In mitochondrial fat oxidation, CPT I is thought to be a major PR-171 regulatory mechanism with numerous regulating factors, both genetic and non-genetic. In the present study, the mRNA level of CPT I was significantly lower in fish fed a high-fat diet than in fish fed a low-fat diet. The reduced expression of CPT I partly accounts for its low activity. It 
is generally accepted that many of the enzymes that are involved in hepatic FA oxidation and metabolism are influenced by PPARs. Expression of CPT I mRNA is thought to be influenced by the PPAR transcription factors because it contains a PPAR response element. The mammalian PPAR isoforms have also been identified in numerous fish species, but their functional roles are different. PPARa activates lipid catabolism by regulating the expression of target genes encoding enzymes involved in peroxisomal and mitochondrial b-oxidation of FAs, mainly in the liver, while PPARc plays an important role in lipid accumulation and adipocyte differentiation. In the present study, the high-fat diet attenuated PPARa gene expression, which may correlate with the down-regulation of CPT I. PPARa mRNA is generally up-regulated by a high-fat diet in mammals, which is in contrast to the current study. In fish, the function of PPARs in lipid metabolism may be even more complicated because whole genome duplication events lead to multiple isoforms of PPARs. Furthermore, their expression may vary across tissues, making genomic and functional studies much more difficult in fish than in mammals. CPT I may also be inhibited by malonyl-CoA, which is produced during the first step of de novo FA synthesis by acetyl-CoA carboxylase. However, in the present study, liver malonyl-CoA levels did not differ significantly between the two CT99021 GSK-3 inhibitor groups. In addition to CPT I, the number of mitochondria is thought to play a role in determining the fat oxidative capacity of a tissue. In grass carp, the rate of mitochondrial FA oxidation per gram of liver tissue decreases following an increase in dietary lipid intake. This is not due to reduced CPT I activity but to a dramatic decrease in mitochondrial protein content per gram of liver tissue. However, the influence of mitochondrial quantity on fat oxidation has received little attention in fish. The ultrastructure and membrane FA composition of mitochondria have been postulated to be strongly related to the metabolic activity of mitochondria. According to the mitochondria structural data presented in this study, there were distinct differences between blunt snout bream fed a high-fat diet and those fed a low-fat diet. In fish fed a 15% fat diet, mitochondria had fewer cristae, less matrix, and altered metrical density with highly hydropic changes. These changes suggest that mitochondria were damaged by exposure to oxidative stress because reactive oxygen species induce damage that impairs organelle integrity. The observation that SOD activity and MDA levels are increased in fish fed a high-fat diet supports the suggestion that the mitochondria are damaged by oxidative stress. There is a considerable amount of information on how manipulating the dietary FA composition changes the FA content of the mitochondrial membrane in fish. However, little is known about how mitochondrial membranes in fish respond to changes in dietary lipid intake.