Effects of dietary hydroxyproline on growth performance, body composition, hydroxyproline and collagen concentrations in tissues in relation to prolyl 4-hydroxylase α(I) gene expression of juvenile turbot, Scophthalmus maximus L. fed high plant protein d

The present study was conducted to evaluate the effects of dietary hydroxyproline (Hyp) on survival, growth, feed utilization, body composition, Hyp and collagen concentrations in tissues, and prolyl 4-hydroxylase α(I) (P4H α(I)) gene expression of juvenile turbot (initial body weight 8.11 ± 0.01 g) fed high plant protein diets. Seven isoproteic (50% crude protein) and isolipidic (12% crude lipid) experimental diets were formulated to contain 0.12, 0.33, 0.51, 0.60, 0.80, 1.03, and 1.23% Hyp, respectively. Quadruplicate groups of 35 fish were fed to apparent satiation twice daily for 10 weeks in indoor seawater recirculating system. The results indicated that growth performance and feed utilization were not significantly different among fish fed diets with graded levels of dietary Hyp (P > 0.05). No significant differences were found in moisture, crude protein, crude lipid, and crude ash contents of whole body among dietary treatments. No significant differences in condition factor, hepatosomatic index, and viscerosomatic index were found among dietary treatments (P > 0.05). Free Hyp level in plasma and total Hyp contents in liver and muscle were significantly increased as dietary Hyp increased (P < 0.05). Free Hyp level in plasma of fish fed diets with equal to or exceeding 0.60% Hyp were significantly higher than fish fed other diets with lower Hyp (P < 0.05). Fish fed diets with 1.03 and 1.23% Hyp had significantly higher total Hyp content in liver compared to fish fed diets containing 0.12 and 0.33% Hyp (P < 0.05). Fish fed diets with equal to or exceeding 0.60% Hyp had significantly higher total Hyp and collagen concentrations in muscle than fish fed other diets (P < 0.05). Total Hyp and collagen concentrations in muscle of fish fed the diet with 1.23% Hyp was significantly higher than other treatments (P < 0.05). No significant difference was observed in total Hyp and collagen concentrations in vertebrae of fish fed diets with graded levels of Hyp (P > 0.05). No significant difference was observed in hepatic P4H α(I) gene expression (0.90-1.02) of fish fed diets with 0.12, 0.60, and 1.23% Hyp (P > 0.05). However, fish fed diets with 0.60 and 1.23% Hyp showed significantly lower P4H α(I) mRNA levels in muscle compared to a fish fed diet with 0.12% Hyp (P < 0.05). It can be concluded that supplementation of crystalline L-Hyp in high plant protein diets did not indicate positive effects on growth performance of juvenile turbot, but could increase tissues Hyp and muscle total collagen concentration. Although there was no significant effect on hepatic P4H α(I) gene expression, muscle P4H α(I) gene expression was decreased with dietary Hyp supplementation.