamine, respectively, were analysed by gas chromatography mass spectrometry; the results indicated that the β-ketoacyl reductase had B stereospecificity and enoyl reductase had A stereospecificity for NADPH.