The study aimed to compare the effects of crystalline L-lysine and L-glutamate (CAA), Lys-Glu dipeptide (KE) on the growth and muscle development of grass carp (Ctenopharyngodon idellus), and related molecular mechanisms. Five experimental diets (CR, 0.5% CAA, 1.5% CAA, 0.5% KE, 1.5% KE) containing Lys and Glu as free (Lys and Glu, CAA) dipeptide (Lys-Glu, KE) forms were prepared, respectively. A total of 450 juvenile grass carp with an initial weight of 10.69 ± 0.07 g were randomly assigned to 15 cages, and 5 treatments with 3 replicates of 30 fish each for 61 days of feeding. The results showed that the group of 0.5% KE exhibited the best growth performances according to the indicator’s weight gain rate (WGR) and specific growth rate (SGR), although no statistically significant occurred among all groups; diet supplemented with 0.5% CAA significantly elevated the condition factor (CF) and viscerasomatic index (VSI) of juvenile grass carp. Diet supplemented with different Lys and Glu co-forms at different levels promoted the muscle amino acid content compared with those of CR group. Comparing with the CR group and other groups, the hardness of 0.5% CAA group significantly increased, and the springiness of 0.5% KE group excelled. Both the muscle fiber diameter and density of 0.5% KE group showed significant difference with those of the CR group, and a negative correlation between them was also observed. To uncover the related molecular mechanism of the differences caused by the different co-forms of Lys and Glu, the effect of different diets on the expressions of protein absorption, muscle quality, and antioxidation-related genes was analyzed. The results suggested that comparing with those of CR group, the dipeptide KE inhibited the expressions of genes associated with protein metabolism, such as AKT, S6K1, and FoxO1a but promoted PCNA expression, while the free style of CAA would improve the FoxO1a expression. Additionally, the muscle development–related genes (MyoD, MyOG, and Myf5) were significantly boosted in CAA co-form groups, and the expressions of fMYHCs were blocked but fMYHCs30 significantly promoted in 0.5% KE group. Finally, the effect of different co-forms of Lys and Glu on muscle antioxidant was examined. The 0.5% CAA diet was verified to increase GPX1a but obstruct Keap1 and GSTP1 expressions, resulting in enhanced SOD activity and reduced MDA levels in plasma. Collectively, the different co-forms of Lys and Glu influenced the growth of juvenile grass carp, and also the muscle development and quality through their different regulation on the protein metabolism, muscle development– and antioxidative-related genes.

本研究旨在比较结晶L-赖氨酸和L-谷氨酸（CAA）、赖氨酸-谷氨酸二肽（KE）对草鱼（Ctenopharyngodon idellus）生长和肌肉发育的影响及相关分子机制。分别制备了五种实验饮食（CR、0.5% CAA、1.5% CAA、0.5% KE、1.5% KE），其中含有游离（Lys 和 Glu，CAA）二肽（Lys-Glu，KE）形式的赖氨酸和谷氨酸。初重10.69±0.07 g的幼草鱼450尾，随机分为15个网箱，5个处理，每组3个重复，每组30尾，饲养61 d。结果显示，从增重率（WGR）和特定生长率（SGR）指标来看，0.5%KE组的生长表现最好，但各组之间没有出现统计学显着性差异；日粮中添加0.5% CAA显着提高了草鱼幼鱼的体况因子（CF）和内脏指数（VSI）。与CR组相比，添加不同水平的Lys和Glu共形式的饮食提高了肌肉氨基酸含量。与CR组及其他组相比，0.5%CAA组硬度显着增加，0.5%KE组弹性优异。0.5%KE组的肌纤维直径和密度均与CR组差异显着，且呈负相关。为了揭示Lys和Glu不同共型引起差异的相关分子机制，分析了不同饮食对蛋白质吸收、肌肉质量和抗氧化相关基因表达的影响。结果提示，与CR组相比，二肽KE抑制了AKT、S6K1、FoxO1a等蛋白代谢相关基因的表达，但促进了PCNA的表达，而游离型CAA则提高了FoxO1a的表达。此外，肌肉发育相关基因（MyoD、MyOG和Myf5）在CAA共型组中显着增强，并且fMYHCs的表达被阻断，但fMYHCs30在0.5%KE组中显着促进。最后，研究了赖氨酸和谷氨酸的不同共形式对肌肉抗氧化的影响。0.5% CAA 饮食经证实可增加GPX1a，但会阻碍Keap1和GSTP1表达，导致血浆中 SOD 活性增强并降低 MDA 水平。总的来说，赖氨酸和谷氨酸的不同共形式通过对蛋白质代谢、肌肉发育和抗氧化相关基因的不同调节，影响草鱼幼鱼的生长以及肌肉发育和质量。