Fatty acid β-oxidation is an essential process in lipid metabolism, providing a catabolic pathway to balance fatty acid synthesis and intake from the diet. This process generates energy for multiple organs, including the cardiac muscle, kidney cortex, and fasted liver, and utilizes fatty acids obtained from the diet, adipose tissue lipolysis, and internal triglyceride stores1,2.
Oxidation of fatty acid through the β-oxidation pathway results in the sequential shortening of the fatty acyl chain by two carbons at a time, released as acetyl-CoA, and this process occurs both in the mitochondria and the peroxisomes. While most fatty acids undergo only β-oxidation, some are oxidized at different carbons before entering this pathway. For example, 3-methyl-substituted fatty acids, such as phytanic acid, undergo removal of one carbon by α-oxidation in the peroxisomes before entering the β-oxidation pathway. Similarly, some fatty acids are first converted to dicarboxylic fatty acids by oxidation of the terminal methyl group (ω-oxidation) in the endoplasmic reticulum before being preferentially oxidized in the peroxisomes by β-oxidation3.
Regardless of the specific organelle, a fatty acid must first be converted to a coenzyme A (CoA) thioester, or acyl-CoA, to be oxidized through the β-oxidation pathway. β-Oxidation of long-chain acyl-CoAs in the mitochondrial matrix requires the carnitine shuttle for their translocation, where carnitine palmitoyltransferase 1 (CPT1) catalyzes the conversion of acyl-CoAs to acylcarnitines and is the rate-limiting enzyme in this process4. Once translocated to the mitochondrial matrix, the acyl-CoAs are re-formed and serve as substrates for the mitochondrial β-oxidation machinery. In the fasted state, the acetyl-CoA produced through β-oxidation in hepatic mitochondria is primarily channeled to ketogenesis. Peroxisomes serve as the primary site for the β-oxidation of very long-chain, branched-chain, and dicarboxylic fatty acids. Peroxisomes do not require the carnitine shuttle to import fatty acid substrates, instead importing the correspondent acyl-CoAs through the activity of the ATP-binding cassette (ABC) transporters ABCD1-35. Within the peroxisomes, acyl-CoAs are then oxidized by a dedicated set of enzymes, distinct from the mitochondrial fatty acid β-oxidation machinery. Both mitochondria and peroxisomes also require a supply of NAD+ and free CoA to oxidize fatty acyl chains. CoA levels in the liver have been shown to increase in response to fasting, supporting the increased rate of fatty acid oxidation which occurs in this state6. Furthermore, increased CoA degradation in the peroxisomes results in a selective decrease in peroxisomal fatty acid oxidation7. Therefore, the process of fatty acid oxidation within the cell is regulated by the expression levels and activities of enzymes involved in the activation, transport, and oxidation of fatty acids, as well as the concentrations of cofactors and other metabolites throughout multiple subcellular compartments.
Procedures using tissue homogenates to measure fatty acid oxidation destroy the cellular architecture regulating and supporting this process, leading to a collection of data that does not accurately reflect the in vivo metabolism. While techniques using plated primary hepatocytes preserve this system, culturing isolated cells for extended periods of time results in a loss of the in vivo gene expression profile that was present in the cells when they were still living within the animal8,9. The following protocol describes a method to isolate primary hepatocytes and assay their capacity for fatty acid β-oxidation immediately after isolation and in suspension, using [1-14C]palmitic acid. The assay is based on the measurement of the radioactivity associated with the acid-soluble metabolites (ASM) or products, like acetyl-CoA, produced by the β-oxidation of [1-14C]palmitic acid10,11.