Effect of dietary poly-γ-glutamic acid on growth, digestive enzyme activity, antioxidant capacity, and TOR pathway gene expression of gibel carp (Carassius auratus gibelio)

This study evaluated the effects of poly-γ-glutamic acid (γ-PGA) diets on the growth, antioxidant capacity, and nutrient absorption of gibel carp. Six groups (Control, PGA-25, PGA-50, PGA-100, PGA-200 and PGA −400) were supplemented with different γ-PGA addition (0, 25, 50, 100, 200, 400 mg/kg), respectively, and were fed for 8 weeks. The results showed that the final weight (FW), weight gain (WG) were quadratically and cubically increased (P < 0.05) with increasing dietary γ-PGA levels. The specific growth rate (SGR) was quadratically and cubically decreased (P < 0.05) with increasing dietary γ-PGA levels. The back muscle moisture, crude protein and fat were not significantly affected (P > 0.05), whereas the crude fat was cubically affected by graded dietary γ-PGA levels (P = 0.049). The serum albumin (ALB) content was cubically increased, whereas the aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, triglyceride (TG) and total cholesterol (TC) were decreased with increasing dietary γ-PGA levels. The alkaline phosphatase (ALP) activity, high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C), however, were not affected by the γ-PGA trail. Digestive enzymes, including amylase, lipase and trypsin in the intestine and liver, were all quadratically or cubically increased with increasing dietary γ-PGA levels (P < 0.05). The increasing γ-PGA levels significantly affected total antioxidant capacity (T-AOC), superoxide dismutase (SOD) and catalase (CAT) activities, and content of malondialdehyde (MDA) and glutathione (GSH). Real-time quantitative PCR (RT-qPCR) analysis revealed that the transcription levels of sod, cat, gsta genes in group PGA-100 were significantly increased by 39.0 %, 59.2 % and 51.6 % compared with group Control, respectively. Meanwhile, the group PGA-100 also up-regulated the transcription levels of TOR, 4E-BP2, eIF4E and GHR. The cubic regression analysis based on FW and WG, the optimal dietary γ-PGA requirement was estimated to be 104 mg/kg for gibel carp.