An 8-week feeding trial was conducted to evaluate the effects of dietary
lysine level on growth performance and
protein metabolism of juvenile leopard coral grouper (Plectropomus leopardus) and thereby obtained the optimal dietary
lysine requirement of P. leopardus. Six isoproteic and isolipidic experimental diets were formulated to contain 1.10%, 1.69%, 2.30%, 3.08%, 3.56%, and 4.36%
lysine of diets, respectively. Each diet was assigned at random to triplicate groups of 25 juveniles (initial mean weight is 10.57 g) per tank in a flow-through mariculture system maintained at 27-30°C. Dietary inclusion of 2.30-3.08%
lysine improved the
weight gain rate (WGR) and specific growth rate and decreased the feed conversion ratio (FCR) of juveniles (P < 0.05). The intestinal digestive
enzyme (
trypsin,
amylase, and
lipase) activities were overall enhanced by dietary inclusion of 3.08-3.56%
lysine (P < 0.05). The
mammalian target of rapamycin (mTOR) signaling pathway was activated in fish fed diets with 1.69-2.30%
lysine by upregulating the relative expression levels of hepatic TOR and S6K1 (p70
ribosomal protein S6 kinase 1) but downregulating the relative expression level of hepatic 4E-BP2 (
eIF4E-
binding protein 2). Conversely, the
amino acid response signaling pathway was inhibited in fish fed diet with 2.30%
lysine by downregulating the relative expression levels of hepatic GCN2 (general control nondepressible 2), ATF3 (
activating transcription factor 3), ATF4a (activating
transcription factor 4a), and ATF4b (activating
transcription factor 4b). Additionally, dietary 1.69-3.08%
lysine enhanced the plasma total
protein level and hepatic
lysine α-ketoglutarate
reductase activity but depressed the blood
urea nitrogen level and hepatic
adenosine monophosphate deaminase activity (P < 0.05). Moreover, dietary 3.08%
lysine increased the contents of whole-body crude
protein and total
amino acids, while 1.69%-4.36%
lysine depressed the whole-body
lipid content (P < 0.05). These results indicated that optimal dietary
lysine increased the digestive
enzyme activities, promoted
protein synthesis but depressed protein degradation, and thereby improved the growth performance of P. leopardus. Based on the second-order polynomial model, the optimal
lysine requirement of juvenile P. leopardus for WGR, FCR, and
lysine deposition was 2.60%-2.97% of diets (4.91%-5.60% of
dietary protein).