Effect of Dietary Methionine on Taurine Distribution and Non-specific Immune Responses in Juvenile Blunt Snout Bream, Megalobrama amblycephala at A Constant Dietary Cystine Level

A 9-week feeding trial was conducted to investigate taurine distribution by supplementation of methionine in juvenile Megalobrama amblycephala at a constant dietary cystine level. Six semi-purified diets were formulated to contain graded dietary methionine levels from 0.39-1.54% in about 0.25% increments. At the end of feeding trial, plasma methionine content significantly increased with increasing dietary methionine level from 0.39 to 1.0% (P < 0.05) and thereafter reached a plateau. The taurine content of muscle, intestine, liver, and eye in the fish fed dietary methionine level ranged from 1.24-1.54% and was significantly higher than that of the control group (P < 0.05). Taurine content of the brain in fish fed 1.54% methionine diet was higher than the fish fed 0.39% (P < 0.05). Cysteine sulfinic acid decarboxylase content significantly increased with increasing dietary methionine level from 0.39-0.85% (P < 0.05) and thereafter stayed at a similar level. Supplementation with methionine significantly increased (P < 0.05) plasma total protein (TP) content, complement 3 (C3) content and superoxide dismutase (SOD) activity (0.85% methionine diet groups), aspartate transaminase (AST) activity (1.24% methionine diet groups) and albumin content (1.0% methionine diet groups). Supplemented groups had significantly decreased plasma urea content (0.85% methionine diet groups) (P < 0.05). In conclusion, these results indicate that crystalline methionine can be utilized efficiently, juvenile blunt snout bream were able to convert methionine to taurine directly, and appropriate dietary methionine supplementation improves non-specific immune responses in blunt snout bream.