Estimation of L-Glutaminase and L-Asparaginase Activities in Soils by the Indophenol Method

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A simple and rapid method was developed for determining L-glutaminase and L-asparaginase activities in soils. It involved colorimetric estimation of NH_4^+ released by the activity of L-glutaminase and L-asparaginase when soil was incubated with 0.25 M L-glutamine or L-asparagine in 0.1 M phosphate buffer(pH 7.6)and toluene used as a germicide. The indophenol method was applied to the assay of NH_4^+ for soil L-glutaminase and L-asparaginase activities. Color development in the indophenol method required one hour. The color development was stable for 2 d and was not appreciably affected by the presence of L-glutamine, L-glutamic acid, L-asparagine, and L-aspartic acid. In addition, no turbidity nor precipitate appeared in the presence of a high concentration(10μg)of NH_4^+-N. The L-glutaminase activity was estimated in three soil samples differing in pH, organic-C content, and land use. The L-glutaminase activity in the three soils ranged from 28 to 1, 470mU(10^<-9> mol min^<-1> g^<-1> of dry soil at 30℃). The optimum pH for the L-glutaminase activity in soils was about 7.5 in phosphate buffer. The incubation time of L-glutaminase and L-asparaginase can be shortened, becase the sensitivity of the indophenol method is about 24 times as high as that of the Nessler method.
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