Studies on acetyl-CoA carboxylase and fatty acid synthase from rat mammary gland and mammary tumours

Mammalian fatty acid synthase activity from crude tissue lysates tracing ¹³C-labeled substrates using gas chromatography-mass spectrometry

Fatty acid synthase (FASN or FAS, EC 2.3.1.85) is the sole mammalian enzyme to synthesize fatty acids de novo from acetyl- and malonyl-coenzyme A (CoA) esters. This article describes a new method that directly quantifies uniformly labeled (¹³C₁₆-labeled palmitate ([¹³C₁₆]palmitate) by tracing [¹³C₂]acetyl-CoA and [¹³C₃]malonyl-CoA using an in vitro FASN assay. This method used gas chromatography-mass spectrometry (GC-MS) to detect [¹³C₁₆]palmitate carboxylate anions (m/z 271) of pentafluorobenzyl (PFB) derivatives and was highly sensitive at femtomole quantities. Uniformly incorporated [¹³C₁₆]palmitate was the primary product of both recombinant and crude tissue lysate FASN. Quantification of FASN protein within crude tissue lysates ensured equal FASN amounts, preserved steady-state kinetics, and enabled calculation of FASN-specific activity. FASN activity determined by [¹³C₁₆]palmitate synthesis was consistent with values obtained from β-nicotinamide adenine dinucleotide 2'-phosphate (NADPH) oxidation assays. Analysis of FASN activity from tissue extracts was not hampered by contaminating enzymes or preexisting fatty acids. Crude mammary gland and liver lysates had significantly different activities at 82 and 65 nmol min⁻¹ mg⁻¹, respectively, suggesting that tissue-specific activity levels differ in a manner unrelated to FASN amount. GC-MS quantification of [¹³C₁₆]palmitate synthesis permits sensitive evaluation of FASN activity from tissues of varied physiological states and of purified FASN activity in the presence of modifying proteins, enzymes, or drugs.

Characterization of fatty acid synthase activity using scintillation proximity

FAS is a 544-kDa dimeric enzyme consisting of seven functional catalytic components that synthesize long-chain fatty acids from acetyl-CoA and malonyl-CoA using NADPH as a cofactor. We have developed a novel radiometric, homogeneous procedure that directly detects FAS activity. The assay determines incorporation of [(3)H]acetyl-CoA into palmitic acid as catalyzed by FAS from rat liver. Radiolabeled palmitic acid is captured on a 384-well phospholipid-coated microtiter plate and is brought into close proximity with embedded scintillant, stimulating the emission of photons. Because it uses acetyl-CoA and malonyl-CoA as substrates, the procedure mimics the classical reductase assay. However, this method eliminates such labor-intensive steps as organic extraction, aspirations and washes, phase separations, and sample transfers. Furthermore, it offers advantages over photometric and fluorometric methods that indirectly measure FAS activity via NADPH absorbance. We present here kinetic and inhibition data for FAS using scintillation proximity. The assay is shown to be robust and reproducible.