Effects of dietary hydroxyproline on collagen metabolism, proline 4-hydroxylase activity, and expression of related gene in swim bladder of juvenile Nibea diacanthus

This study was conducted to investigate the effects of dietary hydroxyproline (Hyp) on tissue collagen level, proline 4-hydroxylase (P4H) activity as well as transcript levels of COL1As (COL1A1 and COL1A2) and P4H alpha s (P4H alpha (I), P4H alpha (II), and P4H alpha (III)) in juvenile Nibea diacanthus. A total of 450 fishes were randomized to six equal groups and fed the diet with graded supplementary Hyp-0, 5, 10, 15, 20, and 25 g kg(-1) of dry matter for 8 weeks. Results showed that fish fed diets with 10 g kg(-1) Hyp had significantly higher acid-soluble collagen (ASC) and total collagen (TC) concentrations in swim bladder than fish fed with the other diets (P<0.05). The activity of P4H in liver and swim bladder showed a similar trend, showing first increase and then decrease with increasing dietary Hyp (P<0.05). The mRNA expression of COL1As in swim bladder and muscle were significantly higher than those in the liver and intestines. Meanwhile, with increasing dietary Hyp, the relative expression of COL1As genes in swim bladder showed a similar pattern with the TC concentrations of swim bladder, increased significantly initially followed by a decrease. Increased dietary Hyp content corresponded with significant decrease in the mRNA level of P4H alpha s in swim bladder. These results indicated that the dietary Hyp promotes the collagen accumulation of swim bladder to some extent, and the promoting action may be related to the expression of COL1As. The optimum supplement of dietary Hyp was estimated from TC of swim bladder with piecewise regression analysis to be 9.66 g kg(-1).