FA levels were significantly higher in fish fed the high-fat diet than in fish fed the low-fat diet, and an increase in n-3 PUFA levels in the diet is AZ 960 thought to increase CPT I Niltubacin activity via increasing mitochondrial membrane fluidity. However, the present study did not detect this enhancing effect of n-3 PUFA on CPT I activity. It is important to note that PUFAs are prone to oxidative damage, which may negatively affect the function of CPT I because of its strong interaction with the outer mitochondrial membrane. UCP 2 is an inner mitochondrial membrane protein that mediates proton leak by uncoupling fuel oxidation from adenosine triphosphate synthesis. Increased UCP 2 expression is thought to promote substrate disposal and limit mitochondrial ROS production by decreasing the redox pressure on the electron transport chain. In the current study, hepatic UCP 2 expression dramatically increased in fish fed the high-fat diet, which implies high ROS production. Mitochondria play a central role in the energy metabolism of cells and provide most of the ATP by oxidative phosphorylation; thus, mitochondrial lesions impair energy metabolism in the cell. In the present study, SDH and Na,K,ATPase activities were lower in fish fed a high-fat diet than in fish fed a low-fat diet. These two enzymes play important roles in energy metabolism, and any abnormalities in these enzymes may indicate a metabolic disorder. Estimating kinetic constants is critical to describe enzymecatalyzed reactions. In the present study, fish fed a high-fat diet had increased Km and decreased Vmax values for CPT I in the liver. Patterns of enzyme Vmax values across tissues are useful to reveal differences in FA oxidation capacity. Enzymaticcatalytic efficiency relates the total enzyme concentration to the interaction between the enzyme and its substrate. Km is defined as the substrate concentration at which the catalyzed reaction occurs at half its maximum velocity. A small Km indicates that the enzyme requires only a small amount of substrate to become saturated. Hence, the maximum velocity is reached at relatively low substrate concentrations. By contrast, a large Km indicates that high substrate concentrations are needed to achieve the maximum reaction velocity. In this study, the Km of CPT I was significantly higher in fish fed the high-fat diet than in fish fed the low-fat diet. Thus, CPT I has a lower ‘affinity’ for 
FAs in fish fed a high-fat diet, which leads to a lower velocity of oxidation. In a previous study, the low hepatic lipid content in juvenile Synechogobius hasta fed with trans-10, cis-12 conjugated linoleic acid was thought to be owing to the increased affinity of CPT I for its substrates and its increased catalytic efficiency. The mechanisms underlying the differences in the Km of CPT I between fish fed a low-fat diet and those fed a high-fat diet are unknown, but might be explained by the following two hypotheses. First, the difference in Km between the two groups may be associated with the expression profiles of CPT I isoforms owing to the co-expression of multiple CPT I isoforms in the liver. In mammals, CPT Ib has a higher Km than CPT Ia for L-carnitine. Zheng et al. reported that four CPT I isoforms are expressed at the mRNA level in the liver of Pelteobagrus fulvidraco.