Pyruvate kinase genes in grass carp Ctenopharyngodon idella: molecular characterization, expression patterns, and effects of dietary carbohydrate levels.

To explore features of carbohydrate metabolism and evolution of carbohydrate metabolism-associated genes in herbivorous fishes, the open reading frames (ORF) of PKL, PKMa, and PKMb genes of grass carp (Ctenopharyngodon idella) were obtained, encoding 538, 528, and 532 amino acids, respectively. Comparative genomic analysis showed that adjacent PK genes were highly conserved between fish and mammals. Gene expression profiles were quite different between the three PK genes in tissues and at developmental stages. PKL, PKMa, and PKMb had the highest expression levels in the liver, heart, and muscle, respectively. During embryogenesis, high expression levels of PKMa and PKMb were detected in unfertilized and fertilized eggs. Following a non-expression period, PKMa and PKMb exhibited high expressions again after the hatching stage. In contrast, PKL transcripts could not be detected in early developmental stages, and expression levels continued to increase from the hatching stage to 144 h post hatching. After the 8-week feeding trial with 18%, 30%, and 42% dietary carbohydrate levels, the concentrations of glucose and insulin in serum, pyruvate kinase enzymes, and gene expression levels in brain, muscle, and liver tissues all increased with the increase in carbohydrate levels in the diets. Furthermore, high carbohydrate levels (30% and 42% carbohydrate diets) had a greater effect on grass carp growth. This indicated that PKL, PKMa, and PKMb genes were not only very important in catalytic enzymes, which can be up-regulated by high carbohydrate dietary conditions, but also exhibited a complex and detailed division of labor in different tissues and developmental stages.