Shiitake mushroom (Lentinula edodes) extract has chloride-activated glutaminase activity that efficiently increases glutamic acid in foods with sodium chloride

Abstract

This study aimed to develop glutamic acid content enzymatically from food ingredients during processing and investigate the activity of glutamic acid-producing enzyme in the water extract of shiitake mushrooms. The glutamic acid content in shiitake mushroom extract considerably increased when incubated at 50 °C in the presence of 1% sodium chloride (NaCl), but showed no increase in the absence of NaCl. Glutaminase activity of the extract was higher in the stipe than in the pileus of shiitake mushrooms. Shiitake stipe extract exhibited 32-fold (0.037 ± 0.003 U/g wet weight) and 14-fold (0.018 ± 0.002 U/g wet weight) glutaminase activity in the presence of 10 and 18% NaCl, respectively, compared to its activity without NaCl. The glutaminase activity of the shiitake extract was the highest at pH 6 and 50 °C. The effects of various chloride compounds and sodium salts indicated that glutaminase activity is chloride-activated. Shiitake extract synergistically increased glutamic acid content during incubation with garlic or onion extract in the presence of 1% NaCl. Since glutamine content in the mixture of shiitake extract and garlic extract largely decreased after incubation with NaCl, it was considered that chloride-activated glutaminase of shiitake extract resulted in efficient production of glutamic acid from glutamine. The findings of this study will lead to the development of umami-enhancing technologies based on the activation of glutamic acid-producing enzymes in food products.