Characterization and dietary regulation of glutamate dehydrogenase in different ploidy fishes.

Glutamate dehydrogenase (GDH) plays a crucial role in amino acid deamination and has been used as an inductor of nutrients metabolism. In this study, we cloned and analyzed the GDH cDNAs in diploids (red crucian carp), triploids and tetraploids and characterized their expression profiles upon dietary treatments. Results showed a high sequence similarity of GDH among the three kinds of ploidy fishes and other vertebrates. Expression analysis revealed that GDH exhibited a distinct spatial pattern of expression in different types of fishes. The triploids and tetraploids had higher levels of expression than diploids in heart, liver, gill, muscle, fore-gut and mid-gut. The GDH expression was also developmentally regulated with a stronger expression around blastula stage. The maternal GDH transcripts were first detected from eggs and their expression dropped down from the gastrula stage to heart beat stage. Adult triploids showed the highest levels of GDH expression in liver during breeding season which may contribute to the good appetite and fast growth. In addition, triploids exhibited high growth rates and excess GDH expression compared with other two types of fishes. The liver GDH enzyme activities were also higher in triploids than red crucian carp and tetraploids. Moreover, GDH expression is regulated by dietary protein levels. Fish fed with either high or low protein diets showed higher levels of GDH expression. In summary, our results demonstrated for the first time that the different ploidy fishes had different patterns of GDH mRNA expression during development, breeding and non-breeding seasons, and as well dietary effects from different protein levels in diet. These data indicate that abundant GDH expression may play an important role in growth rates in triploids.