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  • methane monooxygenase Large yellow croaker Larimichthys croc

    2020-07-15

    Large yellow croaker (Larimichthys crocea) are widely cultured in China but have inferior meat texture compared with wild fish. This result results from excess lipid deposition in the muscle, which reduces mechanical strength (Thakur et al., 2003). The liver is the central of lipid metabolism and storage in this species. Hepatic lipids transfer to muscle when fat intake exceeds capacity of liver lipid storage. On the other hand, fat mobilize from muscle to liver to provide energy by fatty oxidation when fish are in energy deficiency. Therefore, farmed large yellow croaker commonly fast before slaughter in order to improve flesh quality by activating hepatic lipolysis. Considering the crucial role of CPT 1 in fatty methane monooxygenase degradation, understanding the regulatory properties of CPT1 is of significance in practice. However, long-term fasting induced oxidative damage in liver of large yellow croaker (Zheng et al., 2016). Thus, the duration of starvation should be carefully considered. In the present study, we have successfully cloned complete cDNA sequence of cpt1a, and found recombinant protein CPT1A could be expressed in Pichia pastoris. Subsequently, we described the cloning, molecular characterization and the tissue expression profile of cpt1a, and compared kinetic parameters (Vmax and Km) of CPT1 in Pichia pastoris and liver of large yellow croaker. Kinetic parameters, free carnitine (FC) concentrations, CPT1 activity, cpt1a mRNA expression levels, and lipid content were investigated in liver of large yellow croaker during starvation. To the best of our knowledge, this is the first to characterize fish CPT1 function by yeast expression system, and also the first time to suggest the effects of fasting on FC status and CPT1 kinetics in ish. Due to the role of CPT1 in fuel utilization, the fundamental knowledge gained from our study is imperative to understand the mechanism of environmental stressors on lipid metabolism in fish.
    Materials and methods
    Results
    Discussion The cloning of complete cDNA sequences of cpt1 has been reported in several fish species (Boukouvala et al., 2010; Wu et al., 2016; Zheng et al., 2013a, Zheng et al., 2013b). These studies indicate that there are multiple cpt1 genes due to genome duplication (Zheng et al., 2013a). These authors indicate that each cpt1 gene may play a role in the utilization of fatty acids because mRNA expression patterns are responsive to environmental stressors in teleosts. Although CPT1 activity could be detected in tissues, we did not confirm which gene has a biological function due to mRNA co-expression of different genes. In the present study, four cDNA complete sequences from cpt1 genes for large yellow croaker were successfully cloned. However, we failed to get functional production of recombinant proteins except rCPT1A. In the present study, only rCPT1A showed catalytic activity. In addition, bioinformatics analysis demonstrated the presence of conserved functional motifs and amino acid residues in CPT1A protein from large yellow croaker. The kinetic characteristics of CPT1A and CPT1B, encoded by cpt1a and cpt1b respectively, have been well elucidated in mammals. McGarry and Brown (1997) pointed out that CPT1A possessed a low Km for carnitine (0.03 mM) and CPT1B had a high Km (0.50 mM) for carnitine in rat. In the present study, the Km value of rCPT1A for carnitine was about 1.38 mM, which is >40 times of rat CPT1A. According to enzyme kinetics, an increase in Km is often accompanied by a rise in Vmax. Patterns of enzyme Vmax are useful in revealing differences in fatty acid oxidation capacity (Morash et al., 2008). It is widely accepted that fish utilize lipids more efficiently than terrestrial animals (Zhou et al., 2016). Maximal CPT I activities well paralleled with the Km for carnitine in liver and muscle of mammals (McGarry et al., 1983; Morillas et al., 2002, Morillas et al., 2003). There is a significant difference in carnitine Km between CPT1A and CPT1B in mammals. And Zheng et al. (2013a) and Wu et al. (2016) suggested two genes encoding CPT1A and CPT1B showed different Km for carnitine in fish by bioinformatics. We guessed that CPT1A might mainly be expressed in the liver of large yellow croaker, since carnitine Km of rCPT1A was similar with that of liver tissue. In fish, there are multiple genes that encode CPT1A, changes in kinetics could be also explained by changes in expression levels of different isforms (Zheng et al., 2013a).