Phosphoenolpyruvate carboxykinase (PEPCK, E.C.4.1.1.32) is a key gluconeogenic enzyme in aquatic organisms. In this experiment, three marine fish species with different food habits—omnivorous Tilapia (Oreochromis niloticus), moderate carnivorous Derbio (Trachinotus ovatus) and ferocious carnivorous Cobia (Rachycentron canadum) were selected as the experimental subjects. Dextrin was used as the main dietary carbohydrate source to generate three equal-nitrogen groups of diets with different carbohydrate levels (low dextrin LD, middle dextrin MD and high dextrin HD). Sixty juvenile fish with normal size were randomly selected from each species and fed for 8 weeks during the experimental period. To identify the effects of dietary carbohydrate levels on the gene expression and the enzymatic acidity of PEPCK, we also cloned and analyzed the full-length sequence of PEPCK gene cDNA of Trachinotus ovatus. The full-length cDNA of Derbio PEPCK was 2652 bp and encoded 624 amino acids. The bioinformatics study showed that PEPCK gene was highly conserved between the three species and the homology was higher than 90%. The PEPCK activity of Derbio and Cobia were reduced by 28.05% and 26.03% respectively in the HD groups compared to the LD groups (P<0.05), suggesting the increased carbohydrate level may cause decrease in the enzymatic activity. The expression of PEPCK in the liver of Tilapia, Derbio and Cobia was also reduced with the increase in the carbohydrate level. Compared to the LD group, the PEPCK mRNA level of the HD group exhibited a 100-, 4.3- and 4.77-fold decrease in Tilapia, Derbio and Cobia respectively. These results suggested that the gluconeogenic capacity might be related to the food habit of the fish, and that the gene expression and the enzymatic activity of PEPCK of the three fish species could be significantly inhibited by the elevated dietary dextrin level. The degree of inhibition in mRNA expression also varied probably due to different food habits, and the highest inhibition appeared in Tilapia and the lowest in Cobia.%磷酸烯醇式丙酮酸羧激酶(Phosphoenolpyruvate carboxykinase, PEPCK, E.C.4.1.1.32)是水生生物糖异生代谢的关键限速酶。实验以杂食性罗非鱼(Oreochromis niloticus)鲹、温和肉食性卵形鲳(Trachinotus ova-tus)、凶猛肉食性军曹鱼(Rachycentron canadum)三种不同食性海水养殖鱼类为研究对象,以糊精为饲料糖源,分别设置不同饲料糖添加水平(低糖组LD、中糖组MD、高糖组HD)等氮等能饲料,每种鱼分别随机选取60尾体格均匀的幼鱼进行为期8周的饲养实验,同时克隆鲹卵形鲳胞质型PEPCK基因cDNA全长序列,以期探讨不同饲料糖水平对不同食性鱼类PEPCK活性及其mRNA表达的影响。结果显示：鲹卵形鲳 PEPCK基因cDNA共2652 bp,含1个编码624个氨基酸的开放阅读框,三种不同食性海水鱼类PEPCK的生物信息学比较分析显示相似度达90%以上,在结构和功能上具有较高的保守和同源性。养殖实验结果显示：随着饲料糖水平的增加,三种鱼肝脏中PEPCK酶活性均降低,鲹其中卵形鲳、军曹鱼HD组PEPCK活性比LD组分别显著降低28.05%和26.03%(P<0.05)。而其肝脏中PEPCK mRNA表达水平同样均随饲料碳水化合物水平增加而受到抑制,其中鲹罗非鱼、卵形鲳、军曹鱼中 LD 组 PEPCK 的 mRNA分别是 HD 组的100倍、4.3倍和4.77倍。结果表明鱼类的糖异生能力可能与其食性有关,三种鱼PEPCK酶活性与基因表达量随着饲料糖水平的增加而受到显著抑制,且 mRNA 表达抑制程度随食性不同而具有较大差异,以杂食性罗非鱼受抑制程度最高,凶猛肉食性军曹鱼受抑制程度最低。
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