探討酵母菌與乳酸菌轉換苦瓜皂苷之影響

Abstract

苦瓜(Momordica charantia)為葫蘆科一年生草本植物，文獻指出苦瓜皂苷是苦瓜苦味及部分功效成分來源，為一群葫蘆烷型三萜類。本研究目的希望透過酵母菌發酵或乳酸菌之醣苷水解酵素來減少苦瓜中苦味皂苷含量。由於苦瓜皂苷骨架由具長鏈側枝的四環三萜構成，多為不帶醣基之皂苷元或修飾有一至兩個醣基之醣苷態。此類結構之皂苷於培養基質中之溶解度不佳，本實驗首先使用大孔吸附樹脂Diaion™ HP-20針對甲醇萃取物進行純化，收集70-100%梯度甲醇可流洗出之苦瓜皂苷，再透過Tween 20增加其溶解度，經微生物生長曲線及點試驗後，選定以含有0.05% Tween 20之苦瓜皂苷培養基進行後續轉換。經Saccharomyces cerevisiae BY4741和Lactobacillus plantarum BCRC 10069轉換後，觀察到兩者之苦瓜皂苷於轉換前後皆有波鋒消長情形，進一步分析後發現經兩者發酵後苦味皂苷Momordicoside K和Momordicine I皆有顯著下降趨勢。為確認是否為醣苷水解酶作用，於酵母菌選定缺乏合成醣苷水解酶基因之菌株進行發酵，而乳酸菌則透過表現其putative β-glucosidase LpBgl進行試驗，分析後皆發現苦瓜皂苷之轉換與鎖定的目標酵素皆無直接相關。最後為得知苦瓜皂苷之轉換情形，使用菌株發酵單一苦瓜皂苷進行觀察，發現在酵母菌與乳酸菌皆發現Momordicoside K含量之減少除轉換成皂苷元外，尚有一部份會轉換成Momordicoside L，可能為甲基轉移酶(transmethylases)或去甲基酶(demethylases)作用所致。綜合上述結果，可以發現酵母菌及乳酸菌中皆有可轉換苦瓜苦味皂苷之酵素，後續可再針對進行轉換之酵素進行探討及應用。

Momordica charantia is a member of the Curcurbitaceae family and is commonly known as bitter melon. Previous studies indicated that specific bitter melon saponins were contributed to the bitter taste and functional activities. The purpose of this study is to reduce the contents of bitter taste saponins by using yeast fermentation or glucosidase hydrolysis. Enhancing the solubility of bitter melon saponins by adding 0.05% Tween 20 is necessary due to the low polarity of tetracyclic triterpenoids of the bitter melon saponins. The results showed that the composition of saponins decline after fermentation with Saccharomyces cerevisiae or with Lactobacillus plantarum. Specifically, the contents of bitter taste saponins, momordiciside K and momordicine I decreased significantly after fermentation. To dissect the underlying mechanism, we used glucosidase deleted yeast to undergo fermentation and in vitro study with putative glucosidase LpBgl in LAB. Unfortunately, we did not identify specific glucosidase related to the hydrolysis of saponins. However, we discovered that momordicoside K was converted into aglycone and the other became momordicoside L. This results indicated that not only glucosidase involved in the biotransformation of saponins, but also possibly transmethylases or demethylases.