吳郭魚在鹽度轉移過程中肌肉內麩胺酸/麩醯胺酸與糖解作用相關代謝之研究

Abstract

臺灣養殖吳郭魚在出貨前多會進行海水轉移步驟以清除肉質的不良風味。本研究運用廣鹽性莫三比克吳郭魚(Oreochromis mossambicus)探討在不同鹽度處理下，肌肉內麩胺酸(glutamate)/麩醯胺酸(glutamine) 以及糖解反應相關代謝機制之作用。 本論文研究結果顯示在轉移至10 ‰海水處理後，肌肉中麩醯胺酸合成酶 (GLUL)與麩胺酸脫氫酶 (GLUD)的基因表現量有顯著提升，同時麩醯胺酸與其他胺基酸含量亦有上升；而轉移至20 ‰海水處理的結果顯示肝醣磷酸化酶 (PYG)基因表現量有顯著上升，六碳糖激酶 (HK)與甘油醛3-磷酸去氫酶 (GPD)基因表現量則有明顯下降的趨勢，此外葡萄糖亦有累積的結果。而在各鹽度處理下，肌肉中氨含量並無顯著變化。因此我們推測：在10 ‰海水處理後會使吳郭魚肌肉傾向將麩胺酸與銨離子結合生成麩醯胺酸，並累積於肌肉中；而轉移至20 ‰海水處理後由於糖解作用受阻致使葡萄糖逐漸累積於肌肉中。由以上莫三比克吳郭魚適應高滲透度緊迫環境所採用的生理策略，導致肌肉組織中胺基酸與葡萄糖的累積，可能是造成人類在食用味覺上產生「鮮味」的原因。

Transferring to hyperosmotic seawater (SW) is one of the essential steps for tilapia aquaculture to diminish undesirable flavor in muscle. In this study, Mozambique tilapia was utilized to investigate glutamate/glutamine and glycolysis related metabolisms in muscle under various salinity treatment. Transcripts expressions of glutamate-ammonia ligase (GLUL) and glutamate dehydrogenase (GLUD) in muscle were found to be significant increased in 10 ‰ SW. Contents of glutamine and other amino acids in muscle were found to be increased in 10 and 20 ‰ SW as well. And in 20 ‰ SW condition, transcripts expressions of glycogen phosphorylase (PYG) was also significant increased. However, hexokinase (HK) and glyceraldehyde-3-phosphate dehydrogenase (GPD) were found to be significant decreased in 20 ‰ SW condition. In addition, glucose were increased in muscle under 20 ‰ SW condition. Besides, ammonia contents in muscle were not affected in SW condition. Those results inferred that the glutamine accumulation in muscle may be generated by glutamate coupled with ammonia after 10 ‰ SW treatment; moreover, glycolysis would be abolished in muscle under 20 ‰ SW treatment. These physiological appearances in aquaculture tilapia muscle under hyperosmotic stress would cause amino acids and glucose accumulation and further induce the “umami flavor” for human’s taste.